***** File BROWSE.TXT                                                         
                                                                              
     Effective use of the 1,612 images contained in these IHW/Large-Scale     
Phenomena (L-SP) compressed image CD-ROMs requires that the user of the disks 
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.                                
                                                                              
     This was done for the compressed images on this disk and the results are 
located in the BROWSE directory of this CD-ROM.  A set of "browse images" has 
been generated for which the images are uncompressed, subsampled and are 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-S 
imagery.                                                                      
                                                                              
     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, the entire set of 1,612   
digital images exists on the last of the IHW/L-SP dedicated disks in strict   
time order.  The browse images are also interleaved with data from the other  
IHW disciplines in the daily data subdirectories on the "mixed" CD-ROMs.      
                                                                              
     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 a 
MIDGET.FOR in the SOFTWARE directory of this CD-ROM.