***** File HDRFORM.TXT                                                        
                                                                              
                                                                              
                         FITS Header 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 (example:  LSPN0059.HDR).  A short list of this convention by        
Discipline and subnet (or experiment) is given below (for headers only):      
                                                                              
                                                                              
                                                                              
  PDS Object              FITS   Discipline       Subnet      File Ext        
 (description)         NAXIS =                     Code                       
______________________________________________________________________        
                                                                              
text                       1     Astrometry        ASTR        .hdr           
fits_label (no data)       0     IR Stduies        IRSP        .hdr           
table (filter)             0,2   IR    "           IRFT        .hdr           
table(photometry)          0,2   IR    "           IRPH        .hdr           
table(polarimetry)         0,2   IR    "           IRPOL       .hdr           
spectrum(filter)           2     IR    "           IRFC        .hdr           
spectrum                   2     IR    "           IRSP        .hdr           
image                      2     IR    "           IRIM        .hdr           
fits_label (no data)       0     Large Scale Phen  LSPN        .hdr           
image(browse)              2     Large     "       LSPN        .hdr           
image                      2     Near Nucleus      NNSN        .hdr           
table (narrow band)        0,2   Photometry Polar  PFLX        .hdr           
table (broad band)         0,2       "             PMAG        .hdr           
table (polarization)       0,2       "             PPOL        .hdr           
table (Stokes parameters)  0,2       "             PSTOKE      .hdr           
fits_label (no data)       0     Radio Studies     RSCN        .hdr           
fits_label (no data)       0         "             RSSL        .hdr           
spectrum                   1         "             RSSL        .hdr           
spectrum(multiple)      1 or 2       "             RSOH        .hdr           
spectrum(multiple)         2         "             RSRDR       .hdr           
image                      2         "             RSCN        .hdr           
image(multiple)            3         "             RSOC        .hdr           
spectrum (visibility)      6         "             RSCN        .hdr           
spectrum (visibility)      6         "             RSOH        .hdr           
spectrum (visibility)      6         "             RSSL        .hdr           
spectrum                   1     Spectroscopy      SPEC        .hdr           
spectral image qube        2         "             SPEC        .hdr           
image                      2         "             SPEC        .hdr           
fits_label (no data)       0     Amateur           AMDR        .hdr           
fits_label (no data)       0         "             AMPG        .hdr           
fits_label (no data)       0         "             AMSP        .hdr           
table (magnitude)          0,2       "             AMV         .hdr           
table (radar)              0,2   Meteor            MSNRDR      .hdr           
table (visual)             0,2       "             MSNVIS      .hdr           
______________________________________________________________________        
                                                                              
                                                                              
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 IRIM, IRSP, LSPN, 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 header 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 differing  
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 (LSPNNNNN or IRIMNNNN), spectra which can be ordered groups (IRSPNNNN) 
or standard (RSSLNNNN), and existing but not archived data as for AMDRNNNN,   
AMPGNNNN, AMSPNNNN.                                                           
                                                                              
                                                                              
                                                                              
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.