***** File IMAGFORM.TXT                                                       
                                                                              
                             Image Format Conventions                         
                                                                              
     The IHW CD-ROMs with Large-Scale Phenomena (L-SP) images have been       
constructed to allow maximum access to the data using existing software       
developed within NASA, as well as user-supplied programs.   In particular,    
each data file was originally supplied in the FITS format.   The normal       
FITS files have had the headers placed in separate files (with extension      
.HDR) from the data (with extensions .IMQ or .IBG).   A detached PDS          
label (.LBL) has been constructed to describe the basic data structure.       
The headers have been separated to permit unrestricted access to the data     
by non-FITS programs.   The sizes of all header and data files have been      
preserved as integer multiples of 2880 bytes as required by FITS format       
(Wells, D.C., et al. 1981) in order to facilitate the reconstruction of       
the original FITS bytestream by concatenating the data file onto the          
appropriate header file.   The PDS labels files are given in fixed            
length (80 byte) rather than variable length stream format.                   
                                                                              
     In an effort to reduce the space requirements for the full Halley        
Archive, data compression was applied to the images. An algorithm             
based on successive differences between pixels (Previous Pixel) has been      
developed to reduce the digital L-SP images from 16-bit to a coded byte       
string of 8-bit data.  The compression yields files roughly 50% the size      
of the original images.  Details of the algorithm and the accompanying        
FITS proposal are in the file FITSCOMP.TXT in the DOCUMENT directory.         
In order to decompress the image, one can take the pseudocode provided        
in PREVPIX.TXT to write specific code for an operating system.  This was      
done for MS-DOS, using information about the accompanying FITS header file    
(.HDR), to produce an executable (PCDECLSP.COM), which is located in the      
SOFTWARE subdirectory on the CD-ROM.   The output file is in FITS format.     
Decompression of an image costs the user time, so a subsampled set of         
images (browse images) is also supplied. These images are restricted to       
a maximum of 256 pixels on a side and preserve the original sampling          
geometry.  Note, for display the original 10-bit data has been rescaled       
to 8-bits.  These "quick-look" images are especially important for the        
larger set of 1439 images for Comet Halley.                                   
                                                                              
     Some observers have submitted calibration data which include step        
wedges, sensitometer spots, and standard stars.  These were compressed        
and are stored in the CALIB subdirectory; a subsampled image is formed        
for each calibration image.                                                   
                                                                              
     The convention for naming files on the CD-ROM was proposed by the        
IHW to include net/subnet name and a running number as a unique qualifier     
for chronological sorting.    The name for this net has been abbreviated      
to give the appropriate filename LSPNxxxx followed by a three character       
extension.   For calibration data, the filename is LSPN4xxx to distinguish    
from the 3383 images of the Comet.                                            
                                                                              
     The file extensions follow suggestions by the Planetary Data System      
(SPIDS v1.1) for compressed (.IMQ), and subsampled images (.IBG) with PDS     
detached labels (.LBL); for FITS header files, a new designation .HDR was     
introduced.   These extensions are listed below.                              
                                                                              
                 .HDR - FITS header records                                   
                 .IBG - data records for subsampled browse image              
                 .IMQ - compressed byte stream                                
                 .LBL - detached PDS label                                    
                                                                              
Note that in the ISO formatting process, a version number (1 by               
default) is attached to the filename.   This information is ignored           
by MS-DOS but a necessary input for complex systems such as VMS.              
                                                                              
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., Grieson, E.W., and Harten, R.H.: 1981, Astron. Astrophys.        
Suppl. Ser. 44, 363.