PDS_VERSION_ID = PDS3 RECORD_TYPE = FIXED_LENGTH RECORD_BYTES = 80 OBJECT = TEXT PUBLICATION_DATE = 2011-07-31 NOTE = "Old NAVCAM instrument PDS catalog file from Stardust prime mission, provided with Stardust-NExT data set documentation for information and/or comparison." END_OBJECT = TEXT END ######################################################################## ######################################################################## N.B. The PDS keywords keywords below are not functional because they come *AFTER* the PDS_VERSION_ID-END object above. ######################################################################## ######################################################################## PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM LABEL_REVISION_NOTE = "2000-09-14 SDU:Taylor Initial version; 2003-02-29 NAIF:Semenov Pre 2003 peer-review update; 2004-06-10 NAIF:Semenov Post 2003 peer-review update; 2005-03-01 NAIF:Semenov Post 2004 peer-review update 2008-09-04 SBN:Farnham Corrected grammar and spelling;" OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "SDU" INSTRUMENT_ID = "NAVCAM" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "NAVIGATION CAMERA" INSTRUMENT_TYPE = "IMAGING CAMERA" INSTRUMENT_DESC = " The camera state description was provided by the NAVCAM instrument Science Lead, Dr. Raymond L. Newburn, Jr., while the rest of the description was copied from ``STARDUST Navigation Camera Instrument Description Document'' with permission from the Stardust project. Navigation Camera State History =============================== A year after launch the NavCam suffered its one known failure when the filter wheel refused to move when commanded. Fortunately for the overall success of the STARDUST mission, it stuck on one of the two wide bandpass filters, the OpNav (Optical Navigation) filter, a filter which transmits light from about 400 to 900 nm and has the greatest total throughput of any of the eight filters. This filter serves most engineering needs perfectly well. The camera, however, has a Petzval lens system, and over such a wide wavelength range suffers from some chromatic aberration. As a result, the intrinsic point spread function is about 2.3 pixels. (The high resolution filter, by comparison, had a point spread function of a quarter pixel.) Further, this camera lens was manufactured in the early 1970s for the Voyager program, and its antireflection coatings 30 years later leave something to be desired. As a result, all images exhibit a broad shallow skirt of scattered light. When first used after launch, the camera was observed to be heavily contaminated by a coating of unknown source and composition. Total sensitivity was down by a factor of almost 100. A mild heating of the detector to 9 C for 143 hours, utilizing an internal heater, resulted in a slight improvement in performance, reducing the sensitivity loss to about a factor of ten. Every star image still showed a huge halo of scattered light. Turning the spacecraft to place direct sunlight on the radiator, normally used to cool the detector, raised the detector temperature to 24 C for 30 minutes and resulted in great improvement. The camera now showed sensitivity approaching that originally expected and significantly reduced scattered light. Following passage through perihelion and Earth gravity assist, images were acquired of the Moon and of star fields for geometric calibration. It was obvious that some re-contamination had occurred during the previous three months when the spacecraft (but not the cooled detector) was warmest. A third heating cycle resulted in the best images since the camera left the calibration laboratory. A five second exposure reached magnitude 11.7 with a signal to noise of three. The point spread was essentially the 2.3 pixels expected for the filter, though there still was a broad very shallow skirt of scattered light caused by internal reflections in the lens and by residual contamination. Camera performance remained essentially constant for the next six months. After a year in deep space where power was low, communication bandpass limited, and no imaging was attempted, a calibration lamp image once again showed small re-contamination. Interestingly, the periscope, which is not used for most imaging, showed great improvement compared to two years earlier. Before beginning the Annefrank approach, 60 hours of heating to a temperature just above freezing was carried out with the internal heater. No check of the results of this heating cycle has yet been possible. Calibration checks will be performed before and after the Wild 2 encounter, but these were not carried out for the Annefrank encounter, which was conducted as an engineering test and not to gather scientific data. Great caution must therefore be used when attempting to interpret the Annefrank images, since they do contain considerable scattered light. Images acquired May 21, 2003 showed that the camera resolution was still quite good, although a faint halo of scattered light appeared around each image. A calibration lamp image, taken on this date, showed loss in filament resolution, apparently caused principally by the scattered light. An image through the periscope was considerably improved over earlier images, but showed a great deal of scattered light on one side, presumably from the launch adapter ring that actually occults a bit of the periscope on one side. A new feature was a line that was some 10 dn above the background in column 221. This line appeared some time between January 28 and May 21 and has remained to this day. We were not able to take new images until October 8 and October 11, 2003. These indicated that we had acquired some 2.5 magnitudes (a factor of ten) of obscuration over the previous 4.5 months. Another heating cycle reduced this to about 0.5 magnitude. This state would have been adequate for the encounter but not what was desirable. On October 29, 2003 a new problem appeared. An image of the calibration lamp showed nothing. We did not know whether the bulb had burned out (something that had never happened before on any spacecraft) or the shutter didn't open (again something that had never happened before) or, after some thought, the possibility the solar flare that hit us at this time flipped a bit somewhere in the logic circuitry. The next images, taken on November 8, 2003 showed that the shutter was working just fine. There was concern that the failure of the lamp could indicate a short circuit somewhere, so the lamp was not tried again until after the Wild 2 encounter. At that time, January 13, 2004, the lamp was working just fine, leaving us with the somewhat unlikely conclusion that a solar particle had flipped a bit! Three images, each with three second exposures, taken on October 14, 2003 were a first attempt to locate the comet. It hinted at being present on single images, but adding the three together convinced us that we had found 81 P/Wild 2 on the first try. Images three days later with five second exposures absolutely confirmed this. This began the optical navigation effort. Over the next month the images showed some degradation, and there was concern that this might hinder the final days of navigation, to say nothing of the quality of the comet images. So, just five days before the encounter, the Sun was once again turned on the CCD radiator for about 35 minutes. This cleared things up beautifully. As the encounter approached, concern was expressed over the large number of on-off cycles the camera was undergoing for the optical navigation. It was decided to just leave the camera turned on, as a safety measure. Unfortunately this raised the CCD temperature by twelve degrees centigrade, and the images became loaded with hot pixels. (The optics and detector sit on top of the electronics box.) The hot pixels ruined the calibration run that was attempted on December 18, 2003. So, the camera was again turned on and off for every imaging sequence, which caused no problem. The camera went into its encounter activities in a very clean state and completed its imaging with great results. Another calibration sequence was attempted on January 13, 2004 with no problems, except that they were exposed on anything but a rich field. Post encounter images through the periscope showed it to have been heavily damaged by the particle bombardment during encounter, as was expected. And as mentioned previously, the calibration lamp was working just fine. Unfortunately this cal lamp frame was given a one second exposure rather than the appropriate 20 ms. Every pixel was saturated. Navigation Camera Overview ========================== The NAVCAM, an engineering subsystem, is used to optically navigate the spacecraft upon approach to the comet. This allows the spacecraft to achieve the proper flyby distance, near enough to the nucleus, to assure adequate dust collection. The camera also serves as an imaging camera to collect science data. The data include high-resolution images of the comet nucleus at various phase angles during approach and departure. These images can be used to construct a 3-D map of the nucleus in order to better understand its origin, morphology, and mechanisms, to search for mineralogical inhomogeneities on the nucleus, and potentially to supply information on the nucleus rotation state. The camera can provide images, taken through different filters, that give information on the gas and dust coma during approach and departure phases of the mission. These images provide information on gas composition, gas and dust dynamics, and jet phenomena, if they exist. In order to meet these science and optical navigation objectives the NAVCAM design was developed utilizing a Voyager Wide Angle Optical Assembly. Additionally; the NAVCAM has a newly developed scan mirror mechanism to vary the camera viewing angle and a periscope to protect the scanning mirror while the spacecraft flies through the comet coma. The NAVCAM is a framing charge coupled device (CCD) imager with a focal length of 200 mm. The NAVCAM has a focal plane shutter and filter changing mechanism of the Voyager/Galileo type. The detector is a charge coupled device (CCD), cooled to suppress dark current and shielded from protons and electrons. The electronics contain the signal chain and CCD drivers (located in the sensor head), command and control logic, power supplies, mechanism drivers, a digital data compressor and two UARTs too interface with the spacecraft Command and Data Handling (C&DH). NAVCAM command and telemetry functions are also handled by the electronics including storage of science commands, collection of science imaging data and telemetry, transmission of imaging data and telemetry to C&DH and receipt of commands from C&DH. The NAVCAM uses a data rate of 300 kpixels for transferring data to the C&DH. There are also the option for data reduction with 12 bit to 8 bit square root compression, windowing and error free compression within windows. Major Functional Elements ========================= The NAVCAM consists of the following major functional elements (Newburn, 2003): - Optics - Filter Wheel and Shutter Mechanisms - Detector - Scan Mirror Mechanism - Periscope - Electronics and NAVCAM Control Optics ------ The optics subassembly is inherited hardware designed, built and tested for the Voyager Project. It is a Petzval-type refractor lens with a 200 mm focal length, f/3.5 and a spectral range 380 nm - 1000 nm. The optical components, with the exception of the filters, are manufactured from LF5G15 and BK7G14 materials which are radiation resistant. A new field flattener element, located in front of the CCD window, was designed for Stardust to reduce field curvature and to provide additional CCD radiation shielding. The optics are supported on three invar rods that athermalize the system to keep the camera in focus over the operating temperature range. The optical barrel assembly mounts to the filter wheel and shutter assembly utilizing an aluminum truss structure. The housing and truss are also inherited hardware from Voyager. There is a small incandescent lamp, spider mounted in front of the first lens element, that can be used for in-flight calibrations. Because radiation resistant optical materials were used to harden the optics, the lens has a poor broad band modulation transfer function (MTF) performance (axial color). The theoretical MTF for the spectral range 380 nm to 1100 nm is 30% at 32 lp/mm. The thickness of individual filters is optimized to improve the MTF over the filters passband. Optics characteristics are: Focal length 200 mm Relative aperture f/3.5 Spectral Range 380 - 1100 nm Resolution 60 microradian/pixel Field of view 3.5 x 3.5 deg Filter Wheel Subassembly ------------------------ The NAVCAM filter wheel assembly is inherited Flight Spare hardware from the Voyager Project. The assembly contains an eight position filter wheel and a driving mechanism. To actuate the mechanism a pulse is sent that energizes the linear solenoid, thereby rotating the rocker arm by means of the connector rod. The pawl, pivoted on the rocker arm, is driven toward the next wheel cog. At this point the pawl releases latch A from the cog wheel, extends the drive spring and then engages the next cog on the wheel. This puts the mechanism in the cocked position. When the solenoid is de-energized, the rocker arm and pawl are returned to their original positions by the drive spring, which advances the filter wheel one position. During this travel the A latch follows the pawl inward and is in position to stop the filter wheel at the end of the stroke. The back latch B ratchets over the cogs, preventing the wheel from back lashing. A series of photo-diodes are uncovered by a pattern of small apertures in the filter wheel, which are unique for each filter position. Thus the filter that is in the optical path is known for each image taken and is included as part of the engineering telemetry. The spectral response of the camera is controlled by bandpass filters. The bandpass filters for Stardust are new and installed into the filter wheel to replace the Voyager filters. In this table, the filters are identified along with some of their characteristics and their position location in the filter wheel: Filter Name OPNAV -- Optical Navigation Central or Passband (nm) 698.8 FWHM(nm) 400 Transmission 92% Blocking N/A Wheel Position 0 Filter Name NH2 -- NH2 Emission Central or Passband (nm) 665.1 FWHM(nm) 15 Transmission 70% Blocking 10^-2@6500,6800 10^-3@6450,6850 Wheel Position 1 Filter Name OXYGEN -- Oxygen (0[1D]) Emission Central or Passband (nm) 633.6 FWHM(nm) 12 Transmission 60% Blocking 10^-3@6200,6500 Wheel Position 2 Filter Name C2 -- C2 (C2 delta v=0 band) Central or Passband (nm) 513.2 FWHM(nm) 12 Transmission 65% and 52%:5099-5174 Blocking 10^-1@3000,5051 and 10^-1@5230,11000 Wheel Position 3 Filter Name YELLOW -- Yellow Continuum Central or Passband (nm) 580.2 FWHM(nm) 4 Transmission 50% Blocking 10^-2@5750 and 10^-2@5850 Wheel Position 4 Filter Name RED -- Red Continuum Central or Passband (nm) 712.9 FWHM(nm) 6 Transmission 70% Blocking 10^-1@3000-7082 and 10^-1@7175-11000 Wheel Position 5 Filter Name NIR -- NIR Continuum Central or Passband (nm) 874.6 FWHM(nm) 30 Transmission 70% Blocking 10^-2@8400 and 10^-3@9100 Wheel Position 6 Filter Name HIRES -- High Resolution Central or Passband (nm) 596.4 FWHM(nm) 200 Transmission 85% Blocking 10^-3: 3000-480 and 10^-3: 7000-11000 Wheel Position 7 All wavelengths are in nanometers. Shutter Subassembly ------------------- The NAVCAM shutter assembly is also inherited Flight Spare hardware from the Voyager Project. The device is a two-blade focal plane mechanism. Each blade is actuated by its own permanent rotary solenoid. The duration of the exposure is controlled by the time interval between two pulses (an open pulse and a close pulse). The open pulse powers the 'leading' blade and the close pulse powers the 'trailing' blade. The exposure sequence starts with the leading blade covering the aperture. An open pulse moves the leading blade, uncovering the aperture, and the close pulse moves the trailing blade, in the same direction, covering the aperture again. The permanent magnets in the rotary solenoid of each blade hold the blades in a detent position when the shutter is not powered. Exposures can be taken with the blades moving in either direction. A total of 4096 exposure times are available that range from 5 ms to 20 s, in 5 ms increments. There is also a bulb command, for longer exposures, that allows the shutter to be held open for any desired length of time. This double-bladed shutter has the property that in one direction the exposures are 1.65 ms shorter than in the other. Therefore a setting of 5 ms, which is the shortest possible, results in alternate 5 and 3.35 ms exposures, those at 25 ms, alternate 25 and 23.35 ms exposures, and so on. Occasionally bias frames, which do not require shutter action, are transmitted to Earth. This changes the ``parity.'' Detector -------- The NAVCAM uses a charge coupled device (CCD) detector packaged for the Cassini Imaging Science Subsystem (ISS). The operating temperature range is -55 C to -25 C. The CCD is mounted in a hermetically sealed package which is back-filled with argon. An operating temperature of around -35 C is needed for suppression of dark current and to minimize proton gamma and neutron radiation effects. The NAVCAM employs passive radiative cooling to maintain the detector operating temperature. NAVCAM detector characteristics are: Format 1024 x 1024 pixels Pixel size 12 x 12 micrometers Full well >= 100,000 e- Dark current < 0.1 e-/pixel/sec at operating temperature Charge transfer efficiency 0.99996 at operating temperature Read Noise <= 15 e- rms Scan Mirror Mechanism --------------------- This mechanism enables the stationary wide angle optics (flying sidewise during encounter) to keep the comet in view during flyby. The scanning mirror, located some distance forward of the camera lens faces 45 degrees away from the camera viewing axis. Rotating the mirror about the camera axis at the proper rate enables comet tracking during flyby. The mechanism is a single degree of freedom device. It requires proper spacecraft orientation so that the comet can be viewed in a viewing plane originating at the scan mirror and oriented perpendicular to the camera axis. The initial forward looking view (0 degree position) is through a periscope which protects the scan mirror. The mirror's home position is at -20 degrees, at which the camera sees a black object on the spacecraft. Total mirror rotation is 220 degrees, allowing views up to 20 degrees beyond looking straight back. The maximum rotational rate is approximately 3.1 degrees/sec. The mechanism consists of a cylindrical section with mirror and an anti backlash mechanism, the drive unit with motor, gearbox and slip clutch and a base which houses the control electronics. The cylindrical section is coaxial with the camera lens. It consists of the rotational housing containing the mirror and a stationary housing with an anti backlash mechanism attached to it. The sections of the housing which hold the main bearings are made from titanium to enable accurate operations over a 100 degrees C temperature range. A smooth rotational motion is further assured by a duplex bearing pair, by precision gears and an anti backlash mechanism utilizing a negator spring to produce a constant torque against the rotational motion. This should suppress pixel smear to approximately 2 pixels. The mirror, made of zerodur, is bonded to flexures that attach to the rotational housing. Baffling rings along the optical path assure that stray light is being reflected away from the lens. The drive unit next to the housing consists of the following components: A brushless DC motor from American Electronics Inc.: Vmax=36V, T=10oz-in, n~1200rpm. This motor was previously space qualified for the MISR project. The motor is flanged onto the four stage planetary gearbox made by American Technology Consortium: e=252.6:1. The gearbox was previously space qualified for the Mars Pathfinder project. A slip clutch at the gearbox output shaft utilizes a set of Belleville springs to keep the pinion's transmitted torque within a predetermined limit. It prevents mechanical damage in the event of control failures which might cause the mechanism to over-rotate and hit the stops that limit travel. The pinion is engaged with the main gear on the rotational part, providing a fifth transmission stage. The overall gear ratio is 2518.6:1. Periscope --------- The periscope is an optical assembly that allows the scan mirror to look over the protective Whipple shield while it is pointed forward, in a direction parallel to the space craft +X axis. This is to protect the scan mirror from particle impingement, that would significantly degrade its performance, during cruise, upon approach and while flying through the comet coma. The periscope contains two rectangular mirrors mounted at 45 degrees with respect to the space craft +X axis. The mirrors are made out of aluminum to reduce the rate and amount of degradation from particle impacting. For light weighting, the mirrors are fabricated using an aluminum foam core composite material with solid face sheets braised onto the front and back surfaces. Single point diamond turning is used to figure the reflective surface of the mirrors. Since the forward looking mirror is exposed to the impacting particles it is post polished and receives only a very thin protected aluminum coating. While the mirror facing away from the particle stream is nickel coated and post polished with a thin protected aluminum coating. This process achieves a much better mirror figure and smother surface finish but tends to flake off when exposed to particle impact. The periscope structure is a graphite/epoxy composite construction. This material was chosen to make the structure light and to reduce thermally induced distortions from the spacecraft to the periscope assembly. Each mirror is kinematically mounted to the composite structure using three triangular bipod flexures. The periscope is only utilized when the scan mirror is looking forward. After the scan mirror has rotated approximately 15-20 degrees down toward the spacecraft -Z axis it is no longer imaging through the periscope. The periscope was designed so that the images taken while the mirror was partly looking through the periscope could still be used for optical navigation. Electronics and NAVCAM Control ------------------------------ The electronics for the NAVCAM consist of two major parts: the camera electronics and the scan mirror electronics. The sensor head electronics (part of the camera electronics) are mounted on a chassis that is located behind the focal plane of the optics while the rest of the camera electronics and the scan mirror electronics are housed in the baseplate support. The NAVCAM electronics control NAVCAM functions and process NAVCAM commands and telemetry. The NAVCAM electronics are powered from the spacecraft 28 volt regulated and 34 volt unregulated power supplies. The portion of the camera electronics mounted behind the camera is called the sensor head electronics. These electronics support the operation of the CCD detector and the preprocessing of the detector data. The pixel data is quantized to 12 bits giving an intra-frame dynamic range of 4096. Detector readout rate is fixed at 300 kpixels / second. In addition, a direct access port is included in the sensor head electronics to send telemetry to the NAVCAM ground support equipment. This port is used for ground testing only. The remainder of the camera electronics is called the main electronics. The main electronics provide the power and perform all NAVCAM control functions. This includes a CCD clock generator, image compressor, image buffer, mechanism and lamp drivers, telemetry mux and converter, bus controller, UARTs and power supplies. The spacecraft specified RS-422 Bus is used for communication with the Command and Data Handling (C&DH) unit. A high speed bus is used for transmission of image data and a low speed bus is used for sending and receiving commands and telemetry. The NAVCAM scan mirror mechanism has its own interface with the spacecraft. This includes a separate power interface, a bi-directional low speed RS-422 bus for telemetry and commanding transmission, a low speed RS-422 bus for outputting motor rotation pulses, and a discrete output for motor direction. All interfaces with the scan mirror mechanism are done through one 24 pin connector designated J2 that is mounted in the NAVCAM baseplate. NAVCAM Commanding ================= All commands are transmitted and received by the NAVCAM over the low rate RS-422 bus. Commands received by NAVCAM are echoed back to the spacecraft, including parity errors, so that commands with errors can be ignored. This table contains a list of NAVCAM commands: Discrete Commands: Command States/Contents --------------------------- --------------------------------- Camera power off Turn camera power off Camera power on/reset state Turn camera power on/reset camera Camera Function Commands: Sample Analog telemetry 1 of 8 possible channels Sample Digital telemetry 8 registers Move filter wheel 1- 8 positions Take picture (exposure time) shutter exposure and return image data/digital telemetry Select analog telem channel 1 of 8 possible channels Calibration lamp On or Off Data compression On or Off Shutter bulb mode Open or close Scan Mirror Commands: Command States/Contents --------------------------- --------------------------------- Sample telemetry 4 registers Move mirror Mirror is rotated at specified velocity Scan Motor power On or Off Scan Mirror Heater On or Off Telemetry Collection ==================== The NAVCAM collects pixel data, engineering data and status data. This data is divided into three categories: Camera Digital: Image data Shutter exposure time Lamp status (on/off) Compression status (on/off) FIFO status Filter Wheel move steps Filter Wheel position Camera Analog: Filter Wheel voltage CCD Temperature + 5 Volt supply voltage - 5 Volt supply voltage + 12 Volt supply voltage - 12 Volt supply voltage Scan Mirror Mirror velocity Scan motor status (on/off) Heater Status (on/off) Motor direction Motor rotation pulses (tick marks) Housekeeping telemetry consists of engineering and status data only, packetized with appropriate header information into packets called housekeeping packets. This telemetry is used when the NAVCAM is in an ON power state. This telemetry is only used when the NAVCAM is actively taking data. Effective Data Rates ==================== NAVCAM electronics provide a single data rate of 300 kilo-pixels per second. Encoding and Compression ======================== The pixel data from the NAVCAM can be processed within the NAVCAM in several ways. The default processing is to transmit the converted 12 bit data. When data compression is turned on the 12 bit data is compressed to 8 bits using a square-root compression algorithm. This is accomplished via a look-up table stored in ROM. Power Management ================ The camera electronics are required to draw less than 8 watts and the scan mirror less than 10 watts steady state. Operational constraints are placed on the NAVCAM to limit the power drawn by NAVCAM from the spacecraft. This table contains a list of the power operating states. State Definition ------------ ------------------------------------------------ Camera Off 28 volt power to the NAVCAM is off. Heaters powered directly by the spacecraft can still be on. Camera On 28 volt power is applied to the NAVCAM to receive commands, send telemetry and take data. Scan motor Off Power supply to scan mirror is off. Heater can still be on. Scan motor On Power is applied to scan motor to receive commands, send telemetry and scan. At power turn on, the NAVCAM registers are all set to zero. At this point the camera is in an 'idle mode' with all clocks running, waiting to receive commands. The camera remains in this state until the first command is received. The states of all mechanisms are what they were when the camera was last turned off. NAVCAM Safe State ================= In response to a concern that the NAVCAM boresight may, in a spacecraft fault condition, be exposed to the sun (accidentally incident sunlight), a method to protect the shutter and focal plane of the camera was developed. The NAVCAM safe state is defined as placing a narrow band filter in the optical path and opening the shutter. To reset the NAVCAM to a normal operating state a power on reset clears the FPGA lockup." END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "NEWBURNETAL2003" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END