***** 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.