***** File FITSFORM.TXT                                                       
                                                                              
                                                                              
                         FITS 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. In cases where no digital data had been supplied the  
.HDR file carries information about upper limits, values reported by          
observers, references gleaned from the literature, or the characteristics of  
data in analog form. The table below identifies these "dataless" files and    
provides the correspondence between file extension and types of data so that a
concatenated file (.FIT) can be reconstructed.                                
                                                                              
     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.IBG).  A short list of this convention by        
Discipline and subnet (or experiment) is given below:                         
                                                                              
                                                                              
  PDS Object              FITS  Discipline      Subnet    File Extensions     
 (description)         NAXIS =                   Code     (.HDR+.EXT=.FIT)    
_________________________________________________________________________     
                                                                              
text                       1    Astrometry       ASTR      .hdr  .dat         
fits_label (no data)       0    IR Studies       IRSP      .hdr               
table (filter)             0,2        "          IRFT      .hdr  .tab         
table (photometry)         0,2        "          IRPH      .hdr  .tab         
table (polarimetry)        0,2        "          IRPOL     .hdr  .tab         
spectrum (filter)          2          "          IRFC      .hdr  .dat         
spectrum                   2          "          IRSP      .hdr  .dat         
image                      2          "          IRIM      .hdr  .img         
fits_label (no data)       0    Large Scale Phen LSPN      .hdr               
image(browse)              2          "          LSPN      .hdr  .ibg         
image                      2    Near Nucleus     NNSN      .hdr  .img         
table (narrow band)        0,2  Photometry Polar PFLX      .hdr  .tab         
table (broad band)         0,2        "          PMAG      .hdr  .tab         
table (polarization)       0,2        "          PPOL      .hdr  .tab         
table (Stokes parameters)  0,2        "          PSTOKE    .hdr  .tab         
fits_label (no data)       0    Radio Studies    RSCN      .hdr               
fits_label (no data)       0          "          RSSL      .hdr               
spectrum                   1          "          RSSL      .hdr  .dat         
spectrum (multiple)     1 or 2        "          RSOH      .hdr  .dat         
spectrum (multiple)        2          "          RSRDR     .hdr  .dat         
image                      2          "          RSCN      .hdr  .img         
image(multiple)            3          "          RSOC      .hdr  .img         
spectrum (visibility)      6          "          RSCN      .hdr  .dat         
spectrum (visibility)      6          "          RSOH      .hdr  .dat         
spectrum (visibility)      6          "          RSSL      .hdr  .dat         
spectrum                   1    Spectroscopy     SPEC      .hdr  .dat         
spectral image qube        2          "          SPEC      .hdr  .dat         
image (spectrum)           2          "          SPEC      .hdr  .img         
fits_label (no data)       0    Amateur Studies  AMDR      .hdr               
fits_label (no data)       0        "            AMPG      .hdr               
fits_label (no data)       0        "            AMSP      .hdr               
table (magnitude)          0,2      "            AMV       .hdr  .tab         
table (radar)              0,2  Meteor Studies   MSNRDR    .hdr  .tab         
table (visual)             0,2      "            MSNVIS    .hdr  .tab         
_________________________________________________________________________     
                                                                              
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 filename extensions as listed below.                                     
                                                                              
                 .DAT - other non-image and non-tabular data                  
                 .FIT - original FITS file                                    
                 .HDR - FITS header records                                   
                 .IBG - data records for subsampled browse image              
                 .IMG - image data records                                    
                 .TAB - table data records as ASCII                           
                                                                              
     The five PDS objects in this archive are FITS_LABEL (header), IMAGE,     
TABLE, TEXT, and SPECTRUM; a label occurs for each datafile.  Our aim was to  
construct a basic PDS label for each data file on the CD-ROM.   Files that    
remain in the original FITS form do not have a PDS label; these can occur as  
DOCUMENT files and datafiles for Comet Crommelin and Comet Giacobini-Zinner.  
                                                                              
                                                                              
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.