***** 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 <carriage return>, <line feed>
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 <CR><LF> 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.