***** File HDRFORM.TXT                                                        
                                                                              
                         FITS Header Descriptions                             
                                                                              
                                                                              
     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.   PDS labels were constructed to allow     
definition of the data files for the Planetary Data System.   For each        
original data input there must always be a file indicating a FITS header.     
                                                                              
     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 net and subnet for      
each discipline (given by a letter code).  A CD-ROM running number relates    
the information contained in the various indices to the files containing      
the actual data.  A short list of this convention by discipline and           
subnet (or experiment) is given below:                                        
                                                                              
  PDS Object              FITS   Discipline         Subnet    File Ext        
 (description)         NAXIS =                       Code                     
                                                                              
text                       1     Astrometry        ASTR        .hdr           
fits_label (no data)       0     IR Stduies        IRIM        .hdr           
table (filter)             0,2   IR    "           IRFT        .hdr           
table(photometry)          0,2   IR    "           IRPH        .hdr           
table(polarimetry)         0,2   IR    "           IRPOL       .hdr           
spectrum                   2     IR    "           IRSP        .hdr           
image                      2     IR    "           IRIM        .hdr           
fits_label (no data)       0     Large Scale Phen  LSPN        .hdr           
image(compressed)          1     Large     "       LSPN        .hdr           
image(browse)              2     Large     "       LSPN        .hdr           
image                      2     Near Nucleus      NNSN        .hdr           
table (broad band)         0,2   Photometry Polar  PMAG        .hdr           
table (narrow band)        0,2       "             PFLX        .hdr           
table (polarization)       0,2       "             PPOL        .hdr           
fits_label (no data)       0     Radio Studies     RSCN,SL     .hdr           
spectrum                   1         "             RSOH        .hdr           
spectrum(multiple)         2         "             RSOH        .hdr           
image(multiple)            3         "             RSOC        .hdr           
spectrum                   1     Spectroscopy      SPEC        .hdr           
image                      2         "             SPEC        .hdr           
fits_label (no data)       0     Amateur           AMDR,PG     .hdr           
table (magnitude)          0,2       "             AMV         .hdr           
                                                                              
Note that the Subnet coding (explained in the Filenaming Conventions)         
is one way to distinguish file extension and content.                         
                                                                              
     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.                                                   
                                                                              
                 .HDR - FITS header records                                   
                                                                              
     We have conformed with the PDS definition of a specific keyword to       
indicate the presence of a FITS header (the keyword TYPE = FITS) when the     
"data" object is a foreign label (FITS_LABEL).   In FITS, if NAXIS=0,         
then no data records need follow, as in the case of an upper limit.   The     
"dataless" headers can be recognized by the NAXIS value or the IHW keyword    
DAT-FORM = NODATA in the FITS HEADER.   The PDS label (with same filename     
but differring extension) points at the HEADER file as its data object.       
As shown in the above table, the "dataless" header can occur for different    
types of data: images (LSPNNNN or IRIMNNNN), and spectra which can be ordered 
groups (IRSPNNNN) or multiple spectra (RSSLNNNN).