📚 SPECFORM.TXT
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121 lines***** File SPECFORM.TXT
Spectra Data Descriptions
The International Halley Watch agreed early in the project that all data
would be submitted from the individual Disciplines to the Lead Center using
the FITS format (Wells et al., 1981). 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
distinguishable by their filename extensions (.HDR for headers). The file
sizes were preserved as multiples of 2880 bytes, allowing the original FITS
byte stream to be recovered by concatenating the appropriate header and
datafile. PDS labels were constructed to allow definition of the datafiles
for the Planetary Data System. For each datafile there must always be an
associated FITS header.
The convention for naming files on the IHW CD-ROMs was proposed by the
Lead Center and NASA/Goddard Space Flight Center (GSFC) personnel to include a
unique data qualifier for the data. Specifically, a set of subnet codes was
established to enable identification of the IHW Discipline/subdiscipline from
the filename itself. A CD-ROM running number and file extension complete the
filename. A short list of this convention for spectra follows:
PDS Object FITS Discipline Subnet Extensions
(description) NAXIS = Code
______________________________________________________________________
spectrum (filter) 2 IR Studies IRFC .dat
spectrum 2 " IRSP .dat
spectrum 1 Radio Studies RSSL .dat
spectrum (multiple) 1 or 2 " RSOH .dat
spectrum (multiple) 2 " RSRDR .dat
spectrum (visibility) 6 " RSCN .dat
spectrum (visibility) 6 " RSOH .dat
spectrum (visibility) 6 " RSSL .dat
spectrum 1 Spectroscopy SPEC .dat
spectral image qube 2 " SPEC .dat
______________________________________________________________________
A table linking the subnet codes above and the subdiscipline names is given in
Section 5 ('Filenaming Conventions') of the file HALGUIDE.TXT. Concerning the
numeric portion of filenames, calibration files for IRSP and SPEC begin at
4001, whereas the Halley data themselves for all disciplines and
subdisciplines start at 0001.
The file extensions follow suggestions by the Planetary Data System
(SPIDS v1.1; Martin et al., 1988) 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-image or non-table data
.HDR - FITS header records
.LBL - detached PDS stream format
These PDS labels are metadata (as headers describing data submitted to the
archive). There has been no effort to duplicate the documentation contained in
the full FITS headers because the PDS and FITS headers for a given data file
differ only in the filename extension. Instead we have attempted to use the
power of the PDS label syntax to fully describe the data structures and thus
gain access to software by that group. Standards for the Preparation and
Interchange of Data Sets, Martin, T. Z., et al, Document D-4683, Jet
Propulsion Laboratory, California Institute of Technology, Pasadena, CA
document version 1.1 was the primary reference to the Object Description
Language (ODL) necessary to create the PDS labels. (We acknowledge R. Borgen
and M. Martin, PDS-CN, JPL, for assisting the IHW through version 2.0 of the
ODL implementation for SPECTRUM.) The basic PDS descriptors such as
SFDU_LABEL, RECORD_TYPE, RECORD_BYTES, and FILE_RECORDS are explained in the
SPIDS document. The RECORD_TYPE for all data files is FIXED_LENGTH. The PDS
labels have been formed as fixed length (78 byte) plus an embedded CR and LF.
The SPECTRUM class description was refined in v2.0 by working closely
with the PDS group to ensure definition of data groups that included both
uniformly spaced data (as a single array) as well as ordered groups of
observations. From guidelines for dealing with the SPECTRUM data structure,
we consider the spectra as tabular data (COLUMN, NAME, DATA_TYPE, START_BYTE,
BYTES) which are binary. The independent variable (e.g., WAVELENGTH) is
described by the keywords SAMPLING_PARAMETER_NAME, MINIMUM_SAMPLING_PARAMETER,
SAMPLING_PARAMETER _INTERVAL, and SAMPLING_PARAMETER_UNIT. (There are special
cases for Radio or IR data using Doppler VELOCITY, FREQUENCY, or
FREQUENCY_OFFSET.) Another case is a table from the Infrared Studies Network
of ordered sets of data, in which we interpreted the column of signal/noise or
ratios as an associated ERROR. A NOTE about this nonstandard use is included
in the labels for the appropriate datasets. We have also attempted to use the
NOTE keyword to identify the contributing IHW discipline, subnet, and generic
comments about the data. As in the situation for multiple images, we have
subclasses for the spectra indicated by a modifier, e.g.,
LHC_POLARIZATION_SPECTRUM.
A special effort was made to describe 2-dimensional spectra by working
with the PDS to establish a SPECTRAL_IMAGE_QUBE object. The data are reduced
measurements that have the slit oriented either along the tail or
perpendicular to the tail of the comet. To capture the positional
information, a vectorial notation was adopted for the SPECTRAL_IMAGE_QUBE that
could allow for such observational selection. In cases where the derived
units were non-standard, a text DESCRIPTION is embedded in the label.
There was one case of a binary table that was used to describe the UVFITS
data. A hybrid description (VISIBILITY_SPECTRUM), incorporating both the
ordered sets and uniformly spaced data, describes this intermediate processing
step in data reduction; the integer values are Complex numbers, not currently
supported under PDS.
REFERENCES
Martin, T.Z., Martin, M.D., Davis, R.L., Mehlman, R., Braun, M., Johnson
M.: October 3, 1988, Standards for the Preparation and Interchange of
Data Sets, Version 1.1, JPL D-4683.
Wells, D.C., Greisen, E.W., and Harten, R.H.: 1981, Astron. Astrophys.
Suppl. Ser. 44, 363.