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how to determine saturation threshold




  Herb Johnson asks:

> .... how did Tom - and how will
> other TASS members and the software they write - determine a QUANTATIVE
> value for saturation? Chris suggests, in effect, that you eyeball the
> flattops of the pixel images; or you look at a histogram. That was not
> very quantative or methodological, the "messy details" he waived off were
> in fact what I was asking about! 

  I am aware of the following methods of dealing with saturation and
non-linearity in the astronomical community.  

      1. do nothing about it.  Common
         
      2. mark as saturated any pixel with the maximum possible value
               (often 65,535 for a 16-bit image).  Very common

      3. look at pictures by eye, mark as saturated stars with flat tops.
               Common

      4. take a series of images with successively longer exposure times,
               and use the brightness of a set of stars to 
               determine the data value at which the response to light
               goes non-linear.  Write down once, and use forever.
               Uncommon

      5. take a series of images, as above, and use them to 
               a) determine the limit of linear response
               b) fit the non-linear response in a simple way
           Then apply a correction (if necessary) to every pixel value from 
           every image, to bring it to the theoretical linear value.
           Very uncommon

  Very few people I know have actually gone through the steps of methods 
4 or 5.  Most people adopt some value (like 20,000 counts) as a 
"safe" threshold, and try to adjust exposure times so that the target 
of interest is below the threshold on all images.  How do they choose
the value?  Gossip and informal chat with other users of the instrument,
mostly.

                                                    Michael Richmond