***** File NETLARGE.TXT I. SUMMARY This document describes the FITS keywords indexed in the delimited table NETLARGE.IDX in the INDEX directory of this CD-ROM, and provides a brief overview of Large-Scale Phenomena (L-SP) Discipline and Network operations. A more complete discussion of the history of the L-SP Discipline is found in the Discipline Appendix text file on the "mixed CD-ROMs" containing data from all the IHW Networks. NETLARGE.IDX is that index which contains the most information about the L-SP data, and should be used in carrying out very detailed searches for specific images of interest. II. LARGE-SCALE PHENOMENA OPERATIONS Table 1 lists the L-SP Discipline Specialist (DS) personnel who made substantial contributions to the Comet Halley archiving work; their accomplishments, as well as the names of other contributors, are given in the ACKNWLDG.TXT file of the DOCUMENT directory of this CD-ROM. The Large-Scale Phenomena Network (LSPN) observers were extremely productive, and their willingness to ship the DS unique plate and film material for processing at NASA/GSFC was one of the keys to the assembly of this impressively large, and we think successful, archive. We received over 3,500 images of Comet Halley taken with more than 100 observatory/instrument/ location combinations. The observers, their affiliations, the equipment used, and the observing sites, are listed in the file LSPNOBS.TXT in the DOCUMENT directory, and are indexed in the file LSPNOBS.IDX of the INDEX directory. The global LSPN statistics are as follows. A total of 3,383 L-SP images have been archived in these IHW compact discs, either as digital images or as "dataless" headers and listings. A total of 1,612 of the images were digitized at NASA/GSFC (85%) or provided to the DS Team in digital form by the observers (15%), and 1,771 images were not digitized but are listed in several places in the IHW archive. The vast majority of the 1,612 digital images are of Halley (1,439), and the remainder (173) are of various calibration objects. ----------------------------------------------------------------------------- Table 1. THE LARGE-SCALE PHENOMENA DISCIPLINE SPECIALIST TEAM Team Member Affiliation Responsibility ----------------------------------------------------------------------------- Malcolm B. Niedner, Jr. NASA/Goddard Space Flight Discipline Specialist Center, Greenbelt, MD 20771 John C. Brandt Univ. Colorado-Boulder Discipline Specialist Boulder, CO 80309 Jurgen Rahe NASA Headquarters Discipline Specialist Washington, DC 20546 Daniel A. Klinglesmith III NASA/Goddard Space Flight Senior Team Member Center, Greenbelt, MD 20771 Archibald Warnock III ST Systems Corporation Senior Software (STX) Lanham, MD 20784 Specialist Barbara B. Pfarr ST Systems Corporation Archive Manager (STX) Lanham, MD 20784 Joan E. Isensee ST Systems Corporation Data/Software (STX) Lanham, MD 20784 Assistant Edwin J. Grayzeck Interferometrics Inc. CD-ROM Specialist Vienna, VA 22180 Nancy E. Podger ST Systems Corporation Software Assistant (STX) Lanham, MD 20784 Michael R. Greason ST Systems Corporation Microdensitometer (STX) Lanham, MD 20784 Operations Steven B. Howell ST Systems Corporation Software Specialist (STX) Lanham, MD 20784 Lyla L. Taylor ST Systems Corporation Data/Software (STX) Lanham, MD 20784 Assistant Kanav Bhagat ST Systems Corporation Summer Student (STX) Lanham, MD 20784 for 1987-1989 Jennifer Gagliardi ST Systems Corporation Summer Student (STX) Lanham, MD 20784 for 1987-1989 Margaret Perkins ST Systems Corporation Summer Student (STX) Lanham, MD 20784 for 1990 [Margaret ("Meg") Perkins worked for the L-SP DS Team for three weeks before her life was taken in a tragedy which has shocked and saddened all of us. Her memory lives on and inspires us, but we miss her deeply.] ----------------------------------------------------------------------------- ----------------------------------------------------------------------------- III. DESCRIPTION OF THE CONTENTS OF INDEX 'NETLARGE.IDX' Table 2 provides, in FITS header format, a listing of the FITS keywords actually contained in the delimited index NETLARGE.IDX in the INDEX directory. Note that Table 2 is not a full FITS header inasmuch as NETLARGE.IDX does not contain some of the excessively redundant header keywords such as SIMPLE and EQUINOX, nor does it contain any of the COMMENT keywords. It should be obvious, however, that NETLARGE.IDX does contain essentially all of the "data about the data", and this will permit extremely powerful searches to be made for the data of interest without actually reading the headers themselves. Although the meaning of most of the FITS parameters are obvious upon inspection in Table 2, following the table is a detailed description of some of the keywords which require more explanation. A point of explanation is in order before discussing Table 2. The NETLARGE.IDX index includes information not only for the digital images deposited on this CD-ROM, but also for the so-called "dataless headers" which fall in the total time range outlined in the VOLUME.TXT description in this DOCUMENT directory. Dataless headers are the FITS headers for those images which were submitted to the L-SP Discipline but were not digitized. As a result there are large "voids" in NETLARGE.IDX reflecting the fact that many of the keywords in Table 2 (such as those dealing with astrometry) have no value for the dataless images. The dataless headers themselves are to be found in the daily subdirectories of the multi-discipline IHW CD-ROMs, not on these L-SP compressed images discs. We decided in favor of including all images in NETLARGE.IDX so that the archive user would be aware of the full set of imagery submitted to the L-SP Discipline. --------------------------------------------------------- TABLE 2. PARTIAL FITS HEADER SHOWING KEYWORDS CONTAINED IN THE INDEX 'NETLARGE.IDX' ---------------------------------------- BITPIX = 16 / 2-BYTE TWO'S COMPL INTEGERS NAXIS = 2 / NUMBER OF AXES, 2=SINGLE IMAGE, 3=MULTIPLE NAXIS1 = 4096 / PIXELS PER ROW NAXIS2 = 3072 / LINES PER IMAGE OBJECT = 'P/HALLEY ' / NAME OF OBJECT FILE-NUM= 300691 / NUMBER OF FILE SENT TO JPL, DNNNNN DATE-OBS= '20/03/86 ' / DATE OF MID-EXPOSURE DD/MM/YY TIME-OBS= 0.29028 / TIME OF MID-OBSERVATION, DAY FRACTION LONG-OBS= '289/11/07 ' / STATION LONGITUDE DDD/MM/SS LAT--OBS= '-30/09/55 ' / STATION LATITUDE SDD/MM/SS SYSTEM = '38070400 ' / OBSERVING SYSTEM CODE OBSERVER= 'MILLER,F ' / NAME OF OBSERVER WHO TOOK PLATE SUBMITTR= 'NIEDNER,M ' / LSPN SUBMITTER SPEC-EVT= T / SPECIAL EVENTS FLAG DAT-TYPE= 'IMAGE ' / TYPE OF FITS DATA LSPNSEQ = 110331 / SEQUENCE # OF PLATE RECEIVED AT LSPN OBSVTORY= 'UNIVERSITY OF MICHIGAN/CTIO ' / NAME OF OBSERVATORY LOCATION= 'LA SERENA, CHILE ' / STATION LOCATION - ADDRESS TELESCOP= 'CURTIS SCHMIDT ' / TYPE OF TELESCOPE APERTURE= 0.610 / APERTURE, METERS FRATIO = 3.500 / EFFECTIVE TELESCOPE F RATIO FOV1 = 5.000 / RA FIELD OF VIEW, DEGREES FOV2 = 5.000 / DEC FIELD OF VIEW, DEGREES PLTSZE1 = 0.203 / X PLATE SIZE, METERS PLTSZE2 = 0.203 / Y PLATE SIZE, METERS PLTSCALE= 96.661 / PLATE SCALE ARC SECONDS PER MM CALAVL = T / CALIBRATION AVAILABLE HYPSEN = T / HYPERSENSITIZATION EMULSION= 'IIA-O ' / EMULSION TYPE FILTER = 'NONE ' / TYPE OF FILTER USED PLTYPE = T / PLATE TYPE, T=ORIGINAL, F=COPY EXPOSURE= 1200. / EXPOSURE TIME, SECONDS HISTORY OBSLOG = '29953 ' / ORIGINAL OBSERVATORY PLATE BUNIT = 'DENSITY ' / UNITS OF PIXEL VALUES HISTORY SCNAPR = 40 / PDS SCAN APERTURE, MICRONS HISTORY SCNSTP = 40 / PDS SCAN STEP, MICRONS HISTORY SKYDEN = 190 / SKY BKGND PDS DENSITY HISTORY SKYUNF = 'EXCL ' / SKY BKGND POOR/FAIR/GOOD/EXCL CTYPE1 = 'RA---ARC ' / X AXIS UNITS CTYPE2 = 'DEC--ARC ' / Y AXIS UNITS RA--CPME= 0.293700E+03 / RA PLATE CENTER MID-EXPOSURE DEC-CPME= -0.264086E+02 / DEC PLATE CENTER MID-EXPOSURE CRPIX1 = 0.162615E+04 / REF POINT FOR AXIS1, SAMPLES CRPIX2 = 0.148793E+04 / REF POINT FOR AXIS2, LINES CDELT1 = -0.107401E-02 / RA PER PIXEL, DEGREES CDELT2 = -0.107401E-02 / DEC PER PIXEL, DEGREES CROTA1 = 0.121025E+00 / ROTATION ANGLE IN RA, DEGREES CROTA2 = 0.121025E+00 / ROTATION ANGLE IN DEC, DEGREES CRVAL1 = 0.293700E+03 / RA AT REFERENCE POINT, DEGREES CRVAL2 = -0.264086E+02 / DEC AT REFERENCE POINT, DEGREES RA--HEAD= 0.295308E+03 / RA COMET HEAD MID-EXPOSURE DEC-HEAD= -0.261628E+02 / DEC COMET HEAD MID-EXPOSURE HISTORY DATE-PDS= '20/03/87' / DATE DIGITIZED DD/MM/YY HISTORY DATE-PRC= '13/07/88' / DATE PROCESSED/CALIBRATED DD/MM/YY QUALITY = '123 ' / QUAL OF PLATE,GUIDING,TRAILS,SEEING,TRANSP ------------------------------------------------------------------------------ The meanings of many of the FITS keywords are fairly straightforward in Table 2 (see text descriptions to the right of the "/" in each line), and most will not be belabored here. Many keywords near the top of the (partial) header--BITPIX, NAXIS, NAXIS1, NAXIS2, etc.--are standard, mandatory keywords which have been described by Wells et al. (1981). The reader is referred to that paper for further details. SYSTEM is a code denoting the observatory and the instrument used to obtain the image, and the location of the observing site. The reader is referred to the LSPNOBS.TXT file in the DOCUMENT directory of this CD-ROM for detailed information associated with each SYSTEM code, information which is also contained in the delimited index LSPNOBS.IDX in the INDEX directory. SPEC-EVT is a keyword signifying the presence or absence of a "special event", and is either "T" or "F" in value. In nearly all cases it was the L-SP Discipline Specialist (M. Niedner) who evaluated the imagery and set this parameter. Disconnection events (DEs) in the plasma tail, strong bursts of tail ray generation, and large amplitude bends in the tail, are but a few of the situations meriting a SPEC-EVT = T. The classification is obviously somewhat subjective and the DS should not be held responsible for database searches of NETLARGE.IDX keyed solely to SPEC-EVT; it is a coarse parameter that is meant only to signify possible interest in an image. PLTSCL should be used with caution. It does indicate plate scale, but only for original plates or films. Moreover it is derived from the focal length of the telescope, not astrometrically, and hence never varies from one original image to the next taken on the same telescope. In some cases, most notably 35 mm film negatives on Kodak Technical Pan 2415, the digital scan was made of a greatly enlarged film copy of the original small frame. In this case PLTSCL is not remotely close to the scale of the digital image. We have provided a rigorous astrometric plate solution for each scanned image, and the true "digital image scale" is contained in other FITS keywords (CDELT1,2 and SCNSTP, described below). CALAVL is a keyword signifying the existence or absence of calibration data associated with the Halley image. A value of "T" does not, however, necessarily mean that the calibration information resides on these CD-ROMs, because not all calibration plates were digitized by the L-SP Team. Refer to the text file CALIB.TXT in the DOCUMENT directory for a discussion of which calibration data were included in the archive. The filenames of calibration datafiles in the archive are contained in text tables in CALIB.TXT and in delimited index table CALIB.IDX of directory INDEX. PLTYPE designates whether the digitized image was an original plate or film ("T") or a copy ("F") made by either the contributing observer or by the L-SP Team. In the latter category it should be mentioned that in some cases the observer sent the DS a film copy of sufficient quality for scanning, and in other situations it was necessary to enlarge original small images prior to digitization. CRPIX1,2....CRVAL1,2 form a block of "astrometric keywords", the names and meanings of most of which are identical to keywords defined by Wells et al. (1981). Briefly, CRPIX1 and CRPIX2 fix the location of the upper-left (NE) corner of the scan area relative to the reference position, in numbers of samples and lines, respectively. We have adopted the plate center as the reference position, or "reference pixel" as it is called. CDELT1 and CDELT2 are self-explanatory, but it should be noted that the "classical" plate scale in arcsec/mm can be derived from CDELT1(,2) and SCNSTP (scan step in microns) if one wishes it. CROTA1 and CROTA2 were not tightly defined in the original FITS paper (Wells, D.C., et al.,1981), so we wish to be specific here about how the angles are measured. First, we define CROTA1=CROTA2, always. The angle(s) refer to the rotation of the X-Y PDS (microdensitometer) coordinate frame relative to the axes of the celestial coordinate system. To reasonable accuracy, the +Y scanning axis is in the direction of North at the plate center, for the epoch of 1950.0. The +X scanning axis is in the approximate direction of due East (again, at the plate center for 1950.0). As a result of the inevitable human error in positioning the plate on the PDS platen, however, there will be a non-zero rotation angle offset (very small) between two coordinate systems. When the North direction lies in the +X/+Y quadrant, i.e., is rotated in a counterclockwise direction relative to the +Y axis, we say that the rotation angles CROTA1 and CROTA2 are both positive (and the same value). A positive angle for such a counterclockwise rotation is what is meant by SENSE = T, and we adopt SENSE = T throughout this archive. Because of its redundancy, the keyword SENSE is not indexed in NETLARGE.IDX, although it is present in the individual FITS headers. CRVAL1 and CRVAL2 are the right ascension and declination of the reference position, which we have made the plate center. Note that these coordinates are separately given as RA--CPME and DEC-CPME, the formal coordinates of the plate center. RA--HEAD and DEC-HEAD are the measured (approximate) coordinates of the head center, but are not given in either the FITS headers or the appropriate lines in the index table for those plates which did not contain the head region. Note that the head center coordinates are for reference purposes only and are not to be used for accurate positions of the nucleus; well-saturated, wide-field plates are obviously not suitable for that purpose. QUALITY is a keyword which gives a very subjective assessment of the condition of each scanned image in terms of five categories (plate, guiding, trails, seeing, and transparency); the scale is 1-5 (excellent to poor). Not for all plates was it possible to evaluate each category, and hence the archive user will occasionally come across (for example) QUALITY = '123 '. Malcolm B. Niedner, Jr. Discipline Specialist for Large-Scale Phenomena Code 684.1 NASA/Goddard Space Flight Center Greenbelt, MD 20771 ------------------------------------ REFERENCE Wells, D. C., Greisen, E. W., Harten, R. H. 1981, Astron. Astrophys. Suppl., 44, 363.