***** File BROWSE.TXT                                                         
                                                                              
     Effective use of the 1,612 images contained on the IHW/Large-Scale       
Phenomena (L-SP) compressed image CD-ROMs requires that the user of the discs 
be able to "browse through the data" quickly to find those images and         
intervals which are of high scientific interest.  Because of the long         
decompression and transfer times of the full-resolution images with current   
image display hardware, the goal of efficient browsing of the data can be met 
only if the images are placed on the discs at least a second time, in either  
subsampled or filtered form, and uncompressed.                                
                                                                              
     The browse images are actually stored in three places within the total   
set of IHW CD-ROMs.  In addition to the subset of images stored in the BROWSE 
directory of each IHW/L-SP compressed image CD-ROM (HAL_0001 - HAL_0018), the 
entire set of 1,612 digital images exists on the last of the IHW/L-SP         
dedicated discs (HAL_0018) in the "volume subdirectories" of the              
SUMMARY\BROWSE directory (sample path is SUMMARY\BROWSE\HAL_0006). The browse 
images are also interleaved with data from the other IHW disciplines in the   
daily data subdirectories on these "mixed data" CD-ROMs (HAL_0019 - HAL_0023).
                                                                              
     A "browsed image" is one that has been generated from the original       
uncompressed image. It has been subsampled and is no larger than 256 pixels in
either dimension.  In addition, the digital data have been scaled into a      
numerical range of 0 to 255 (one byte per pixel; the precision for most of the
original images is 10 bits, requiring two bytes per pixel).  The BROWSE       
directory of each L-SP compressed image CD-ROM contain datafiles, FITS        
headers, and PDS labels for the compressed images on that CD-ROM;  this       
includes both images of Comet Halley and of calibration objects.  Note that   
the 1,612 total L-SP digital images (1,439 of the comet, 173 of calibration   
objects) are deposited on multiple CD-ROMs "dedicated" to the L-SP imagery.   
                                                                              
     The browse data were obtained by taking the "n"th row and column for the 
original image starting at row "n/2" and column "n/2".  The value for "n" was 
determined from the larger of the two axes such that the quantity (original   
length / n) was less than or equal to 256.  For the images which were         
digitized at GSFC, the original densitometer values ranged between 0 and 1023.
The density values in the images were divided by 4 in order to convert the    
density to a single byte.  For those images digitized elsewhere, the density  
scaling factor was chosen so that the density in the browse image was less    
than or equal to 255.                                                         
                                                                              
     The FITS header records for the browse images have had their astrometric 
information adjusted to reflect the change both in pixel spacing and image    
origin.  Thus, should the user wish, (crude) astrometry can be performed with 
the browse images.  In addition, HISTORY keywords have been inserted to       
document the linear scale and density scale changes.  The creation of the     
browse images was accomplished using the program MIDGET, which can be found as
MIDGET.FOR in the SOFTWARE\BROWSE\FORTRAN directory of the compressed image   
CD-ROMs.                                                                      
                                                                              
     The filename extension for the files of the browse data (.IBG = image,   
.HDR = header, .LBL = PDS label) follow the IHW filename conventions.  To     
reconstruct the original FITS byte stream, the .HDR and .IBG files for the    
appropriate observation should be concatenated.