PDS_VERSION_ID         = PDS3
RECORD_TYPE            = STREAM
OBJECT                 = TEXT
  PUBLICATION_DATE     = 2004-12-07
  INTERCHANGE_FORMAT   = ASCII
  NOTE                 = "Documented changes to NAC slope files."
END_OBJECT             = TEXT
END
   
Slope Files Corrected for Anti-blooming Pairs
---------------------------------------------

Note: this document refers to the following list of NAC slope files:

slope.slp109  slope.slp133  slope.slp159  slope.slp185  slope.slp220
slope.slp110  slope.slp136  slope.slp160  slope.slp187  slope.slp221
slope.slp111  slope.slp137  slope.slp161  slope.slp188  slope.slp225
slope.slp112  slope.slp138  slope.slp165  slope.slp189  slope.slp228
slope.slp113  slope.slp139  slope.slp166  slope.slp190  slope.slp232
slope.slp114  slope.slp140  slope.slp167  slope.slp196  slope.slp233
slope.slp121  slope.slp141  slope.slp168  slope.slp201  slope.slp235
slope.slp122  slope.slp146  slope.slp172  slope.slp202  slope.slp239
slope.slp125  slope.slp150  slope.slp173  slope.slp209  slope.slp240
slope.slp129  slope.slp151  slope.slp174  slope.slp215  slope.slp242
slope.slp130  slope.slp152  slope.slp175  slope.slp216  slope.slp245
slope.slp131  slope.slp153  slope.slp178  slope.slp217  slope.slp253
slope.slp132  slope.slp158  slope.slp181  slope.slp218

This set of slope files was derived from long exposure Thermal/Vac
images with Anti-blooming ON. This resulted in the formation of 
the bright-dark pixel pairs of various degrees of intensity.  In 
order to use these slope files to correct Anti-blooming OFF data 
or data with short exposures, the pairs had to be removed.

The pair-removal process involved:
1. Determination of which slope files needed fixed
   - This was a visual inspection process
2. Determination of a criteria for which pairs to remove
   - Trial and error gave a useful threshhold of
     (pair delta DN >= 5% of mean DN)
3. Generation of a procedure to remove the pairs
   - A VICAR procedure was generated (see description below)
4. Validation that nothing bad happened to the good data
   - Visual inspection of the corrected files and differencing
     vs. the original files
     
Removal Procedure
The slope file generation process begins with raw flat-field data.
These data files are then summed by exposure level before being
processed into slope files.  Due to software limitations, it was 
determined that the removal process had to act upon the summed
input data.

The summed files were corrected in the following way:
1. A filter was applied to identify each pixel which had the
   characteristic of a large positive difference between itself
   and its neighbor above.
2. Areas of known natural large differences not due to pairs
   were then excluded.
3. The remaining pairs were replaced by the median value of the
   surrounding pixels
   
The corrected sums were then processed with the original slope
file generation process.  The current set was generated on Unix,
so the floating point numbers may differ insignificantly from 
the old files produced on the VMS OS in the 1990's.

Charlie Avis
10/2003