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Flats and Pixelmath

Jim Savarese
in PixInsight
I was stumped when I took some flats and they showed a relatively even field instead of showing some vignetting. It took me a few days to figure out that I used the wrong function in SharpCap and the flats were calibrated by a master flat and ended up being unusable as they were. In addition since I used the normal capture function in SharpCap instead of the Flats function, the files had the FITS keyword OBJECT which caused them to be unusable  in WBPP without removing the keyword, since I was processing by OBJECT. WBPP wasn't going to use the Flats anyway to calibrate my lights, which were of multiple objects.
I decided I might be able to use Pixelmath to multiply the two images, as I remembered that Adam had said that Lights were divided by Flats and a scaling factor was used. I tried rescaling the result but the resultant image was at the .2 level in the center and I know I was hitting the middle of the histogram when I took the exposures so the resultant flats should be roughly around the .5 level. Can I scale the result arbitrarily, or is the scaling factor used in calibration a function of the statistics of the two images used in the division?

Comments

  • BlockHeadBlockHead
    Sorry...I did get a little lost.
    You said you want to "multiply the two images.." 
    I am not certain which two images are you talking about. 
    But...I can say that the flatted flats are not going to be useful in any form.
    A flatted flat is very nearly one (the value) (with some scaling... making it whatever the light level was).
    The issue is that all of the useful information about the variation of illumination is gone. You really do need the original/correct flats. 

    -the Blockhead
  • Jim Savarese
    I used sharpcap to make my flat images. Due to my error, sharpcap calibrated the flats I took with a flat I used to calibrate lights. I multiplied that flat with one of the flats I took with sharpcap. 
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