LASCO HANDBOOK FOR SCIENTIFIC INVESTIGATORS Version 1.0 November 1994 Edited by J.W. Cook E-mail: cook@hrts.nrl.navy.mil The LASCO Consortium: NRL E.O. Hulburt Center for Space Research, Naval Research Laboratory, Washington, DC 20375-5352, USA MPAe Max-Planck-Institut fur Aeronomie, 37189 Katlenburg-Lindau, Germany LAS Laboratoire d'Astronomie Spatiale, 13376 Marseille, France DSR Space Research Group, School of Physics and Space Research, University of Birmingham, Birmingham B15 2TT, UK ------------------------------------------------------------------------ Table of Contents Chapter 0. Introduction Chapter 1. The LASCO Consortium for a Coronagraph on the SOHO Mission Chapter 2. Scientific Objectives of LASCO Chapter 3. Outline of the LASCO Instrument Chapter 4. C1 Telescope Chapter 5. C2 Telescope Chapter 6. C3 Telescope Chapter 7. Mechanical Design Chapter 8. The CCD Cameras Chapter 9. Electronics Chapter 10. Onboard Software: Image Compression Chapter 11. Onboard Software: LASCO Observing Sequences Chapter 12. Data Reduction and Distribution Chapter 13. LASCO Data Access and Publication Policies Chapter 14. Sources for Further Information Introduction The purpose of this handbook is to give LASCO investigators from outside the immediate experiment groups a background on the LASCO instrument and operations. It gathers together in one place information that a scientist would need to know in order to intelligently plan a data analysis project. It is not a comprehensive reference book, and the reader can expect further questions to arise while working on any individual project. An immense amount of information about LASCO exists in formal documents, informal memos and notebooks, and in undocumented human memories. This handbook is not meant to be an archival repository, but tries to draw a balance between level of detail, and accessibility to nonspecialist scientific readers. A modest attempt has been made to make each chapter self-contained, by a certain amount of limited repetition, so that they can be read individually for background on a particular topic. The plan of this handbook is the following: Chapters 1-2 discuss the collaborative efforts which produced LASCO, and the scientific goals of the experiment; Chapters 3-9 describe the hardware comprising LASCO; and Chapters 10-13 cover instrument operations, over which the investigator can have some control. The final chapter alerts the reader to another source for information on LASCO and the SOHO mission in general: the MOSAIC browser on the INTERNET. The LASCO home page on the World Wide Web directs the user to constantly expanded and updated material, and puts all potential LASCO investigators on a more equal basis as regards timely information. Instructions for using this resource are given in Chapter 14. This version was completed in November 1994. At this time the launch of SOHO is officially scheduled for September 1995, and LASCO has been delivered to the ESA spacecraft contractor for integration, after full testing at Goddard Space Flight Center and the Naval Research Laboratory. This document was prepared by John Cook, with extensive, greatly appreciated help from Russ Howard. Written contributions and corrections were also provided by Marty Koomen, Clarence Korendyke, Don Michels, Allen Oliver, and Dennis Socker. The LASCO Consortium for a Coronagraph on the SOHO Mission The Solar and Heliospheric Observatory (SOHO) is an unmanned scientific space mission developed by ESA and NASA. It supports two classes of scientific investigations, centered around (1) helioseismology, or the study of the Sun's natural seismic vibrational modes, with the objective of advancing knowledge of the properties and structure of the solar interior, and (2) the processes that account for the heating and acceleration of the solar wind; more broadly, the nature and modes of evolutionary change in the Sun's outer atmosphere. To accomplish these investigations, the SOHO spacecraft will be placed in an orbit about the first Lagrangian, or L1, libration point, that point along the Earth-Sun line at which the gravitational attraction of the spacecraft to the Earth is just balanced by its attraction to the Sun. Located approximately 1.5 million kilometers from Earth, SOHO will require a "cruise phase" of about 4 months to settle into its orbit about L1. The planned orbit allows for observations of very low-frequency helioseismologic oscillations, and for continuous observation of the Sun's outer atmosphere, but suffers some disadvantage in that the distance takes a toll on allowable telemetry data rates. The orbit is only quasi-stable, and periodic station keeping is required. The nominal lifetime for the SOHO mission is two years after arrival on station. Expendables will be carried, however, sufficient for six years of operation. LASCO Instrument Development The Large Angle Spectrometric Coronagraph (LASCO) is a wide-field white light and spectrometric coronagraph consisting of three optical systems having nested fields of view, that together observe the solar corona from just above the limb at 1.1 Rsun, out to very great elongations. LASCO was developed jointly by the Naval Research Laboratory (USA), the Max-Planck-Institut fur Aeronomie (Germany), The Laboratoire d'Astronomie Spatiale (France), and the University of Birmingham (UK), for flight in 1995 on SOHO. The three telescopes comprising LASCO are designated the C1, with coverage from 1.1 to 3.0 Rsun, the C2, with coverage deliberately overlapping parts of both C1 and C3, and extending from 2.0 to 6.0 Rsun, and the C3, which spans the outer corona from about 3.7 to 32 Rsun. The inner corona instrument, C1, is a newly developed mirror version of the classic Lyot internally occulted coronagraph, while the C2 and C3 are externally occulted instruments. In addition, the C1 is fitted with an imaging Fabry-Perot interferometer, making possible spatially resolved high-resolution coronal spectroscopy in selected spectral emission and absorption lines, between 1.1 and 3.0 Rsun. High definition CCD cameras in each telescope provide detailed images with exceptional dynamic range, while large digital memories and a high-speed microprocessor support extensive on-board image processing and image data compression by large factors, that will allow transmission of up to 10 full coronal images per hour. The reader should note that NRL is also a co-investigator institute responsible for major hardware components in the SOHO Extreme_ultraviolet Imaging Telescope (EIT) experiment, that will provide high-resolution images of the emission-line corona over the solar disk and at the limb. There is an important consequence from this: LASCO and EIT are supported by the same electronics command and data handling package. From the operational point of view, this one electronics package, the LASCO Electronics Box (LEB), supports four coronal telescopes having similar technical characteristics, C1, C2, C3, and EIT. At the command and data level, these four instruments are synchronized in highly coordinated operations, with their uplinked commands and downlinked data interleaved into a single, common signal stream. The primary responsibilities of the four LASCO Consortium institutions for the final LASCO flight instrument are listed: Naval Research Laboratory: Principal Investigator: Guenter Brueckner Program Scientist: Don Michels Project Scientist for US: Russ Howard Overall system design and development Design, fabrication, test, and calibration of C3 Design, fabrication, and test of the Fabry-Perot Interferometer Design, fabrication, test, and calibration of the CCD camera systems for C1, C2, C3, and EIT Design and fabrication of the shutters for C1, C2, and C3 Design, fabrication, and test of the shutter mechanism for C2 Design, fabrication, and test of the main electronics system (LASCO Electronics Box) Design and development of the LASCO/EIT flight software Design and development of the LASCO Experiment Ground Support Equipment Design, development, and implementation of the data reduction, archiving, and distribution system LASCO Program management Max-Planck-Institut fur Aeronomie: Project Scientist for Europe: Rainer Schwenn Design, fabrication, test, and calibration of the C1 optical system, including all of its optical parts, mounting cells, interior baffles, and articulated M1 mirror mount, except for the optical bench, CCD Camera, and Fabry-Perot interferometer Design, fabrication, and test of the aperture door mechanisms for C1, C2, and C3 Design, fabrication, and test of the focus mechanisms for C1 and C2 Support for design, development, and implementation of the data reduction, archiving, and distribution system Laboratoire d'Astronomie Spatiale: Science Coordinator for France: Philippe Lamy Design, fabrication, test, and calibration of the C2 optical system, including all of its optical parts, mounting cells, and interior baffles, except for the CCD Camera and optical bench Design, fabrication, and test of the inner occulter mechanism for C2 Design, fabrication, and test of the filter/polarizer wheel mechanisms for C1, C2, and C3 Design, fabrication, and test of the shutter mechanisms for C1 and C3 Support for design, development, and implementation of the data reduction, archiving, and distribution system School of Physics and Space Research, University of Birmingham: Co-Investigator: George Simnett Design, fabrication, and test of the Coronagraph Optics Box (COB), its internal covers, and the mechanical interface to the SOHO spacecraft Design, fabrication, and test of the adjustable and non-adjustable mounting legs Design, fabrication, and test of the boresighters and their associated electronics for the optical alignment system Support for design, development, and implementation of the data reduction, archiving, and distribution system Return to Title Page or Go to Next Chapter Scientific Objectives of LASCO The central scientific questions to be addressed by LASCO are: * How is the corona heated? * Where and how is the solar wind accelerated? * What causes coronal mass ejections, and what role do they play in the evolutionary development of the large-scale magnetic field? * What are the distribution and properties of the zodiacal dust cloud, and what are the effects on it of the small "sungrazing" comets? To answer these questions, the LASCO investigation will study the transport of mass, momentum, and energy through the corona and into the solar wind, by measuring: * Global distributions of key plasma parameters: temperature, density, bulk and nonthermal (turbulent) velocities, and direction of the magnetic field * Time sequences of coronal dynamical events, especially processes that occur in coronal mass ejections, and the conditions that trigger them * The spatial distribution and properties of circumsolar dust particles, including those newly released from sungrazing comets An important feature of the LASCO design is that it will routinely obtain the required synoptic observations simultaneously in the inner (C1), middle (C2), and outer (C3) parts of the field. Consequently, it will be able to trace both the origin and outward extension of coronal structures, and to monitor their temporal evolution. In addition to quantitative measurements of temperature, density, velocity, and magnetic field direction, C1 will also provide a new and important link between white light outer coronal images (C2, C3), and correlative images of the disk and inner emission-line corona from the SOHO EIT. The SOHO spectrometers (CDS, SUMER) observing on the disk and near the limb will benefit from LASCO imagery depicting the global setting to which their measurements apply. Their off-limb spectral measurements, as well as those by the UVCS of the ultraviolet corona, will have available during scientific analysis, and perhaps for mission operations planning, independent measurements of the electron density by LASCO. Detailed Scientific Goals 2.1.1 Coronal Heating and Acceleration of the Solar Wind How is the corona heated? This is perhaps the major unsolved problem in solar coronal physics. Current theories of coronal heating center around either heating by waves guided by the magnetic field, or heating by small-scale reconnection. If wave heating is taking place in the corona, it should be possible to detect it by measuring root mean square (RMS) velocity fluctuations in emission lines formed at coronal temperatures. UV coronal observations show that these velocities should be in the range 20- 30 km/s. We would like to measure the nonthermal velocities of large active region loops, which frequently extend to heights > 10E5 km, with an accuracy of at least 10 km/s. Measurement to this accuracy will provide a critical test of wave heating theories. In addition to being able to measure velocities, it is necessary to image the corona with sufficient spatial resolution to distinguish the major structural elements of the low corona. Images from Skylab, SMM, and Yohkoh in soft X-rays and the XUV show that a large active region loop has a cross-sectional diameter of about 10,000 km. Thus, detailed measurements along a loop requires a spatial resolution in the low corona of roughly 10 arc sec (7200 km). For an isolated large loop, a spatial resolution of 20 arc sec should be adequate for comparing line widths with the typical quiet corona. If heating by small-scale reconnection is taking place, then theory predicts that the heating rate should drop off rapidly with loop length. Testing this model requires the ability to measure the temperatures, densities, velocities and, hence, energy losses of the coronal plasma as a function of height. To determine the lengths of the loops being observed, it is important to be able to distinguish the large scale structure of the inner corona. As with the wave heating observations, this requires measurements with a spatial resolution in the low corona of 10 to 20 arc sec. It is not clear that waves alone account for the energy input to the solar wind. Magnetic loops are sometimes ejected during coronal eruptions and flares, and it has been conjectured that many small magnetic loops may be continually emitted outward through coronal holes, heating the gas and perhaps imparting some outward momentum as well. LASCO will measure, in the first few solar radii above the surface of the Sun where the acceleration moves the gas up to supersonic speeds and where most of the thermal energy is consumed, the electron density directly and the electron and ion "temperatures" of the observed outward flow indirectly. It will also measure the direction of the magnetic field. Line intensity ratios utilizing iron lines can provide a measure of the "frozen-in" temperature of the gas, and line widths provide a measure of the unresolved small-scale RMS fluid motions. The variation of temperature and motions with radial distance is related to the energy budget and gas pressure for the expansion. By combining all the observations from LASCO, the equations for conservation of matter, momentum, and energy for steady flow along a smooth magnetic field can be examined term by term. 2.1.2 Coronal Evolution (Coronal Mass Ejections and Magnetic Field) In addition to the fundamental questions of what heats the corona and what accelerates the wind, there are a host of additional important questions that LASCO will address. These concern primarily the large-scale structure and evolution of the outer corona and its extension into the interplanetary medium. Four such questions are: What is the effect of emerging magnetic flux on large scale coronal features? Observations of the lowest extent of the corona can be made from the ground. To understand fully the effect of emerging flux, it is necessary to go from observations of emerging flux at the surface, through the innermost corona, to the far outer corona. Thus, LASCO must be able to bridge the gap between the low coronal observations made from the ground and the traditional space borne coronagraphs, by observing the inner and middle corona at the same time. Moreover, it is vital to image the corona outward as far as possible to track the propagation of plasma disturbances which are expected to accompany reconnection processes. It is also important to be able to compare coronagraph observations with simultaneous images obtained at X-ray and XUV wavelengths. Following the effects of emerging flux on large scale coronal features, therefore, requires simultaneous imaging over the solar corona from just above the limb to the far outer corona. The innermost observable distance above the limb should be close enough to the limb to provide some overlap with observations from other SOHO imaging instruments (about 1.1 Rsun), while the outermost observable distance from the limb should be as far out as possible to provide the maximum radial extent to track plasma disturbances (about 30 Rsun). What are the properties of helmet streamers? Streamers are evolving structures. By comparing observations of individual streamers over a large range of radii as a function of time, we will be able to determine the extent to which the streamer pattern is affected by individual new "condensations" in the lower corona. In addition, by obtaining profiles in the emission lines observable in the inner corona, we will be able to determine the density, temperature, and flow speeds in the legs of helmet streamers. At higher atmospheric levels, changes in the widths and shapes of streamers should reveal the possible existence of magnetic neutral points. Thus, we desire both extended spatial resolution to allow measurements of the shapes of streamers (10 to 20 arc sec in the inner corona, 20 to 30 arc sec in the middle corona), and also the ability to measure line profiles with the same precision outlined in section 2.1.1. What physical processes are responsible for coronal evolution? The close spatial relation between the global pattern of coronal intensity and the large-scale surface magnetic field clearly shows that coronal evolution reflects evolution of the field. Observations in the inner corona are necessary to determine whether material ejected in a coronal mass ejection (CME) originated in hot coronal condensations over new active regions, or in larger-scale structures which evolved gradually. Coverage of both a large azimuthal and radial extent of the corona is necessary to obtain a complete mass budget of coronal material, and to show the origin of ejected material. To identify the physical mechanism responsible for coronal mass ejections, it is critical to trace the ejected mass back to its source in the low corona. How does a CME evolve as it moves into the heliosphere? A field-of-view extending from the inner corona near 1.1 Rsun to the far outer corona at about 30 Rsun, will allow us to address key issues about the CME mechanism, and the ultimate assimilation of the CME into the solar wind. For example, it should be possible to determine whether fast CMEs continue to accelerate out to 20-30 Rsun, as radio-scintillation observations have suggested. Increased spatial coverage and increased sensitivity will help to resolve the question of the existence of "forerunners," as well as the occurrence and relative positions of associated shock waves. Moreover, increased spatial coverage makes it possible to determine whether slow CMEs, with speeds less than 400 km/s within 10 Rsun, can accelerate to super fast-mode speeds at greater radial distances. 2.1.3 F Corona and Comets The white light corona is composed of two components, the Thomson scattered light from free electrons (K corona) and the light diffracted by interplanetary dust particles (F corona). The radial gradients of these two components are such that the K corona dominates inside about 2.5 Rsun, while the F corona dominates beyond. An accurate determination of the F and K coronae, and of the stray light contribution to intensity, is required to derive, in particular, radial profiles of electron density. The separation will be accomplished using a combination of three independent methods: 1. The Grotrian method, which uses a Fraunhofer absorption line, will be used with C1 to separate the K corona from stray light, which dominates the F corona at these radial distances. 2. The traditional polarization analysis, which will have to be generalized, since the classical assumption that the polarization of the F corona pF is equal to 0, while valid for radial distances inside of 3 Rsun, becomes less valid for increasing radial distances. For example, pF = 0 leads to a factor of two error for the brightness of the K corona at 20 Rsun. 3. A coronal color analysis will be used as a criterion of consistency, since the K corona has neutral color, while the F corona is redder than the Sun in a way which should be consistent with the zodiacal light. Chapter 3. Outline of the LASCO Instrument In 1930 B. Lyot (Ref. 1) invented the coronagraph, which views the solar corona outside of times of total solar eclipse. This instrument is essentially a telescope with an occulting disk in the focal plane to eclipse the image of the solar disk, and with other features to reduce stray sunlight to a level where the corona surrounding the occulting disk can be observed. Because of the residual stray sunlight within the instrument, the corona can be observed only to about 1.3 Rsun from the center of the Sun, and then most successfully in the coronal emission lines. J.W. Evans (Ref. 2) modified the basic Lyot design by placing a circular disk at a substantial distance in front of the entrance aperture, to eclipse the Sun externally and to shade the entrance aperture from direct sunlight. This reduces the instrumental stray light by several orders of magnitude, to a level where the outer corona can be viewed by rocket or satellite borne coronagraphs at altitudes where skylight is absent. R. Tousey (Ref 3) first flew an externally occulted coronagraph on a sounding rocket in 1963. Since then, several externally occulted coronagraphs, all based ultimately on the earlier work of Lyot and Evans, have been flown on satellites for long-term continuous coronal observations: OSO-7 (1971-72) (Ref 4), Skylab (1973-74) (Ref 5), P78-1 (1979-1985) (Ref. 4, 6), and the Solar Maximum Mission (1980-1989) (Ref. 7). These coronagraphs brought improved spatial resolution, time resolution, and mission duration, but were limited to observing the corona over a limited range of elongation. 3.1 Optical configuration A wide field-of-view for LASCO was chosen because of the vast range of distance over which coronal activity influences the solar wind. Within this domain, the K corona brightness varies by about eight orders of magnitude, and relevant spatial scales vary from features the size of photospheric granules (1-2 arc sec), to coronal holes, streamers, and explosive plasma ejections that frequently exceed the size of the Sun itself. To cover these extended ranges of brightness and spatial scale, the LASCO field- of-view is divided into three concentric annular rings, covered by three independent optical systems, each optimized for its observing range, and miniaturized to fit into a single instrument package of reasonable size. The two outer telescopes, C2 and C3, are both externally occulted. This design, however, has a basic limitation. For the necessary distance from the occulting disk to the objective lens, such an instrument can only provide images of the corona from a distance well beyond the Sun's limb (&>1.5 Rsun). The spatial resolution at the inner edge is poor, because most of the objective lens is shadowed by the external occulter, which results in very small effective apertures at the inner edge of the field-of-view. In addition, because of practical size limitations on the instrument length, the objective lens aperture usually cannot exceed a few centimeters. The LASCO design overcomes these problems by covering the inner corona, from 1.1 Rsun to 3 Rsun, by the C1 mirror version of the classic Lyot coronagraph, without an external occulter, thus preserving the full resolution of the instrument over its whole field-of-view. The C2 telescope images the corona from 2 Rsun to 6 Rsun, overlapping the outer field-of-view of C1 from 2 Rsun to 3 Rsun. The C3 telescope extends the field-of-view to 32 Rsun, from a previous maximum of around 10 Rsun with the SOLWIND coronagraph. The overlapping is essential for intercalibration of the three telescopes, including cross calibration on orbit, and to assist in later reconstruction of composite wide-field images. Table 3-1 summarizes the design parameters of the three coronagraphs. LASCO will be the first space borne coronagraph with spectrometric capabilities. The C1 telescope is equipped with a Fabry-Perot interferometer that can take monochromatic images over the whole field-of-view with a spectral resolution of 0.07 nm. By stepping the bandpass across a spectral feature, line profiles can be recorded by a 1024x1024 pixel CCD camera. C1 is internally occulted to obtain coronal images with full instrumental resolution over the entire field-of-view, as close to the solar limb as possible. The resolution of C1 is determined by the detector pixel size, not the theoretical optical resolution of the 4 cm objective mirror, approximately 3 arc sec. The 1024x1024 pixel CCD and 3 Rsun field-of-view give a pixel scale of 5.6 arc seconds. The spatial resolution, taken as two pixels, is approximately 11 arc sec. Over most of their coverage, the spatial resolutions (taken as 2 pixels) of C2 (23 arc sec) and C3 (112 arc sec) are also set by the detector pixel scale, except toward the inner part of their ranges. As discussed, the external occultors cause vignetting at the inner edges of the fields-of-view, and the spatial resolution is degraded. 3.2 Stray Light Test Results Independent tests of an initial configuration of C1 were carried out at the Max-Planck-Institut fur Aeronomie in Lindau and at the Institute d'Astrophysique in Paris. Different test methods produced very similar stray light levels: 10E-6 Bsun at 1.1 Rsun, and 10E-7 Bsun at 3 Rsun, where Bsun is the disk average solar brightness. It was demonstrated that only the scattered light from the first mirror contributes to the total instrument stray light at these levels. C1 was then equipped with improved versions of the earlier mirrors. The stray light measurements for C1 shown in Figure 3-1 are from system level tests performed at NRL and at MPAe, and show an improvement by a factor of two over the results for the earlier mirrors. Stray light levels for C2 shown in Figure 3-1 are based on component measurements, not on system tests. This version of C2 had an inner field limit of 1.5 Rsun, which has been increased to 2.0 Rsun in the flight version. Stray light levels for C3 are based on system tests at NRL, and are an order of magnitude better than the levels suggested in the original proposal. The large field-of-view of C3 (16 degrees total) makes these measurements extremely difficult. When comparing the C2 or C3 stray light levels with the average equatorial coronal brightness at any radial distance, it must be remembered that both telescopes are increasingly vignetted toward their inner field limits, and so their CCD detectors would not see the coronal brightness level of the equatorial curve, but the vignetted brightness level. 3.3 Expected Images From the Coronagraphs A green line coronal image was obtained with a prototype of C1 at the Sacramento Peak Observatory. Figure 3-2 shows the Fe XIV image with a nearby continuum image subtracted. The sky background, which dominated the instrumental stray light, was 40x10E-6 Bsun. At the inner edge of the field-of- view, the instrumental stray light in orbit will be less than 1 x 10E-6 Bsun, an improvement of a factor of 40 over the ground-based image, and images from orbit should be a vast improvement over the ground-based images. In Figure 3-3, an eclipse image by Koutchmy has been adapted to the C1 resolution of 11 arc seconds, to demonstrate the expected C1 image quality from orbit. Similarly, Figure 3-4, an eclipse photo by Keller, shows an image of the corona as it is expected from C2. Note that the full resolution will be achieved only between 3 Rsun and 6 Rsun; resolution inside the 3 Rsun circle is reduced by vignetting. Since the preparation of this figure, the inner radius of the C2 field-of-view has been increased to 2 Rsun. The corona beyond 10 Rsun has never been seen, and the images from C3 will give the first views of this region. The individual, and overlapping, fields-of-view of the three LASCO telescopes are shown in Figure 3- 5 (low resolution) or Figure 3- 5 (high resolution) . The square blocks within which the fields of C1, C2, and C3 are shown give an approximate indication of the spatial coverage of the 1024x1024 CCD detector for each coronagraph. 3.4 Mechanical Design The instrument mechanical design is conceptually similar to the solar instruments at many ground-based observatories. A "spar," or rigid structure (in the case of LASCO, the box containing the optics) provides mechanical support, alignment, and thermal control, and is serviced by a microprocessor-controlled electronics system. Within the spar structure is the cluster of three compact optical systems, each specially designed and optimized for its particular range of operation, and all held in precise co-alignment through proper mechanical and thermal design. LASCO consists of two boxes. The first is the coronagraph optical box (COB). It contains the three optical systems and cameras, and provides alignment, mechanical support, and enclosure against contamination and unwanted stray light. It is mounted to the spacecraft instrument pylon with isostatic mounting legs. The rear set of these legs is equipped with motor drives, controllable in open-loop fashion by ground command, for the purpose of removing any launch-induced misalignments of the LASCO optical axis and the spacecraft pointing direction. The LASCO optical axis is taken to be the C2 optical axis. The second box is the LASCO electronics box (LEB). It contains microprocessors for instrument control and image processing, memory, power conditioning circuitry, and the command and telemetry interface serving C1, C2, and C3, and the Extreme_ultraviolet Imaging Telescope (EIT). 3.5 Electronics The LEB supports three camera/data channels for the three LASCO telescopes, one for the EIT, and a command/data channel for the Fabry-Perot. The camera and Fabry-Perot electronics contain all of the functions required to operate the CCD and Fabry-Perot as a "smart" peripheral, with only high level commands from the LEB being required. Clock-driver, preamplifier, and conditioning circuits for the CCDs are part of the camera units, and are contained on the (plug-in) camera modules. Mechanism motor drivers are also identical in all four channels, with the only difference being the exact number of motors in each. Similarly, monitors are of similar type in each channel. A block-redundant CPU/power converter/memory subsystem approach is employed. Since either processor subsystem can receive the image data from any of the four cameras, considerable flexibility and redundancy exists. Motor control and status information are passed along one interface to the COB, and the camera control and video output are passed along a different interface. Thus, an operational restriction is that only one motor can be driven at the same time, and only one camera can be read out or commanded at the same time. Since in a typical five minute interval, a camera will be read out for about 30 seconds and mechanisms driven for about 30 seconds, neither restriction presents any difficulty to the LASCO/EIT operation. 3.6 Thermal Design The thermal environments of both the COB and LEB are individually controlled. Design of the isostatic mounting legs for the COB, and the attachment of the LEB, includes thermal isolation. Thermal balance and stability are controlled by a multi-layer thermal blanket, Sun shields, radiators, and (in the case of the COB) small balance heaters. A critical item in the thermal design is the inclusion of a passive radiant cooler for each CCD detector chip. In order to minimize thermal dark current noise and to reduce the effects of radiation damage on the CCDs, the chips operate at about -80 degrees C. Each camera has a suitable radiator structure as part of its design. The three are similar in concept, but the details of the shielding baffles depend on the environment and view angle for each camera. References: 1 Lyot, B. 1930, La couronne solair etudiee en dehors des eclipses, C.R. Acad. Sci. Paris 191, 834. 2 Evans, J. W. 1948, A photometer for measurement of sky brightness near the Sun, J.O.S.A. 38, 1083. 3 Tousey, R. 1965, Observations of the white-light corona by rocket, Ann. Astrophys. 28, 600. 4 Koomen, M. J., Detwiler, C. R., Brueckner, G. E., Cooper, H. W., and Tousey, R. 1975, White light coronagraph in OSO-7, Appl. Opt. 14, 743. 5 MacQueen, R. M., Gosling, J. T., Hildner, E., Munro, R. H., Poland, A. I., and Ross, C. L. 1974, The High Altitude Observatory white light coronagraph, Soc. Photo-Opt. Instrum. Eng. 44, 207. 6 Sheeley, N. R., Jr., Michels, D. J., Howard, R. A., and Koomen, M. J. 1980, Initial observations with the Solwind coronagraph, Ap. J. 237, L99. 7 MacQueen, R. M., Csoeke-Poeckh, A., Hildner, E., House, L., Reynolds, R., Stanger, A., Tepoel, H., and Wagner, W. 1980, The High altitude Observatory coronagraph polarimeter on the Solar Maximum Mission, Solar Phys., 65, 91. Chapter 4. C1 Telescope In his pioneering work on the design principles of the coronagraph, Lyot identified five sources of stray light (diffracted and scattered light) in a simple objective lens telescope: * the diffraction pattern from the aperture of the objective lens * a spurious solar image produced by multiple interreflections in the objective lens * macroscopic inhomogeneities in the glass surface inhomogeneities of the lens (pits, scratches) * body-scattering within the glass Lyot designed the coronagraph to allow viewing of the faint solar corona from the ground outside of times of total solar eclipse. In the usual lens version of the coronagraph, a solar image is formed by the objective lens. Photospheric light from the disk is blocked by a blackened occulting disk. A field lens re- images the objective lens and its diffraction pattern, but a "Lyot stop" prevents the diffraction ring from reaching the focal plane, and a "Lyot spot" blocks the spurious image caused by multiple interreflections in the objective lens. With this design, Lyot found that he could eliminate stray light caused by aperture diffraction and by multiple intrreflections in the objective lens, which are the major contributors to telescope stray light. The other three stray light sources mentioned above remain, but can be reduced by selecting very clear glass, and polishing it well. Lyot constructed coronagraphs with a residual stray light level of 5 x 10E-6 Bsun, where Bsun is the disk average solar brightness. The scattered skylight level, even on a very high mountain top, is seldom less than 10 x 10E-6 Bsun, and Lyot's coronagraphs were able to observe the corona from the ground out to radial distances of around 1.3 Rsun. The cover of this Handbook illustrates an image taken during the 1981 July 31 solar eclipse with a normal telescope; of course, the coronagraph obtains images outside of times of total solar eclipse. Coronal observations from space are limited only by the stray light generated in the instrument. So far, all space-borne coronagraphs have been "externally occulted" instruments that use a principle first described by J.W. Evans (see Chapter 5 and Chapter 6 on C2 and C3). The inner corona, however, has not been observed with high spatial resolution because of the increasing distance in front of the objective lens at which the external occulter must be placed, because of vignetting at the inner edge of the field, as the occulted area becomes smaller in angular radius. With the advent of superpolished mirrors and extremely smooth coatings, a new modification of the internally occulted Lyot coronagraph is possible, here called the "Lyot mirror coronagraph." 4.1 C1 Optics In order to image the corona with high spatial resolution very close to the limb, an internally occulted system is necessary. C1 implements this requirement with a mirror telescope design. An off-axis, superpolished, parabolic mirror forms an image of the Sun on a convex mirror. All photospheric light enters through a hole in the convex mirror, which serves as the internal occulter and dumps the light to the outside of the instrument. A Lyot stop blocks the diffracted light arising from the entrance aperture. Since there are no multiple interreflections in a mirror Lyot coronagraph, the Lyot spot can be omitted. In addition, a Fabry- Perot interferometer acts as a narrow bandpass, tunable filter to isolate individual coronal emission lines, further reducing the effect of the stray light background. The filter itself is located behind the Lyot stop, reducing the light that would otherwise be scattered from its surfaces. Figure 4-1 (low resolution) or Figure 4-1 (high resolution) shows a conceptual optical diagram for C1. Light enters a 4.7 cm diameter entrance aperture A0. The objective mirror M1 is an off-axis paraboloid of 75 cm focal length, located 127 cm behind A0. The solar image is formed at the prime focus, on a convex annular mirror M2. This mirror provides a field stop, and also performs the function of the Lyot internal occulter by removing the unwanted photospheric image from the system. It does this by means of a smooth-edged central hole through which the disk image falls, to be subsequently ejected from the system by a diagonal rejection mirror. The field mirror M2 determines both the inner and outer radius of the field-of-view. The field mirror relays the light of the coronal image onto a second off-axis paraboloid collimating mirror M3. M3 is identical to M1, and is in principle a second segment cut from the same superpolished telescope mirror as M1. It is positioned to have the same axis and focal point. This fully symmetric arrangement accomplishes complete cancellation of coma in the system. The field mirror M2, together with M1 and M3, forms an image of the A0 entrance aperture in a plane 70 cm in front of M3. Here the 4.0 cm diameter Lyot stop A1 is placed to intercept the bright ring of light formed by diffraction of the sunlight incident on A0. The divergence of the image beam passing through A1 is a 0.8-degree half- angle, corresponding to the 3.0 Rsun field limit. This small divergence makes possible satisfactory performance of the Fabry- Perot filter. The collimated beam of coronal light from M3 passes through the Lyot stop, and is sent through a narrow bandpass, tunable Fabry- Perot (FP) cavity filter, where the coronal light is analyzed spectroscopically. The FP is an interference (comb type passband) filter, which passes many interference orders. It works in conjunction with a broader blocking filter, with a bandpass sufficiently narrow to eliminate all but one selected order passed by the FP, but broad enough to allow tuning of the FP over nearly a full free spectral range (the wavelength range between successive orders). A second, very broad bandpass filter (110 nm) passes many orders, and serves as a "white light" channel. During a series of exposures, the FP bandpass is stepped across a part of the free spectral range to cover a desired spectral line, with the scanning actuated by piezoelectric drivers. The beam from the FP is sent to a telephoto lens. Filter and polarizer wheels, and the shutter, are located between the telephoto lens and the focal plane. The telephoto lens focuses the final coronal image onto a 1024x1024 pixel CCD camera. The 21 micron square pixel size subtends an angle of 5.6 arc seconds in the coronal image, for a resolution of 11.2 arc seconds (2 pixels). Laboratory measurements of the C1 system demonstrated adequate imaging of the solar disk for sharp occultation at 1.1 Rsun, imaging of A0 on A1 with sufficient quality for the required stray light reduction, and spatial resolution over the entire field to 3.0 Rsun of better than 3 arc sec (although the actual instrumental resolution is set by the CCD pixel resolution). The M1 objective mirror is able to perform small angular movements (see Chapter 7). One use of this ability is in dynamic imaging, where a series of four images is obtained in which the M1 mirror is moved through an angle corresponding to half a CCD pixel in both dimensions. This is equivalent to oversampling the image, and can potentially improve the spatial resolution by a factor of 2. With the door closed, the coronagraph will see the rear of a Sun-illuminated diffuser set in the door. This simulates a flat field source at the corona, with the same spectral character as the stray disk intensity, providing a known level of solar disk illumination at the CCD. In addition, internal calibration lamps (redundant) located near the front door can be used to illuminate the diffuser to provide an additional, calibrated incremental intensity. A photodiode (redundant) will monitor the absolute level of illumination from the lamp to detect any changes in the door surface characteristics or lamp degradation. Both illumination sources are used to calibrate the telescope optics and instrumentation. 4.2 The Fabry-Perot Interferometer The scientific objectives of LASCO required a spectrometric instrument, capable of recording the inner E (emission line) and K (electron scattered) corona over a three solar diameter field-of- view, with high spatial, spectral, and temporal resolution. In the C1 design, a Fabry-Perot interferometer is used as a narrow passband, tunable filter. Sets of individual monochromatic images can be obtained by scanning over a range of wavelengths around each observed coronal emission line. The tunable passband also allows the faint coronal signal to be accurately separated from the instrumental stray light background using differential imaging techniques. The low coronal signal levels, in combination with the high stray light level of an internally occulted coronagraph, result in background noise - limited signal detection over most of the field- of-view. The Fabry-Perot has a bandpass (FWHM) of approximately 0.07 nm, and a tunable range of approximately +/- 1 nm (+/- 500 km/s doppler velocity), both differing slightly for each of the chosen lines. Blocking filters, which are used as order sorters, have a free spectral range (interorder separation) of 3.5 nm. With the 0.07 nm bandpass, this can only be achieved if the finesse (the ratio of the free spectral range to the spectral bandpass) is larger than 50. Cavity length and wedge defect (departure from parallelism) of the interferometer plates are controlled from a microprocessor that uses capacitance micrometers for position sensing. Three piezoelectric transducers determine the cavity length. This combination allows automatic adjustment of wavelength and finesse. The coronal spectrum is referenced to the solar disk Fraunhofer spectrum using three optical control channels, which are located on the perimeter of the interferometer plates. The coatings of the interferometer plates limit the useful wavelength range to approximately 120 nm. The coating was selected to cover the range from Fe XIV at 530.3 nm to H-alpha at 656.2 nm. Two additional channels have been selected for the coronal emission lines of Fe X at 637.4 nm and Ca XV at 569.4 nm. The Na I D line channel at 589.0 nm allows absorption line spectroscopy in the corona for separation of the K coronal and stray light components of intensity. A channel from 530 nm to 640 nm, covering many orders of the interferometer, is used for white light imaging. As discussed previously, blocking filters are used to eliminate all but one selected order passed by the Fabry-Perot. There are four of these blocking filters, one for each of the spectral lines of Fe XIV, Ca XV, Na I D, and Fe X, in the filter wheel. The last filter wheel position contains the broad bandpass "white light" filter. The polarizer wheel contains three polarizers at 120 degrees, a blocking filter for H-alpha, and a clear glass position. Observations of the Fe XIV, Ca XV, Na I D, and Fe X lines are made using the appropriate blocking filter, and the clear glass polarizer wheel position or one of the three polarizers. Observations of H-alpha are made using the H-alpha blocking filter in the polarizer wheel, and the "white light" filter in the filter wheel. The white light observations are made using the "white light" filter in the filter wheel, and the clear glass polarizer wheel position or one of the three polarizers. Table 4-1 summarizes the available channels using the Fabry-Perot. The Grotrian technique will be used to separate the K coronal and stray light components of intensity, using the Na I D Fraunhofer absorption line. Intensity measurements at the Na I D line center and at a nearby continuum wavelength are made with the door open and closed. With the door closed, the diffuser plate (discussed above) introduces a photospheric solar disk spectrum, with an instrumental stray light level which is ignorable. Its Na I D line profile is similar in its wavelength behavior to the instrumental stray photospheric light which is present with the door open. With the door open, a measurement of the coronal spectrum gives the K coronal and stray light components of intensity at the Na I D line center wavelength, and the K coronal and stray light component at the nearby wavelength. With the door closed, the ratio (Na I D line center) / (continuum) is found, and with the door open, the ratio (K corona+Na I D line center stray light) / (K corona+continuum wavelength stray light). Assuming that the first ratio also gives the ratio (Na I D line center stray light) / (continuum stray light) for the open door ratio, the K corona intensity can be separated from the stray light component. The E corona intensity component of the coronal emission lines observed by C1 can be determined by normalizing two sets of measurements, with the door open and closed, at wavelengths outside the emission line, followed by subtraction of the closed door (diffuser) set from the open door (coronal) set at all passband central wavelengths where the emission line is present. Given a reasonably stable instrument, this method should provide coronal measurements whose precision is determined primarily by the background noise - limited observation statistics. Table 4-2 lists the expected precision of two of the emission line measurements, calculated using estimated stray light levels which were taken from the original proposal. These have since been further reduced, and a significant improvement can be expected. Table 4-3 lists potential diagnostic methods for use by C1. Coronal temperatures (assuming T(e) = T(ion)) can be deduced from line intensity ratios using the three forbidden lines, but Ca XV will be useful only in active regions. The three lines can also be used in density diagnostic line intensity ratios. Nonthermal velocities can be derived from line profiles, and coronal dynamics from doppler shifts. Magnetic field directions can be determined from the Hanle effect polarization in Fe XIV. Intensities of the Na I D line can be used to separate the K corona from the stray light component of intensity. Outflows in the corona may be measurable by the doppler dimming in H-alpha, but this remains to be demonstrated. Chapter 5. C2 Telescope The C2 optical design is adapted from earlier, well developed coronagraph designs, ultimately deriving from the fundamental work of B. Lyot and J.W. Evans. A particular source was a miniaturized coronagraph concept, designed and tested but ultimately not flown, for the ISPM (Solar Polar, now Ulysses) imaging experiment. The C2, like C3, is externally occulted. This approach, in which an external occulting disk assembly completely shadows the objective lens from photospheric light, has the advantage of dramatically lowering levels of stray scattered light. This is necessary for observations at elongations from the Sun greater than those viewed by C1, because the intrinsic brightness of the corona decreases very rapidly (about rE-3.5). The main disadvantage of external occultation is that the imaging properties of the objective lens are degraded by vignetting (partial obscuration of the aperture by the external occulter) for object points not far away from the occulter shadow. In other words, the instrument achieves full resolution at the outer part of its field, but relatively poor resolution nearer the inner edge. It is considerations such as this that drive the requirement for extensive overlap of all three of the LASCO telescopes. Where one is weak, another excels, and all are mutually dependent on cross calibration in the overlap regions. 5.1 C2 optics A conceptual diagram for C2 is illustrated in Figure 5-1 (low resolution) or Figure 5-1 (high resolution). The top diagram traces a selected ray bundle for the coronal image, while the lower diagram illustrates the optical elements and ray paths involved in the suppression of stray light. Beginning at the left of each diagram, the external occulter D1 completely shadows the entrance aperture A1 from direct sunlight. The D1 occulter is a threaded conical cylinder, which acts similarly to a stack of closely spaced disks. The objective lens O1 is an achromatic doublet whose elements have been joined in optical contact to avoid possible scattered light from optical cement. The objective lens images the corona in its rear focal plane. A field stop here defines the 6.0 Rsun outer field limit. The O1 objective lens also images D1 onto a stop D2 at a distance behind the coronal image. This D2 internal occulter intercepts residual diffracted light originating at the tapered multiple edges of D1. A short distance behind D2 is a field lens O2, which collimates the primary coronal image, and also images A1 onto the Lyot stop A3. The Lyot stop intercepts diffracted light originating at the entrance aperture A1. Finally, a relay lens O3 behind A3 re-images the primary coronal image onto the 1024x1024 pixel CCD camera at the image plane. The front surface of the O3 relay lens also carries the Lyot spot. This is a small, opaque spot on the optical axis which intercepts a small ghost image of D1 produced by interreflections in the O1 objective lens, a non- negligible source of stray light. The 0.021 mm square pixel size of the CCD subtends an angle of 11.4 arc seconds in the coronal image. The 6.0 Rsun outer field approximately circumscribes the CCD square imaging surface. The total length of the C2 optical path requires the use of folding mirrors to fit it into two adjacents quarters of the Coronagraph Optics Box (see further in Chapter 7, and Figure 7-1). Two flat mirrors, M1 and M2, are used to fold the optical beam. They are coated with a low-polarizing film to avoid introducing significant instrumental polarization. Filter and polarizer wheels, and the shutter, are located in the space between the relay lens and CCD camera. C2 does not have a narrowband, spectroscopic quality filter. As an aid to separation of F from K coronal light, however, it has broadband color filters and polarizers for polarization analysis, as does C3. A moderately narrow (2 nm) H-alpha filter is included. The filter wheel contains the blue, orange, light red, and H-alpha filters, and a lens in the last position which is used to image the external occulter for certain calibration measurements. The polarizer wheel contains three polarizers at 120 degrees, a clear glass position, and a neutral density filter used in conjunction with the filter wheel lens. The four filters are used with the clear glass polarizer wheel position or one of the three polarizers. Table 5-1 lists the bandpasses (FWHM) of these filters. Internal calibration lamps (redundant) are located behind the shutter. When powered, light from the lamps will be reflected diffusely from the rear of the shutter blade, pass through a filter and a polarizer, and then illuminate the CCD. The signal level of the CCD detector can be monitored when the front door is closed. In this case, the coronagraph will see the rear of a small, Sun- illuminated diffuser set in the door. This patch will be out of focus at the CCD, but provides a known level of solar disk illumination. A critical consideration for C2 is precise alignment of the optical axis to the center of the Sun. The axis is taken as a line drawn through the centers of the external occulting assembly D1 and the entrance aperture A1. Thus, when the axis is pointed directly at Sun center, the external occulter shadow falls precisely centered on A1. This condition will be taken as the criterion for proper alignment of LASCO. A further description of the alignment process for LASCO is given in Chapter 7. Chapter 6. C3 Telescope The C3 optical design derives from the NRL coronagraph flown on OSO-7 (1971-74), and the similar SOLWIND coronagraph flown on the P78-1 satellite (1979-84). It is similar in conceptual approach to C2 (externally occulted), and both ultimately derive from the fundamental work of B. Lyot and J.W. Evans. However, C3 is designed to function at extremely large solar elongation angles, and therefore differs in a number of respects that optimize it for this region. In particular, the field lens, which determines the radial extent of the field of view, is quite large compared to the other C2 optics. 6.1 C3 optics The conceptual diagram for C3 illustrated in Figure 6-1 (low resolution) or Figure 6-1 (high resolution) is identical to the diagram for C2 in the previous chapter. The top diagram traces a selected ray bundle for the coronal image, while the lower diagram illustrates the optical elements and ray paths involved in the suppression of stray light. Beginning at the left of each diagram, a 110 mm diameter front aperture A0 admits the coronal light. The external occulter D1 shadows the small 9.6 mm entrance aperture A1 from direct sunlight. The D1 occulter is an assembly of three equally spaced disks on a common spindle, with each disk sized to intercept diffracted light from the edge of the previous one. This configuration reduces the amount of scattered light falling into the umbra where the A1 aperture is located. It is similar in concept to the C2 external occulter, whose threaded conical cylinder can be considered as a stack of closely spaced disks. The objective lens O1 immediately behind the A1 entrance aperture is a superpolished, plano-convex singlet, to obtain absolutely minimal scattering. It forms the primary image of the corona in its rear focal plane. A field stop here defines the 32 Rsun outer field limit. This is followed by the internal occulter D2, which intercepts the image of the D1 external occulter and its halo of diffracted sunlight. Its supporting pylon intercepts diffracted light from the external occulter pylon. Behind this occulter is the field lens O2, whose large diameter is necessary to maintain the outer field limit of 32 Rsun. Because it operates at high relative aperture (f/1.5), it requires a number of elements of different optical glass type to keep aberrations under control. However, its function is the same as that in C2, to image the entrance aperture A1 upon a Lyot stop A3, in order to intercept diffracted light from the A1 entrance aperture edges. It also collimates the primary image, and presents it to a relay lens O3, placed behind the Lyot stop A3 to avoid diffracted light that would otherwise fall upon its surfaces. The relay lens is also a complex lens that controls the residual aberrations from the O1 singlet objective lens, and also provides a long working distance in which to install the shutter and the filter and polarizer wheels, while still maintaining a small image that will fit onto the 21.5 mm square CCD detector. Just before the front surface of the O3 relay lens is a glass plate with a small, opaque Lyot spot, which intercepts a small ghost image of D1 and its diffraction halo produced by interreflections in the O1 objective lens, a non- negligible potential source of stray light. The relay lens transfers the image onto the 1024x1024 pixel CCD camera. The 21 micron CCD pixel size subtends an angle of 56 arc seconds to the solar corona. The CCD square is circumscribed by a 30 Rsun radius image, so that portions of the 32 Rsun optical field-of-view are lost off the top, bottom, left, and right edges of the CCD surface (see Figure 3-5). Several further design features are used in stray light suppression. The contribution to diffracted light from the out-of- field sunlit edge of the A0 front aperture is uncertain, but it has been provided with smooth-edged serrations to direct high order diffracted light away from the A1 entrance aperture. In addition, its out of focus image is intercepted by the field stop at the O1 objective lens image plane. Another stray light source is surface and volume scattered light from the O1 objective lens, which is superpolished to keep scattered light to a minimum. This light cannot be intercepted downstream in the optical path of the telescope, and appears at the focal plane. Stray light arises from internal interreflections in the O1 objective lens, which produce a small ghost image of the external occulter and its ring of diffracted sunlight. This light is propagated down the system, but is re-imaged at the front of the relay lens O3 and intercepted by the small, opaque Lyot spot on a glass plate. The remaining scattered light arises by reflections from edges and from walls. All edges are made sharp, polished to a 25 micron radius, and blackened. Baffles are located so that optical elements view only the rear surfaces of baffles, or shadowed walls. After these efforts, stray light in the system is measured to be 10E-12 Bsun, where Bsun is the disk average solar brightness. This low level is partly achieved by over-occulting the solar disk and corona to 3.7 Rsun. The outer circular field, 32 Rsun, is slightly larger than the 30Rsun circular field which is circumscribed by the 1024x1024 CCD, as illustrated in Figure 3-6. Because of the relatively short distance between the external occulter D1 and the entrance aperture A1, the coronal image is strongly vignetted. A compensating factor is that the brightness range at the focal plane is reduced, so that the entire coronal image can be well recorded with a single exposure. The corona becomes unvignetted beyond 10 Rsun. Filter and polarizer wheels, and the shutter, are located in the space between the relay lens and CCD camera. C3 does not have a narrowband, spectroscopic quality filter. As an aid to separation of F from K coronal light, however, it has broadband color filters and polarizers for polarization analysis, as does C2. A moderately narrow (2 nm) H-alpha filter is included. The filter wheel contains the blue, orange, deep red, and infrared filters, and a clear glass position. The polarizer wheel contains three polarizers at 120 degrees, the H-alpha filter, and a clear glass position. The four color filters are used with the clear glass polarizer wheel position or one of the three polarizers. The H-alpha filter on the polarizer wheel is used with the filter wheel clear glass position. Table 6-1 lists the bandpasses (FWHM) of these filters. Internal calibration lamps (redundant) are located behind the shutter. When powered, light from the lamp will be reflected diffusely from the rear of the shutter blade, pass through a filter and a polarizer, and then illuminate the CCD. The signal level of the CCD detector can be monitored when the front door is closed. In this case, the coronagraph will see the rear of a small, Sun- illuminated diffuser set in the door. This patch will be out of focus at the CCD, but provides a known level of solar disk illumination. Chapter 7. Mechanical Design The LASCO experiment consists of two boxes, the Coronagraph Optical Box (COB) and the LASCO Electronics Box (LEB). The COB contains the three coronagraph optical systems, C1, C2 and C3, each with its optical axis pointed at Sun center. The COB has dimensions of 135 x 34 x 32 cm. Optical axes are parallel to the long axis of the box, and to the solar vector. View apertures for each telescope are at the sunward end of the box. The COB is separable into two sections, illustrated in Figure 7-1. One section contains C1, and the other both C2 and C3. Each section is milled from heavy aluminum 6082-T651 plate, selected for its low stress corrosion and for dimensional stability, and forms the structure upon which lens mounts, baffles, filter wheels, etc. are mounted. Each section is milled with a system of ribs, bulkheads and mounting pads, with thin walls between. When assembled, the interlocking edges, plus ribs and bulkheads, form a trussed structure for maximum stiffness. Lens mounts and baffles are of aluminum alloy (e.g. 6061-T6), and blackened with materials of low vapor pressure: either chemically blackened, black anodized (vacuum baked to drive off condensables), or black teflon coated. The COB experiment layout allowed three co-aligned instruments to be assembled, aligned, and tested at three separate consortium institutions. After assembly, co-alignment was checked and corrected by minor adjustments, such as adjustment of external occulting disks, or internal adjustments reached through small access ports. The design allowed the three systems to be thermally coupled, so that all would operate in the same temperature range. The COB is isostatically mounted using three stress-relieving legs, an adjustable bipod in back and two non-adjustable monopod and tripod legs in the front. The LASCO Electronics Box (LEB) is hard mounted to the spacecraft. Isostatic mounting, with position adjustments by the rear leg, permits the COB to achieve the required pointing accuracy and stability. The mounts of both the COB and LEB provide thermal isolation from the Spacecraft Payload Module. 7.1 Experiment Alignment Once on station, the pointing of the COB will be determined by the C2 optical system, to maximize the stray light rejection in the C2 optics. Each of the three optical systems (C1, C2, and C3) have their own optical axes. Prior to delivery of LASCO, boresighters were co-aligned with auto-collimating telescopes to align the boxes to each other to within 15 arc sec, and to the reference axis. Shims were used to adjust the COB alignment at mounting to the spacecraft to within 3 arc min. Launch and thermal distortions may add up to 5 arc min to the misalignment between the spacecraft and the LASCO pointing axes. To compensate for this misalignment, the rear leg actuators can move the entire COB to Sun center in steps of 2 arc sec. The total range of motion is greater than 15 arc min. 7.2 M1 Mirror Mount The M1 mirror mount can perform small angular movements of the M1 objective mirror of the C1 telescope, to compensate for spacecraft pointing errors or other flight-induced misalignments. It will also be used to increase the resolution of C1 through dynamic imaging (described in Chapter 4). The M1 mirror is held by a three-point mount. Orthogonal tip, tilt, and fore-and-aft motions are provided by low voltage piezo-electric transducers. This will be used occasionally for pointing corrections, once after launch to bring the C1 instrument into optimal alignment, and later when thermal or other changes occur. 7.3 Aperture Doors Each of the three telescopes in the COB has its own front aperture door, to seal the instrument against contamination during storage and launch, and during spacecraft station-keeping maneuvers. The door is moved by a stepper motor driving a lead screw inside the door hinge. All bearings are of low abrasion type, to avoid generation of contaminating particles. The door mechanism is designed to open and close the door at least 1000 times. Since the opening mechanism obviously carries the risk of a single point failure, a redundant one-time opening mechanism is provided. With this, the door arms are released from the lead screw by a paraffin actuator on ground command, and the door is permanently rotated back by means of a helical torsion spring. In addition, the LEB contains redundant motor driver circuits for the COB opening mechanism. 7.4 Filter/Polarizer Wheels and Shutters Each of the three telescopes has a filter wheel, a polarizer wheel, and a shutter. Both wheel diameters are 10 cm, and they are driven directly by 1.8 degree stepper motors. The mechanical shutter is a blade shutter with a 90 degree stepper motor. The shutter blade is indexed in or out of the light path by the motor. Redundant motor driver circuits are provided by the LEB. The shutter must be opened and closed for every exposure. About 120 cycles a day are planned for the C1 and C2 telescopes. The filter wheels will be indexed less frequently, about 50 times per day for C1, and 25 per day for C2 and C3. The filter wheels are located in an area of the COB less sensitive to particulate contamination, but sealed motors are still used. 7.5 Internal Occulter (D2) Centering System The internal occulter D2 of the C2 telescope is movable in the plane perpendicular to the optical axis, to compensate for small misalignments. Small sealed stepper motors with eccentric shafts provide travel in the micron range. Redundant motor driver circuits are provided by the LEB. This mechanism is used to compensate for small but very important misalignments that may occur during launch, or as a result of thermal distortions. 7.6 Focus Mechanisms The C1 and C2 telescopes each include a focus mechanism. A lens mount is moved along the optical axis by a mechanical linkage to a stepper motor. This motor is the same type described for the previous mechanisms. Redundant motor driver circuits are provided by the LEB. The focus mechanisms will be used infrequently, to compensate for changes in the thermal equilibrium which may defocus the optics. The depth of focus of the C3 telescope is greater than the defocusing that would occur for any realistic temperature departure from the nominal 20 C. Chapter 8. The CCD Cameras 8.1 The CCD Chip Each of the LASCO telescopes uses a front-side illuminated, 1024x1024 pixel CCD manufactured by Tektronix for recording images. The imaging area is a 21.5 mm square, and each pixel is a square, 0.021 mm per side. The device is operated in the Multi-Phase-Pinned (MPP) mode. While the MPP mode reduces the full well, it keeps thermally generated noise (dark current) to a minimum. The noise immunity of the MPP implant also helps to avoid the effects of noise generated by energetic particle radiation. The quantum efficiency of the CCD is about 0.3-0.5 in the 500 to 700 nm spectral region. The flight candidate CCDs have very few defects, less than 10 hot or dark pixels over the entire array, and no column defects. Since the full well capacity is between 150,000 and 250,000 electrons, and the read noise of the output amplifier is approximately 5 electrons, a dynamic range exceeding 30,000 is possible. Both vertical and horizontal charge transfer efficiencies (CTEs) are better than 0.999999 for signal levels greater than 0.1 of full well. At low signal levels (0.01 of full well), the CTE drops to about 0.999995. The CCD is mounted on a custom multi-layer ceramic package that satisfies several requirements. The optical focal plane must be located very accurately in all dimensions. Dowel pin holes were provided in the copper-tungsten frame to accurately position the CCD. Measurements of the focal plane were made in a mechanical/optical jig, allowing a custom mounting plate to be made for the CCD to remove any manufacturing tolerances. The multi-layer ceramic package is able to heat the CCD, either to control the operating temperature of the CCD, or to increase its temperature above the ambient temperature to drive off any condensed vapors that might have collected on the surface. A temperature sensor is integrated onto the package, and another on the CCD die itself. The package has provisions for a non-flight cover over the CCD to be attached, that protects the surface of the CCD during handling. The cover shorts all of the pins to a large ground plane, to protect against damage caused by electrostatic discharge, a common failure mechanism for CCDs. This grounding cover remained in place until after the CCD was installed into the camera circuitry, after which discharge damage is minimal. 8.2 The CCD Cameras The CCD camera was designed to be very conservative in the critical spacecraft resources of power and mass, using about 5 watts power, and weighing about 3 kg. The CCD is operated at -80 C. It is susceptible to condensation, and needs to be kept very clean. Thus, the camera electronics are housed in a compartment separated from the CCD, and vented to the box exterior. The CCDs are cooled by passively radiating heat to deep space. A temperature of -80 C was chosen to reduce the effects of permanent proton radiation damage to the bulk silicon, which causes a drastic drop in the ability of the CCD to transfer charge. The CTE can then drop to 0.999, which would virtually destroy the image quality. The effect of this CTE difference is that for a point in the center of the array, the photoelectron charge packet would be reduced by 64% for a CTE of 0.999, compared to only 0.1% for the nondamaged CCD. Providing a mechanical shield only reduces the damage effects by a factor of about 3-6 for a reasonable shield thickness, which is insufficient, but cooling to below -70 C can avoid almost completely the CTE damage produced by radiation effects. The camera accepts commands to initiate various setup parameters, to initiate the clearing cycle, and then to initiate readout. The readout rate is 50,000 pixels/sec, or about 22 sec for a full image readout. The camera provides all of the clock signals and voltages required by the CCD and the analog signal processing chain. The camera can also control the CCD. Unwanted lines can be dumped at the rate of 0.060 msec per line. Thus any line on the CCD can be accessed within about 60 msec. Additional capabilities of the cameras include setting the number of cycles for clearing, setting slow or fast line dumping, reading out of any one of the four CCD chip output ports, and setting of certain voltages for radiation damage compensation. While it is possible to use any of the CCD output ports and their associated electronics lines for an image readout, the camera design has resulted in the readout electronics associated with the CCD chip "D" output port having the best noise performance, less than 1 DN (Digital Number) without the CCD in place. The best output port on the CCD chips generally has a noise performance of 1 DN also, but this is not necessarily from the chip D port. The readout line with the best combined readout electronics noise and CCD port noise performance will be used until a failure occurs, forcing the use of another port. The analog signal processing amplifies the output of the CCD (about 1.5-2.0 x 10E-6 volts per electron) by a factor of about 30, and uses the usual double-correlated sampling technique to sample the photoelectron charge packet. In this technique, the CCD output is measured just prior to injection of the electron charge packet, establishing a reference level. The CCD output is again sampled after the charge packet is injected, and the difference from the reference level is measured. The analog signal is digitized to 14 bits by an analog- to-digital converter, with a quantization step of about 15-20 electrons. The noise of the entire process is less than 10 electrons, so that the entire dynamic range of the system is the full 14 bits, or about 16,000. Note that this does not cover the full dynamic range that the CCD can achieve. Since the total system noise has been measured to be about 25 electrons, the noise will be dominated by photon noise. While the CCD chip has an imaging area of 1024x1024 pixels, output of any image line from the readout register will contain 20 additional pixels at the beginning which are not used for imaging, but provide useful calibration information. These pixels in the readout register are called underscan pixels. The pixel addresses assigned to the image pixels will include these underscan pixels, so that the first true imaging pixel column is 21. In addition, calibration information on the horizontal and vertical charge transfer efficiency can be obtained by commanding the readout of virtual pixels beyond the imaging area, called overscan pixels, both horizontally and vertically. Normally, only the imaging pixels are downlinked to the ground, but the first image column is still numbered 21. The pixel numbering convention is always referenced to the CCD readout port, and column 21 will always be the column closest to the readout port being used. If the readout port changes, for example from A to D, the image as read out will be inverted. This will be corrected during the data reduction process. Chapter 9. Electronics The LASCO and EIT experiments are both controlled by a single electronics unit, the LASCO Electronics Box (LEB). The main LEB central processing unit (CPU) is a Sandia Lab SA3000, a radiation-hardened, 32-bit processor based on the National 32C016. Three other CPUs in the LEB are Intel 8031 processors integrated onto Application Specific Integrated Circuits (ASICs), which are used to offload some control tasks from the main CPU. The LEB receives commands from the spacecraft, and provides science and housekeeping data to the spacecraft for telemetry. The LEB acts as the main controller for the electronics components of the experiment, including the CCD cameras, the Fabry-Perot, telescope mechanisms such as shutters, filter wheels, and polarizer wheels, and the heaters and thermistors. Two instrument controller modules provide redundancy. In addition, the LEB contains the power converters used to supply operating voltages to the LEB, the cameras, Fabry-Perot, pointing eyes, boresighter electronics, and M1 mirror controller. 9.1 Primary and Backup Sides The LEB has a primary and a backup side to increase system reliability. The two sides have identical capability except for the amount of memory. The primary side has 12 Mbytes of random access memory (RAM), while the backup side has 6 Mbytes. The memory is organized into three areas: program space (0.5 Mbytes), an output buffer (1.5 Mbytes), and temporary image buffers (4-10 Mbytes). The redundancy concept is based on a block redundancy approach. The power converter, CPU, and memory units are duplicated for the primary and redundant sides. The primary CPU interfaces to the spacecraft computer primary side, while the backup CPU interfaces to the spacecraft computer redundant side. Similarly, the primary power converter receives power from the primary spacecraft power distribution, while the backup power converter receives power from the redundant side. There are no connections between the primary and backup power busses. The determination of which side of the LEB (primary or backup) will be used is made on the ground. In addition to the block redundant approach for the CPU, memory, and power converter, the instrument controller modules are cross-strapped to allow either CPU subsystem to operate either instrument controller. The redundancy concept allows partial failures in the instrument controllers to be overcome. 9.2 Experiment Control The LEB controls the experiment operations, manages the interface to the spacecraft computer, and performs image processing of the CCD camera data. In addition, it monitors the operation of the experiments and, via an analog-to-digital converter, provides measured values of voltages, temperatures, and other parameters to the experiment housekeeping data stream. The CCD cameras contain their own processors, but act as slaves to the LEB. They perform complex operations when commanded by the LEB, while reducing LEB processing requirements, allowing the LEB to act as the overall operational coordinator. The communication link between the LEB and the cameras is by opto-isolated (to mitigate power surges, noise, etc.), Manchester-encoded ( a digital format), serial channels. The CCD cameras provide images with 1024x024 pixel resolution. The four cameras are identical electronically (the EIT camera is unique only in its mechanical interface). Switched power is supplied to the cameras by the LEB, and power returns from the cameras are brought back to the internal LEB single-point ground. Similarly, the C1 Fabry-Perot contains its own processor acting as a slave to the LEB. Its primary function is to control the Fabry-Perot optical plate position, in order to perform scans of the solar spectrum. It receives commands from the LEB, and provides status reports back to the LEB during operation. The communications link between the LEB and the Fabry-Perot processor is identical to the CCD camera link. The same command and data transfer protocol is used in both the camera and Fabry-Perot links. Switched power is supplied to the Fabry-Perot processor by the LEB, and power returns from the Fabry-Perot are brought back to the internal LEB single-point ground. 9.3 Power Converter The LEB power converter subsystem provides the secondary voltages to all elements of the experiment except some spacecraft- powered heaters and thermistors. There are over 20 separate mechanisms in LASCO and EIT. The instrument controller allows only one mechanism motor to be driven at a time. This limits the power requirements to a reasonable peak value, and allows for adequate power for individual motor operation. Each mechanism motor can be driven by either or both of the two instrument controller modules in the LEB. Encoders are redundant for each mechanism. The primary encoder set is connected to one instrument controller, and the other set to the other controller. Since either or both of the instrument controllers can be powered-on at any given time, partial failures on either or both controllers can be overcome. The LEB also provides thermal control of the five heater zones of the Coronagraph Optics Box (COB), and the two zones in EIT. Thermal control of the COB is required to limit the lateral gradients which would tend to distort the telescope images. (In the EIT, the two heater zones are used for focus control). Either instrument controller can drive a given zone heater (the heaters themselves are not redundant), while each instrument controller drives its own set of thermistors (thermistors are redundant) Chapter 10. Onboard Software: Image Compression The major functions of the flight software are command processing, instrument control, image processing and compression, status monitoring, and telemetry control. In this chapter we describe the image processing and compression options available for LASCO data. The low spacecraft telemetry rate (4.2 Kbps) would result in a transmission time of about 60 minutes for a full 1024x1024, 2 bytes per pixel image. (We note that the individual pixel intensities from the camera analog-to-digital converters are actually represented by only 14 bits, leaving a factor of 4 headroom in the 16 bits which are reserved in the data format.) Thus, image compression is desirable. A number of image processing techniques have been included in the flight software, from simple square root through transform encoding. The types of processing and compression utilized will be determined by ground command and executed through stored sequences, with parameters stored in tables. After a camera image is stored in a 2 Mbyte image buffer, the appropriate algorithms are applied. Image columns that are known to be bad will be replaced by the average intensities of the adjacent, or nearest good, columns. The locations of the bad columns (for each CCD) will be stored in a bitmap located in RAM, which can be updated as necessary. Two general classes of techniques are included in the flight software. The first class consists of image processing techniques which, although by themselves they do not directly compress the telemetry required to transmit an image, do transform the image into a format which is more suitable for true image compression. The second class are true image compression techniques which directly reduce the telemetry load. These consist of both geometrical techniques which reduce the total number of pixels composing an image, and coding techniques which reduce the number of bits necessary to transmit an image, which may already have had other techniques applied previously. Transform encoding yields the highest compression ratio, but is very computationally intensive. Additional compression is obtained by transmitting only the pixels that are not obscured by the occulting disk or the aperture stop. For some situations, the microprocessor will compute intensities only along a radial spoke, thereby saving up to 89% of the telemetry for a full image. Time resolution can be traded against field coverage to further reduce the data download requirement. 10.1 Image Processing Techniques These techniques prepare the image for true compression by transforming the original image into an new image whose intensity histogram distribution has more intensity values falling in fewer histogram bins than did the original intensity histogram distribution: DIV2: The image intensities are divided by 2. This can be repeated to obtain an even smaller range of intensities. This is equivalent to representing the intensities with one less bit for each division. Square Root: The square roots of the intensities are computed. This reduces the number of bits required to represent the maximum intensity by a factor of 2. Difference: A current image is differenced from a second image taken earlier in time, and the differences are transmitted. In a series of images, the second image can be either a constant image, or the immediately previous image to the current image in the series. In either case, original images can be reconstructed from differenced images on the ground. The potential compression factor depends upon the extent of variation between the two images. These variations are due to photon noise, and to real coronal temporal evolution. Summed: The sum of a sequence of scaled images is transmitted. Since the intensity of an individual pixel in an original camera image is represented by only 14 bits, there is potential headroom in the 16 bit output format to add together 4 original images, but for greater than 4 images it is possible to exceed the 16 bit limit and to wrap the output. The scale factor, often set to division by 2, avoids wrapping in the final image. In addition, individual scale factors can be negative to perform differencing. This technique replaces a series of images by one image, and so can be considered a true compression technique, with the compression factor the number of images in the sum. However, the individual images cannot be recovered on the ground. 10.3 Image Compression Techniques The geometric data compression techniques for LASCO are: Geometric: The pixels that are beyond the field limit or that are occulted by the occulting disk are not transmitted. Depending upon the telescope, the compression factor is between 1.3 and 1.5. Subregion: Any subregion in the 1024x1024 CCD may be read out. This will be used to trade field coverage for time resolution. The only restriction is that the subregion must be a multiple of 32 pixels on a side, and must begin at a pixel location which is divisible by 32. The subregion may be rectangular. PIXSUM: The LEB can form pixel sums (binning) of any rectangular size n x m, where n and m are positive integers. This feature is also available on the CCD chip itself, but is then limited to the dynamic range (14 bits) of the analog-to-digital converter. Radial Spoke: An image is transformed into polar coordinates. Along 1-degree wide sectors (pie shaped pieces), the averages along the 512 perpendicular chords are computed. Each of the 360 sectors is replaced by the 512 average values along a spoke through the center of the sector. This alone would produce a compression factor of 1024x1024/(360x512) = 5.7. An annular ring is then specified between an inner radius and an outer radius, and transmitted. By throwing out the pixels beyond the field limit or occulted by the occulting disk ("Geometric"), the compression factor becomes 6-9, depending upon the telescope. The coding compression techniques can be divided into two categories, lossless or lossy, depending upon whether the image can be reconstructed on the ground with no loss of information, or with some (small or negligible) loss. The LASCO options are: Rice: The Rice algorithm is a lossless scheme that creates a unique code for each intensity value that occurs in the image intensity histogram distribution. This code has a variable number of bits, depending upon the frequency of that intensity value, and the most frequent intensity is coded with the least number of bits. The use of a unique code, with no subsection being the code for a more frequent intensity, eliminates the need for a marker between individual intensity codes. However, less frequent intensity values can have codes which are many more bits in length than the actual uncoded value. The Rice technique compares three different schemes for forming the unique code, and also a fourth uncompressed code. It then outputs the image using the most efficient coding scheme. It also outputs a code indicating the coding scheme selected. The analysis is done in blocks of 32x32 pixels. The compression factor is variable, but is often about 2. ADCT (The Adaptive Discrete Cosine Transform): This is a lossy scheme. It is one of the most efficient transforms at compressing the information content into the fewest number of bits. The adaptive feature examines the statistics of each image to determine the coefficients of the cosine transform matrix which represents the image intensities most efficiently in the least number of bits. Compression is achieved by then eliminating higher order coefficients. The compression factor is not fixed, but is selectable to be up to 100. Of course, the higher the compression factor, the higher the information losses; compression factors higher than about 15-20 generally introduce unacceptable errors. The ADCT is computationally intensive, but does not depend upon the degree of compression. The transform is performed on blocks of 32x32 pixels. These compression schemes may be combined to achieve a higher compression. The average compression factor is expected to be about 10. Thus on average, the readout time should be about 6 minutes, which would allow around 200 images each day to be transmitted. Figure 10-1 illustrates at top left an eclipse image, which is compressed by a factor of 10.5 and then reconstructed using three different compression techniques. The top right image uses the ADCT technique described above, while the bottom two images use techniques (Adaptive Hadamard Technique and Block Truncation Coding) which were finally not implemented for the LASCO software. The image quality losses for all three techniques are minimal, as seen by the small values of the normalized mean square errors of the three reconstructed images from the original. Chapter 11. Onboard Software: LASCO Observing Sequences The LASCO observing program has been constructed using a building block approach. The fundamental operational modes of the instrument are pre-programmed, with the variables of exposure times, filter settings, compression techniques, etc., stored in parameter tables. Layers of observing sequence software are built from the fundamental blocks. The observing program will then consist of high level commands building a sequence of observing programs that begin at a specific time of day. Patches to the software stored in ROM and modifications to the table parameters, such as exposure time for each filter, will initially be stored in electrically erasable programmable read-only-memory (EEPROM). This technology is not radiation-hard, and will degrade over some time period after launch. However, it will be acceptable during the initial post-launch checkout, eliminating the need to upload large numbers of commands after any power outage. The length of time until the EEPROM fails is not exactly known, since the failure mechanism depends on the amount of time the EEPROM is powered, which in this case is very short since it is only powered when the code is being transferred to RAM. After the EEPROM fails, the software will be loaded from ROM, and patches will be uploaded from the ground. 11.1 LASCO Electronics Box (LEB) Programs A LEB Program (LP) is a software program that is part of the Observation Executive (OBE) in the LEB. It performs a single observation or set of observations. The LP controls the entire procedure of collecting and processing an image. First it configures the telescope mechanisms, Fabry-Perot interferometer, and cameras. Then it commands an exposure-readout cycle, including commanding the shutter to open and close as appropriate. The camera data is then transferred to the LEB, where it is compressed and passed to the telemetry system. An LP can be scheduled to run immediately, or at some future time. As part of the scheduling process, the LP duration must be specified, as a number of iterations, as a delta time measured from the beginning of the observing program, or as an absolute time. Various instrument parameters such as exposure time, readout coordinates, etc., are specified through tables stored in RAM. The contents of the tables can be modified by ground command. These table content commands can also be scheduled much like an LP. This permits an observation to be made with one set of table entries, and then later the same LP can be performed with new table entries. There are three tables for exposure times: primary, alternate, and calibration. This distinction between the tables is meant for clarification only, and each table is functionally the same. The primary and alternate tables provide the optimum exposure times for full field and reduced field readout, respectively. The calibration table provides the exposure times for calibrations using the internal lamps. Three tables, primary, alternate and calibration, provide the parameters for all of the camera modes, including readout port, dump speed, preamplifier power status, etc. Again the designation of primary, alternate, or calibration is for clarification only. The LPs have been classified into four categories, depending upon the number of images that the LP generates in a single iteration, and how many telescopes participate in the LP (as discussed in previous chapters, the telescopes included in the LP complement include the EIT). These categories are: * Those that take a single image from a single telescope * Those that take multiple images from a single telescope * Those that take images from more than one telescope * Others 11.2 Single Image from a Single Telescope The LPs that take a single image from a single telescope are the following. In all cases the tables defining the camera parameters, the exposure times, the method of compressing the image, and, in the case of C1, the Fabry-Perot wavelengths, must be specified. NORMAL: A single image is taken from a specified telescope (C1, C2, C3, or EIT) for a specified polarizer wheel and filter wheel position. The camera parameter table and exposure time table are also specified. No configuration of the C1 Fabry-Perot is performed. Thus, this program would only be used if the Fabry- Perot had already been commanded into the proper state, or if the image did not depend upon the Fabry-Perot configuration. DARK: A dark exposure is taken from a specified telescope. The shutter is not activated. The camera parameter table and exposure time table are specified. CAL LAMP: A calibration lamp exposure is taken for a specified polarizer wheel and filter wheel position. The camera parameter table and exposure time table are specified. CONT: A calibration exposure is taken from a specified telescope (C1, C2, C3, or EIT), in the pseudo continuous readout mode, for a specified polarizer wheel and filter wheel position. The shutter is opened before the readout starts, and is closed after the readout stops. Thus, the CCD is exposed continuously during readout. The camera parameter table and exposure time table are specified. This pseudo-continuous mode is generally used for calibration of the CCD readout system. 11.3 Multiple Images from a Single Telescope The LPs that take multiple images from a single telescope are the following. In all cases the tables defining the camera parameters, the exposure times, the method of compressing the image, and, in the case of C1, the Fabry-Perot wavelengths, must be specified. FP SCAN LINE: An image sequence is taken from the C1 Fabry-Perot for a specified polarizer wheel and filter wheel position. POLARIZATION SEQUENCE: A sequence of images is taken from a specified telescope using a single filter wheel position, where for each image the polarizer wheel position is specified. Any polarizer wheel position of the five available may be used. WOBBLE OUTSIDE: A dynamic imaging and Fabry-Perot sequence is taken with C1, in which FP SCAN LINE is performed at the four M1 mirror pointing positions. "Dynamic imaging" means that the M1 mirror is moved to the corners of a square 2.8 arc sec (1/2 pixel) on a side. WOBBLE INSIDE: A dynamic imaging and Fabry-Perot sequence is taken with C1, in which an image is obtained at the four M1 mirror pointing positions for each wavelength step of the Fabry-Perot sequence. "Dynamic imaging" means that the M1 mirror is moved to the corners of a square 2.8 arc sec (1/2 pixel) on a side. INTER-SUM: A sequence of images is taken, from either C1, C2, C3, or EIT. Any image processing or compression technique can be applied individually to any of the images, as described by a processing table. Subsequent images individually can be added to or differenced from the first image or the immediately previous image, as determined by the LP processing table. Any combination of images or product images can be sent to ground. This FP allows almost any conceivable succession of images to be formed. An example would be to form and send to ground the simple average of the series of images, to improve signal-to-noise. No Fabry-Perot configuration is performed. 11.4 Images from More than One Telescope The observing programs that use more that one telescope are the following. In all cases the tables defining the camera parameters, the exposure times, the method of compressing the image, and, in the case of C1, the Fabry-Perot wavelengths, must be specified. SEQUENCE: Takes a sequence of images, in any order of the telescopes (C1, C2, C3, EIT). If C1 is specified, a Fabry-Perot sequence is commanded. CONCURRENT: Takes a series of up to four exposures (one from each of the four telescopes) in which the start of the exposures is as closely spaced in time as possible. A table defines the offset times at which the various events in the exposure-readout readout cycle are to be performed, for each of the telescopes from which an image is desired. These events are: when the camera is cleared, when the shutter is opened, when the shutter is closed, and when the camera readout and image processing begins. An image may be kept temporarily on one telescope camera CCD while an image from another telescope is being processed by the LEB. 11.5 Others An observing program that does not fit into one of the above categories is the following: CAM_FP_COORD: Coordinated calibration activities using the C1 camera and Fabry-Perot are performed in which the transmittance of the Fabry- Perot is measured by the CCD. Three calibration tasks are performed by the Fabry-Perot electronics in a semi-autonomous mode: finesse optimization, central aperture scan, and finesse measurement. These tests will be performed by the C1 instrument team. 11.6 General LASCO Operations During the Mission The SOHO satellite will take about four months after launch to reach, and attain station at, the L1 point. During this phase the SOHO instruments, including LASCO, will undergo engineering checkout. Toward the end of this phase some scientific observations will probably be made by LASCO, and it should be ready for full scientific operations once on station. Previous sections described a number of available LEB Programs. The LPs of actual interest for science operations are basically NORMAL for single images from C1 (and the Fabry-Perot if no configuring is required), C2, or C3; FP SCAN LINE for images from the Fabry-Perot alone; POLARIZATION SEQUENCE for images from one telescope; SEQUENCE for general multi-telescope operations; and CONCURRENT for multi-telescope exposures taken together at nearest the same sarting time. From the basic LPs, using individualized parameter tables, higher level observing programs can be constructed. The reader should realize that LASCO is basically telemetry limited in its operations. After taking and processing an image, controlled by the LEB, the data is passed to a 2 Mbyte telemetry buffer. Its storage capacity is approximately 10 compressed images, and with the low available telemetry rate, 4.2 Kbps, it would take approximately one hour to empty the buffer by telemetry to ground. We will call this the "One Hour" buffer. A goal of daily planning is to keep this One Hour buffer full but not overloaded, allowing approximately 200 images a day to be downlinked. A small group of synoptic programs will probably occupy most of the observing day. The synoptic programs will obtain observations of the white light corona at least hourly in each telescope, and also obtain a complete description of the emission line corona several times a day, to generate synoptic maps of turbulence, velocity, temperature, etc. These observations will be designed to thoroughly use the capabilities of all three coronagraphs, and will collect a data set which can satisfy a wide range of scientific objectives. The most direct measure of the observational resources required by an observing program is the telemetry necessary for it. The synoptic programs will utilize something like 85% of the available daily telemetry. Within the synoptic programs, approximately 50% of the telemetry will be devoted to C1 programs, with thorough exercise of the Fabry-Perot capabilities, and 30% to C1 and C2 programs. The EIT telescope, which is controlled by the LEB, receives an allocation of about 20% of available resources (including telemetry) for all of its programs. The C1 spectral line synoptic program will take a complete spectral line description of the Fe XIV and Fe X coronae, 2-3 times per day. The Ca XV corona will be imaged once per day. The polarization of the Fe XIV corona will be determined 1-2 times per day. The white light synoptic program will obtain an image from C1 and C2 every 30 minutes, while C3 will take an image every hour. In addition, a polarization sequence using C2 and C3 will be taken 1-2 times per day. A color sequence will be taken with C2 and C3 once per day. The polarization and color sequences are used to help separate the F corona from the K corona. The remaining available telemetry can be devoted to special observations. These will consist of both special observing programs which will optimize some particular instrumental capability, such as increased temporal resolution, and programs for special targets, such as individual CMEs or streamers. 11.7 Exposure Times We give here a preliminary table of nominal exposures times for various filter-polarizer combinations for the three coronagraphs. The actual optimal exposures times will be determined during the early instrument operations. We also refer the reader to the previous Tables 4-1, 5-1, and 6-1. Table 11-1: Nominal Exposure Times for LASCO Telescope Spectral Line Polarizer Exposure Time Filter (minutes) C1 Fe XIV Yes 3 No 1 Ca XV Yes 5 No 1.5 Na I D Yes 4.5 No 1.5 Fe X Yes 7 No 2 H alpha No 2 White light Yes 3 No 1 C2 Blue Yes 7 No 2 Orange Yes 3 No 1 Light Red Yes 6 No 2 H alpha No 5 C3 Blue Yes 7 No 2 Orange Yes 3 No 1 Deep Red Yes 5 No 1.5 Infrared Yes 6 No 2 H alpha No 5 Clear Yes 1.5 No 0.5 Chapter 12. Data Reduction and Distribution The SOHO operations center will be located at the NASA Goddard Space Flight Center (GSFC) in Greenbelt, Maryland. The experiment teams will be located in the Experiment Operations Facility (EOF) and the Extended Analysis Facility (EAF). After the initial instrument checkout period, only the coronal instrument teams will be permanently resident at the EOF/EAF. The coronal instruments are LASCO, EIT, CDS, SUMER, and UVCS. The MDI helioseismology experiment will be producing photospheric magnetograms regularly, which will be available at the EOF, but may not have representatives at the EOF/EAF to support daily operations and collaborative programs. All of the data from SOHO will be sent to the EOF daily. There will be 4 real-time contacts with the spacecraft each day. One of the contacts will be about 8 hours long, and the other three about 1.5 hours each, for a daily total contact of just over 12 hours. During these contacts, real-time experiment data will flow into the EOF as they are being received at the ground station. LASCO experiment data acquired at other times will be recorded by an on board tape recorder and downlinked during the shorter contact periods (the longer 8 hour period will be used for playback by other SOHO experiments, principally MDI). These playback data will be available at the EOF several hours after being downlinked. These two sets of data, the real time and the playback, are called the Quick Look data. The final experiment data, called the Level 0 data, will be mailed from the GSFC Data Distribution Facility (DDF) to the four consortium institutions about one month after the data are collected. The Level 0 data will be stored on optical disk CD- ROMS, and will be a duplicate of the Quick Look data, but with transmission errors removed. The CD-ROM will also contain summary data from each of the SOHO instruments, spacecraft ephemeris data, command logs, and other ancillary information. The summary data are representative daily images, and catalogs of available data. These summary data will also be available by computer for access by the general scientific community. They will not be used for scientific analysis, since they are processed from the Quick Look data within one day of being received, and may not be properly calibrated, etc. LASCO workstations at the EOF will be linked to the nearby EAF, and to NRL computers, by a high speed network. The EOF computers will acquire the Quick Look data as they are made available. This direct telemetry data will be reformatted and sent to NRL for further processing, including image decompression, calibration, and cataloging. From NRL the data will be sent back to the EOF for use in planning future observations. Data will be transmitted over a high speed link, at 1.544 Mbps (T1 link). Transfer of a complete image (about 1.7 Mbit compressed) should take approximately 1-2 seconds from the EOF to NRL, and about 10-20 seconds for processing at NRL. Since the equivalent of over 200 images will be collected each day, this transfer cycle between the EOF and NRL will be repeated on average every 7 minutes, and utilize about 6% of the time for the Quick Look data. The remainder of the link capacity will be used to query the data base management system, and to transfer images from the data base archive kept at NRL. The LASCO workstations at the EOF will have the capability to decompress the images themselves in case the network link to NRL is interrupted, but will normally leave the image processing to NRL computers. The LASCO Level 0 data will be processed at NRL similarly to the Quick Look data. Image data and catalogs will be generally available over the network, and also distributed on CD-ROMS to the three European consortium members weekly. Each of the four consortium institutions will participate in the overall data analysis program. They will each become the central repository for the LASCO data in their own respective countries, and will administer the access of the scientific investigators from their own countries, including those who are not affiliated with the consortium institution, according to the policies described in Chapter 13. 12.1 Naval Research Laboratory (NRL) NRL will be the primary institution for performing the routine processing tasks, and will be responsible for the Experiment Operations Facility (EOF) at the Goddard Space Flight Center. As discussed, the EOF workstations will be linked to the NRL computers via a high speed network. Real-time and tape recorder playback (Quick Look) data will be acquired by the EOF workstations, reorganized into image and housekeeping files, and transmitted to the NRL computers for decompression, calibration, and cataloging into the data base management system at NRL. Images at various stages of calibration will be transmitted back to the EOF to support the mission operations. The final, Level 0 data will be mailed to NRL from the Data Distribution Facility at GSFC for processing. NRL will then perform the routine processing tasks and disseminate the data to the four consortium institutions. NRL will also be responsible for submitting the final data products to the NSSDC. NRL will provide the calibration data for the C3 optical system, the CCD camera subsystems, the Fabry-Perot interferometer calibration, and the C1 filter and polarizer calibrations. 12.2 European Consortium Institutions The co-investigator institutions (MPAe, LAS, and UB) will provide the algorithms for the routine processing of data from their respective optical systems at NRL. They will be linked to the EOF and to NRL by computer, and can query the data base management system and down-load any items in the data base such as instrument status, processing status, LASCO images, etc. They will perform the detailed analyses of the performance of their respective subsystems, and provide updates to the calibration tables as necessary. European consortium team members may be in residence at the EOF during special observing campaigns, and at other times of interest. 12.3 Image Calibration and Evaluation Following restoration of the compressed data (perhaps with some loss of information) at NRL, the images will be characterized statistically in the following ways: * Noise standard deviation * Histogram * Minimum and maximum intensity * Average intensity The following calibration procedures will be applied to the images: * Locate stars, determine roll angle and absolute pointing * Remove CCD fixed pattern noise * Perform photometric calibration * Remove instrumental vignetting, stray light, and polarization * Calibrate narrow band color images. The images will be evaluated to determine the effectiveness of the compression algorithm that was used. Stellar and planetary objects (whose positions are well known) that are imaged in the fields of view of each of the telescopes, will provide roll angle, absolute pointing, and geometric distortion determinations. Similarly, the radiometric variations of stars and planets in transit through the fields of view, will yield vignetting and photometric checks. Individual instrument calibration sources will be used to track transmission of filters and optics, and sensitivity of CCDs. 12.4 Standard Data Products The data will be searched to locate dynamic events. This search will utilize difference images and an image pyramid scheme to perform an initial screening of the data. If a possible dynamic event is seen, then the images will be searched visually to verify the presence of a real event. As events are found, they will be entered into the event data base. Selected images each day will be used to compute the K-brightness values, polarization, electron density, doppler velocity, green and red line intensity, and green line polarization. These images will be used to produce synoptic maps in each of the quantities. Plots will be generated of the radial brightness variations, line profiles, and daily parameters. Data will be archived on optical disk CD-ROMs to ensure longevity. Sufficient storage capacity on the NRL computers will be available to enable images collected from the most recent 2 weeks to be available over the network. Storage technologies are evolving very rapidly, so that more capacity may be available for the same cost when SOHO actually flies. Juke box storage of optical disk platters could provide 6-12 months of on-line data. Older data will be available on-line upon request. Catalogs of the entire data base will always be available over the network. For the white light science program the following procedures will be available on demand: * Remove F corona * Convert to electron density and mass * Polarization analysis * Electron density profiles of streamers, holes, etc. * Identification of CMEs and/or comets For the emission line science program, the following procedures will be available on demand: * Wavelength calibration * Compute first, second, and third moments of line profiles for any location in an image * Compute doppler shifts and temperature for any location in an image * Plot line profiles for selected locations in an image * Construct monthly synoptic maps of any of the above physical parameters Chapter 13. LASCO Data Access and Publication Policies The LASCO data access and publication policies are intended to encourage the active participation of the space science community. The image data will be archived at the four consortium institutions (NRL, MPAe, LAS, and UB). In addition to the images themselves, catalogs of information about the images will also be available, maintained by a data base management system which can be searched to help the user find the particular images needed for analysis. Both the catalogs and the images may be accessed by computer from an investigator's home institution. There are several classes of potential LASCO scientific investigators. First are the members of the consortium institutions who produced the LASCO instrument. Next are the official co- investigators and associated scientists of the original LASCO proposal. Finally, there are the general members of the scientific community (guests), including scientists associated with other SOHO instruments, who would like to participate in using the LASCO data. This chapter describes the SOHO data access policy, the LASCO data access policy, and the LASCO publication policy. 13.1 SOHO Data Access Policy The SOHO scientific program has issued the following guidelines related to individual experiment data: * The SOHO experiments will be operated in a coordinated manner designed to collect a comprehensive data set on problems defined by the SOHO Science Working Team. * In order to enhance scientific observations and scientific return, data will be shared among the observers at the Experiment Operations Facility (EOF) and the European SOHO Data Archive to aid in the planning of future observations, and for quick analysis of current observations. * Synoptic data will be distributed to all the PI teams one month after collection. These data are defined in the SOHO Operations Plan, and consist of a daily representative image from each of the imaging instruments, time series averages from particle experiments, and catalogs of data from all of the experiments. * The PI teams retain exclusive rights to publish their own data for one year after collection. Thus, data made available by another experiment for planning purposes, as well as for the synoptic data, are not to be used in publications without the consent of the experiment PI. * The PIs are responsible for establishing analysis policies and procedures for coordinated analyses utilizing their instrument's data. 13.2 LASCO Data Access Policy The LASCO data access policy incorporates the SOHO concepts outlined above. Access to data from LASCO will be provided during the mission to all SOHO instruments as necessary to enable planning for future observations, thus enhancing the scientific return from SOHO. However, data provided for this purpose may not be used for independent individual analyses, to avoid uncontrolled overlap with existing projects. Thus, the policy on access to LASCO data will differ for operational purposes and analysis purposes. 13.2.1 Operations Access SOHO experimenter operations scientists located at the EOF/EAF may access recent, uncalibrated images from the LASCO instrument for planning future observations. Recent data will be available on LASCO workstations located at the EOF, and may be accessed directly. For data older than a few days (3-4 days), the data may not be available from the EOF workstation, but will be available from the LASCO archive located at NRL. The LASCO team will provide an image for the daily summary data distribution planned by ESA to all the PI teams. The summary data will be very useful for planning synoptic studies. Note that the summary data will also be available to the general scientific community on the network. Data obtained for operational planning, or in the daily summary distribution, may not be used for independent analysis or publication within the one year exclusive period. 13.2.2 Analysis Access Complete archives of LASCO observations and processing software will be available at the four consortium scientific institutions. The LASCO team will take full advantage of electronic communication to allow easy access to the data, and to facilitate scientific cooperation. Access to the archive is controlled by a data base management system. There are three levels of access: catalog, browse image, and full data. Anyone with INTERNET access may use the catalogs, and any information that has been declared public. A registered user of the LASCO data base management system can also access the browse images, which are reduced resolution images, but which can be used for quick browsing through all of the data. Registration is essentially open to anyone. In order to access full data, a user must first submit an analysis proposal, consisting of an abstract, a list of co-investigators, and a list of required images. This procedure must be followed by all investigators, including consortium scientists, to maintain the coherence of the total analysis effort. After an analysis proposal has been submitted and approved, the proposal PI and co- investigators will be able to access the requested image data sets. Approved analysis proposals will be publicly available through the data base management system. Each approved analysis proposal by a non-consortium scientist (official LASCO co-investigators and guests) will be assigned a LASCO consortium co-investigator to participate in the investigation. This co-investigator will act as an on-site contact/advocate for the guest scientists, serve as their guide to the LASCO data base management system, and participate in the scientific analysis. This direct participation is meant to facilitate data access, and reduce the inherent frustration of using unfamiliar data, particularly for the novice LASCO data user. More experienced guest scientists should be more autonomous, and the LASCO consortium co-investigator participation will be concerned more with the scientific analysis. 13.3 Publication Policy To ensure scientific coordination and coherence in the analysis of the LASCO observations, especially in its early phases, the data access and publication policies will be supervised by an oversight committee, and administered by a LASCO data scientist appointed by the committee. The LASCO Executive Committee that has advised and assisted the PI in the hardware development phase will act as the oversight committee. The Executive Committee will consist of the PI, the Program scientist, and a representative from each of the four consortium institutions. The PI, or the Program scientist in his absence, will act as chairperson. The Executive Committee will establish policies relating to the use of LASCO data and the publication of papers utilizing LASCO data. The committee may change these policies as needed to reflect new conditions. An attempt will always be made to reach consensus on decisions. However, if consensus cannot be reached, the PI has the responsibility and authority to resolve disputes. The committee will meet as frequently as necessary to perform its role. The Executive Committee will appoint the LASCO data scientist. Since the duties of this position will be demanding, the position is intended to rotate among the various LASCO co-investigators. The LASCO data scientist will be responsible for implementing the decisions of the committee. Any disputes arising from decisions made by the LASCO data scientist will be resolved by the committee. The data scientist will be free to refer especially controversial issues to the oversight committee immediately. In the event of a personal conflict of interest, he/she will recuse himself/herself and also refer the issue to the oversight committee. 13.3.1 Categories of Publications A number of potential types of publications can be envisioned. There will probably be some overlap between the examples listed below, but the purpose of this suggested classification is to help the potential LASCO investigator to understand the rationale in the discussion of authorship which follows. LASCO publications may include: LASCO Instrument/System Technical Details This category includes system and subsystem descriptions and/or performance characteristics. It includes both hardware and non-hardware systems. It also includes the individual telescopes. LASCO Scientific Capabilities This category includes papers which describe the scientific capabilities of LASCO. Initial Observational Results These are the initial journal papers that present the first LASCO observational results in a given science area. Major Observational Results This category includes publications on those LASCO observations which are recognized as major new observational discoveries. The point is that the discovery was really the result of the observational capabilities of the LASCO instrument, to which a large number of people made significant contributions. General Observational Results This category is for those papers that report LASCO observational results that do not fall into either of the first two categories. Interpretative Results This category is for those papers whose major emphasis is on the interpretation of previously published observations. Additional new observations which support the interpretations may be included. Invited Papers and Talks This category includes invited presentations at professional meetings and any resulting published paper, and invited papers such as review articles. Collaborative Papers This category includes those papers that are prepared in collaboration with other SOHO instrumenters, SOHO guest investigators, guest scientists, LASCO co-investigators, etc. IAU Circulars This category includes those announcements of comets, etc., that are made as IAU circulars. Press Releases This category includes the announcement of LASCO observations to the press. It can include the announcement of a major discovery. It does not include a press release that may be associated with a paper presented at a scientific meeting. In all cases, the source of the data must be cited with acknowledgments of the ESA/NASA joint mission and the role of the four consortium institutions. A sample attribution follows: "These coronal images were obtained by the Large Angle Spectrometric Coronagraph instrument (LASCO), constructed by a consortium consisting of the Naval Research Laboratory (Washington DC, USA), the Max-Planck Institute for Aeronomy (Lindau, Germany), the Laboratoire d'Astronomie Spatiale (Marseille, France), and the Space Research Group at the University of Birmingham (Birmingham, United Kingdom). LASCO is one of a complement of instruments on the ESA/NASA Solar Heliospheric Observatory satellite." 13.3.2 Authorship Guidelines The lead author on a paper generally should be the individual primarily responsible for the scientific analysis of the observations and the preparation of the results for publication. For some publications, several persons may make major contributions, or different individuals may play the dominant role in different phases of the analysis (and in such situations, the lead authorship could rotate if two or more papers are to be prepared). This is the normal type of authorship question which arises with any multi-authored publication, and is settled by mutual agreement. However, with publications using data produced by an instrument with a large experimenter group, an additional consideration arises: how to be fair in attributing scientific credit to the individuals who made the instrument possible in the first place. This section presents guidelines for appropriate authorship, using the publication classification scheme given previously. An early general paper describing the LASCO instrument and its scientific capabilities will be written by the principal investigator as lead author, with contributions from the consortium co-investigators. This paper will serve as a framework for publishing more detailed subsystem descriptions and subsystem scientific descriptions in additional papers. The publication of papers describing subsystem performance is encouraged, to document in the open literature the LASCO experiment technical achievements. Such papers should be written by the lead individual for the particular subsystem, and should include coauthors who shared in the subsystem development effort. The coauthors may or may not be required to actually contribute to the paper. The accomplishments of the engineering personnel and skilled technicians in designing, developing, and testing a particular subsystem should be acknowledged and appropriately recognized. Papers describing the LASCO scientific capabilities should be written by the lead consortium scientist responsible for developing the portion of the system being described. The papers should include coauthors who shared in the development effort of the subsystem. In certain cases, contractor institutions provided significant assistance to the LASCO personnel, and this should be acknowledged as appropriate. The authorship of these papers will be arrived at on a consensus basis, with the principal investigator and oversight committee resolving any conflicts. For papers describing initial results or major new observational discoveries in the sense defined previously, co-authorship rights may be extended to key LASCO development personnel, and to other consortium members who have significantly contributed to the operations, observations, analysis, or interpretation. Lead authorship may be assigned on the basis of contribution to the overall instrument or subsystem critical to the discovery. In such cases, however, the lead author should play a role, not necessarily dominant, in the data analysis effort, and have primary responsibility for editing the paper. On some guest investigator proposals and resulting publications, the LASCO data scientist may recommend that official LASCO co-investigators or consortium members be added as part of the guest investigator team and as co-authors, for consistency with the LASCO publication policy, and for the inclusion and recognition of key individuals in the LASCO development. Several LASCO consortium team members will be given special recognition due to their major contributions during the development of the LASCO instrument, data analysis system, and mission operations system. These team members may be added as co-authors for selected papers. It is always best to discuss these aspects as early as possible in any guest investigator analysis. Interpretive papers, which as defined previously do not present major new observational discoveries, shall include only those consortium team members who have made significant contributions to the interpretation or related analysis. Such co-authors on papers describing observational results should make significant contributions to a publication, and should fully understand the results. Initial papers describing results obtained during major observational campaigns should include an official LASCO co- investigator or consortium member responsible for obtaining the results as a co-author or lead author. Additionally, if a particular individual went to a great deal of trouble to arrange the observations, he/she should be included as a co-author. Invited talks at major scientific meetings may perhaps have a single formal author, but if a talk presents the work of a large group, this should be clearly acknowledged. Professional courtesy should be utilized in selecting lead and co-authors of all scientific papers at all times. 13.3.3 Internal Review All papers to be submitted for publication should first be submitted to the LASCO data scientist, by computer or by paper copy. These papers will be made available to various members of the LASCO team for review. Reviewer comments should be sent via e- mail or letter to the first author of the paper, and also to the LASCO data scientist. Depending on the nature of the comments, the LASCO data scientist, in consultation with the lead author, may recommend the paper be changed to accommodate the particular concerns. Invited talks at major scientific meetings should also be submitted by title, authors, and abstract to the LASCO data scientist, in order to keep track of LASCO invitations and to alert the LASCO group to planned major presentations. In addition, any resulting paper would go through the normal review process. Scientific results heavily dependent on an accurate photometric or other calibration of the experiment would be expected to receive serious review, particularly early in the mission. This review and feedback process is intended to avoid invalid results reaching the open literature by assuring a wide circulation of the paper prior to publication. To avoid an undue burden on the author, all comments must be made within four weeks of receipt of the paper by the LASCO data scientist. After the paper has been submitted for publication, the paper will be available from the LASCO data base management system. 13.3.5 Publication of unauthorized material In the event of an unauthorized paper reaching publication, the LASCO data scientist and/or oversight committee will impose penalties ranging from barring access to certain data for a certain period of time, to loss of all data access privileges. Return to Title Page or Go to Next Chapter Chapter 14. Sources for Further Information The best existing publically available technical description of the LASCO instrument is in the proceedings of the First SOHO Workshop: "The Large Angle Spectroscopic Coronagraph (LASCO): Visible LIGHT Coronal Imaging and Spectroscopy," by G.E. Brueckner, R.A. Howard, M.J. Koomen, C. Korendyke, D.J. Michels, D.G. Socker, P. Lamy, A. Llebaria, J. Maucherat, R Schwenn, G.M. Simnett, D.K. Bedford, and C.J. Eyles, in Coronal Streamers, Coronal Loops, and Coronal and Solar Wind Composition (ESA SP-348, 1992), pp. 27-34. This article is now somewhat out of date. An issue of Solar Physics devoted to SOHO has been proposed, similar to the issue on SMM which appeared just before its launch. An updated description of LASCO is being prepared for this occasion. The most immediately available source of information and updates on LASCO and the SOHO mission is through INTERNET, using the MOSAIC browser. This information is constantly undergoing expansion and update, and is available to a wide audience. An enormous range of material is potentially available to individuals with access to INTERNET, and tools for searching and recovering information are constantly under development. One of the most useful is MOSAIC, which is a graphical browser used to access information on the World Wide Web (WWW). The basic concepts of the WWW are address protocols which can point to the location of any publically accessible file on any computer on the INTERNET, and hypertext links in recovered data files. These hypertext links are marked keywords in a file which point to further files of potential interest, which can be text, graphics, or even video. A single file can have many hypertext links to other files, each of which itself is linked to further files, and a web of connections is present throughout the WWW. Any desired piece of information (file) is accessible from many different starting points. MOSAIC is a tool to access information (a browser) on the WWW, using a client-server model. Server software is on any computer offering information. Client software runs on the user's computer to recognize user requests and display the information recovered from the server. The easiest way to learn to use MOSAIC is simply to start. Your home computer must be able to access the INTERNET, support windows, and have installed the MOSAIC client program, which is available for UNIX systems, PC Windows, Macs, and recently for VAX VMS systems. For a UNIX version the user can anonymously ftp a binary executable file from the site ftp.ncsa.uiuc.edu, in the directory /Mosaic/Mosaic-binaries/. The WWW uses a file reference system employing URLs (Uniform Resource Locators). MOSAIC will want a URL from the user, or can access URLs embedded in hypertext links in files which are recovered by the user. However, you don't have to search if you know what you want! Once starting MOSAIC (on a UNIX system probably with "xmosaic &"), select "Open URL" in the File menu, and enter the URL for the LASCO home page: http://lasco-www.nrl.navy.mil/lasco.html Other URLs of interest are the Solar Data Analysis Center (SDAC) at Goddard Space Flight Center: http://umbra.gsfc.nasa.gov This will lead the user to a SOHO home page, which directly is http://umbra.gsfc.nasa.gov/soho/anglais/soho Use of the MOSAIC tool may become in the future, especially after launch of SOHO, the most important source of information available to the general LASCO community. Downloaded: http://lasco-www.nrl.navy.mil/handbook/hndbk.html Modified: Matthew Knight 22 February 2006