Re: automatic processing of data

[Stupendous Man <richmond@a188-l009.rit.edu>]

  In Tech Note 62, I scaled the darks before combining them because
the temperature of (at least one of) the chips wasn't controlled 
very well during the night.  I should have simply averaged the 
dark frames together.  For some reason (and probably incorrectly),
I decided to rescale them to a common mean before taking the median.

  Perhaps my reasoning was "I must use a median, not an average, 
when I combine dark frames; if I use an average, then cosmic rays
will contaminate the output pixel values.  But wait -- I can't
simply combine all these dark frames with a median, because their
overall levels are changing due to temperature changes.  Rats.
Well, if I rescale them all to have the same mean value, then I 
can use a median filter on them."

  The proper thing to do in this case, with a bunch of dark frames
taken at different temperatures, is ... throw them all out and
take a new set of dark frames.  

> Any thoughts on minimum number of frames for a good dark/flats?  Or trying
> to quantatize how you reject images for flat creation?

  The more darks and/or flats, the better.  Thirty to fifty dark frames
would be nice.  It's harder to get good flatfield frames, but a similar
number would be good.


                                           Michael Richmond