***** File GZGUIDE.TXT Comet P/Giacobini-Zinner User's Guide to the Compact Disc Read Only Memory E. Grayzeck (SBN/PDS - U Maryland) A. Warnock (LSPN - NASA/GSFC) M. Aronsson (IHW - JPL) Contents 1. Background and Scope of Compact Disc 2. Accessing the Disc 3. Data Products 4. Directory Structure 5. Filenaming Convention 6. Available Software 7. References 1. BACKGROUND AND SCOPE OF COMPACT DISC This compact disc contains observations of comet Giacobini-Zinner from the apparition in 1984-1987. During that period, the International Halley Watch (IHW) mounted a campaign to observe the comet in support of the NASA project to fly the International Cometary Explorer (ICE) spacecraft through the comet's tail. The data from that apparition and encounter form a database which is being placed on this compact disc as a test for the larger project to catalogue the data collected on comet Halley. 2. ACCESSING THE DISC These discs have been manufactured following the normal specifications for Compact Disc Read Only Memory (CD-ROM) as well as the logical structure outlined by the International Standards Organization (ISO) in the No. 9660 document. A basic system requirement demands that the CD-ROM reader and software driver support this standard. At this writing (July 1990), primary access to the disc will be using IBM-PC or compatible computers. However, software support from vendors for the Apple MacIntosh II and some workstations (SUN, MicroVAX) is now available. If you are using a PC style machine, the reader that you bought should include the appropriate hardware (a PC-bus or SCSI controller board and cable) and software to address your device. Software packages should include a device driver for your machine and extensions to DOS that allow the reader to "look" like another random access disk drive; the de facto standard here is the Microsoft Extensions (MSCDEX 2.0 or higher). Install the specific device driver software as directed by the documentation accompanying the hardware. Usually, this will involve specifying the device driver in a DEVICE= line in the CONFIG.SYS file and invoke the extensions package from your AUTOEXEC.BAT file. This will automatically configure the CD-ROM reader when the computer is booted. Remember that the CD-ROM reader is assigned a drive name which you have either chosen with the appropriate command line switch (/L:H assigns the CD- ROM reader to drive H:) or allowed to default to the next available device letter in your system. Typical PC configurations will have two floppies (A: and B:), and a hard disk (C:). Put your drive at some letter higher than that. Remember that many DOS commands work on the CD-ROM but that it is a read-only medium. Some of the useful DOS functions are: DIR - directory listing CHDIR - change directory (also CD) TYPE - list the contents of a file on the screen; useful for normal text with , delimiters COPY - copy file(s) to another device PRINT - print file on hard copy device Notice that commands such as DEL and MKDIR are not available since the CD-ROM is a read-only medium. Furthermore, some compact disc software packages will invoke screen plots that may depend on the DOS program GRAPHICS.COM. In this case, execute it in your AUTOEXEC.BAT file or before you run the software package. The IHW CD-ROMs have been designed to allow maximum access to the data using existing software developed within NASA, as well as user-supplied programs. In particular, each data file was originally supplied in the FITS format, and also has a detached PDS label to describe the data structure. Furthermore, the FITS (.FIT) files have had the headers placed in separate files (with extension .HDR) from the data (.DAT, .IBG, .IMG, .IMQ, .TAB). The headers have been separated to allow unrestricted access to the data by non-FITS programs. The sizes of all header and data files have been preserved as integer multiples of 2880 bytes (required by FITS) in order to facilitate the reconstruction of the original FITS bytestream by concatenating the data file with the appropriate header file. 3. DATA PRODUCTS A large number of scientists have contributed to this archive project. The contributing observatories, with their IHW system codes, are summarized in a separate file (OBSCODES.TXT). Individual observers and their various instruments are identified in the header that accompanies the data. The ground-based data is located in chronological directories. The directory names are based on date, specified by UT year, month, day and hour. We have attempted to restrict directories to a reasonable size while allowing enough information for useful browsing. The average number of files in a directory is less than 70, although in one case (August 12, 1985) splitting the directory at the hour level was necessary to reduce the number of files. The average subdirectory size is less than 0.5 MByte. No subdirectory was created for days on which no observations were made. For the images from the Large Scale Phenomena Network (LSPN), the data has been compressed and subsampled. The latter occurs twice on the disc: in the chronological listing and in a separate directory. The LSPN images are in the COMPRESS and BROWSE subdirectories, respectively. The associated FITS headers and PDS labels also appear in each subdirectory. In an effort to reduce the space requirements for the full Halley Archive, data compression was applied to the images. It has been used on the images of P/Giacobini-Zinner included on this disk. An algorithm based on successive differences between pixels (Previous Pixel) has been developed to reduce the digital images from the LSPN to a coded byte string of 8-bit data. The compression yields files roughly 60% the size of the original images. Details of the algorithm and the accompanying FITS proposal are in the file FITSCOMP.TXT in this directory. Decompression of data is limited by transfer rates, so a subsampled set of images (browse images) is also supplied. These images are restricted to a maximum of 256 pixels on a side and preserve the original sampling geometry. In addition, the original 10-bit data has been rescaled to 8 bits. Some networks have submitted supplemental data which include filter tables, non-comet images, flat fields, and laboratory spectra. These are stored in the CALIB subdirectory and except for the Infrared Filter Tables (IRFT) appear in the chronological directories. 4. DIRECTORY STRUCTURE The volume and directory structure of this disk conform to the Level-1 standard specified by the ISO. This format is widely accepted and used on a variety of machines. The AAREADME.TXT file in the root directory introduces the user to the full extent of the archive and the contents of the individual subdirectories. There are three directories (DOCUMENT, INDEX and SOFTWARE) on the CD-ROM that contain supplementary files. In addition, to correlate the chronological observations with physical location of the comet, some recent ephemeris information has been included (in the subdirectory EPHEM). The table (EPHEM.TAB) contains information at daily intervals, so an interpolation routine is also provided; more information is in the text (EPHEM.TXT) file. The DOCUMENT subdirectory contains text files that give the background to this CD-ROM project and a general guide to its use. An explanatory discussion of the FITS and PDS formats used specifically for the P/Giacobini-Zinner data is located in the files *INFO.TXT. This file (GZGUIDE.TXT) is an overview. Some files may also appear as a printed volume of the data (see Preface). Detailed appendices, written by the individual Disciplines of the IHW, contain information on the data collection and processing. Finally, some documentation has been duplicated at the beginning of special directories, e.g., CROMMELN. Tables of useful index information have been collected in three forms in order to allow automated searching of the data. The QUIK tables contain the full set of mandatory FITS keywords from all disciplines; the two tables cover the pre- and post-perihelion passage of the comet. A second set of tables contains the data from the proposed printed archive (in the subdirectory NETABLES), split by discipline and subnet, and finally chronologically ordered in each table. The full set of keywords (COMMENTS, HISTORY) for four networks (Amateur (Visual), Large-Scale Phenomena, Radio, Spectroscopy) are in additional tables as specified by each group. These tables are in files NETAMATV.IDX, NETLARGE.IDX, NETSPECT.IDX and NETRADIO.IDX. We have also made the index tables transportable to relational DBMS by delimiting the tables and providing structure, program, and dBASE-compatible .DBF files. Further information is contained in the file INDXINFO.TXT. The SOFTWARE subdirectory contains program code for a few utilities, in this case, decompression of images and the interpolation of ephemeris tables for the comet. The Previous Pixel algorithm used to compress the images is described in the file FITSCOMP.TXT. It was developed and implemented by the LSPN at Goddard Space Flight Center. Pseudocode for this algorithm is listed (COMPRESS.TXT) as well as the IDL procedure (PREVPIX.PRO) originally used to perform the compression. Fortran and C code are provided for image decompression, but there has been no attempt to supply fully executable programs. An interpolation program was developed by the Astrometry network group for the ephemeris table. The Fortran code, an equivalent C version, and executable file for VAX/VMS and MS-DOS are provided. 5. FILENAMING CONVENTION The International Halley Watch agreed early on in the project that all data would be submitted from the individual disciplines to the Lead Center using the FITS format. When the decision was made to distribute this information on CD-ROM, it was determined that the data had to have even broader accessibility. For this reason, the original FITS files, with contiguous headers and data, were split into separate files. The original FITS byte stream could then be recovered by concatenating the appropriate header and data files. In addition, detached PDS labels were constructed to allow parallel, definition of the data files for the Planetary Data System. (We thankfully acknowledge the help of M. Martin, PDS, for assisting the IHW through version 1.1 of the Object Description Language to describe the SPECTRUM object.) The convention for naming files on the CD-ROM was proposed by the IHW Lead Center to include a unique data qualifier for the data. A specific set of rules was established to identify the network/subnetwork for each discipline (given by a letter code). A CD-ROM running number relates the information contained in the various indexes to the files. In the case of calibration data, the same network/subnetwork code is used but the number starts at 8001. A short list organized by Discipline (Number) and listing network/subnetwork is given below: Discipline Number Subnet (Exp) Letter Code Astrometry 1 none ASTR Infrared Studies 2 Photometry IRPH Polarimetry IRPOL Spectroscopy IRSP Image IRIM Filter Table IRFT Large-Scale Phenomena 3 none LSPN Near Nucleus Studies 4 none NNSN Photometry Polarimetry 5 Broad Band Mag PMAG Narrow Band Flux PFLX Polarimetry PPOL Radio Studies 6 Continuum RSCN Occultation RSOC OH RSOH Spectral Line RSSL Spectroscopy Spectrophot 7 none SPEC Amateur Observations 8 Drawing AMDR Photography AMPG Visual Magnitude AMV The file extensions follow suggestions by the Planetary Data System (SPIDS v1.1) for tabular and image data. In addition, for IHW FITS, the original headers and data were split into separate files, with filename extensions as listed below. .DAT - other non-imaging or non-tabular data .FIT - original FITS records .HDR - FITS header records .IBG - data records for subsampled images .IMG - imaging data records .IMQ - data records for compressed images .LBL - detached PDS stream format .TAB - tabular data records In re-organizing the original FITS files, we have attempted to make the data available to a larger community. For transporting data to analysis packages that demand FITS, the header and data files may be concatenated to create a fully valid FITS byte stream, as the original structure of the FITS headers and data has been preserved. The tabular data, even though presented as valid FITS data records, are really just ASCII byte strings that can be exported to many machines. Inclusion of the PDS labels allows use of CD-ROM imaging software already available from PDS. The PDS labels are text files, with lines terminated by the characters. The total size of the files is maintained in the required SFDU_LABEL. 6. AVAILABLE SOFTWARE At the present, no executable software is included with the CD-ROM except for programs supporting the ephemeris. We have designed the data formats to be compatible with many existing packages and plan to distribute two public domain packages, IMDISP (from JPL) and FITSREAD (from the ADC) on an accompanying floppy disk. We may also supplement this release with a printed manual and some support programs for manipulating the data (FITSUTIL, FITSCONV). 7. REFERENCES Martin, T., Martin, M., Braun, M., Johnson, T., Davis, R., and Mehlman, R., SPIDS: Standards for the Preparation and Interchange of Data Sets, JPL D-4683: October 3, 1988. "Information Processing -- Volume and File Structure of CD-ROM for Information Interchange", Reference Number ISO 9660:1988(E), Developed in coordination with the National Information Standards Organization, National Bureau of Standards, Administration 101, Library E-106, Gaithersburg, MD.