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Sky Subtraction Troubleshooting

dborncampdborncamp Member, Institute_Staff Posts: 7
I used a large dither in my program, and the drizzled products show a clear offset in the sky background. How do I correct this?


  • avilaavila Member, Institute_Staff Posts: 10
    If the default parameters aren't working for you, the best bet would be to calculate the sky values yourself and feed them to Astrodrizzle using the 'skyfile' parameter. Here's what the help file says regarding this:

    skyfile : str (Default = ‘’)
    Name of file containing user-computed sky values to be used with each input image. This ASCII file should only contain 2 columns: image filename in column 1 and sky value in column 2. The sky value should be provided in units that match the units of the input image and for multi-chip images, the same value will be applied to all chips.

    Another option would be to try Drizzlepac 2.0, which has new algorithms in the Astrodrizzle task to match backgrounds in a situation like yours.
  • mutchlermutchler Member, Institute_Staff, Moderator_DrizzlePac Posts: 6
    I'll have to try the skyfile parameter -- haven't yet. I usually measure the sky myself (find a nice "empty" spot in the flt images to measure sky), then paste my values into the MDRIZSKY keyword of the input flt images, so that value then gets subtracted during drizzling. Just supplying a simple skyfile sounds easier, but I assume it does the same thing (puts my values into MDRIZSKY)?
  • oleholeh Member Posts: 10
    avila, which method of sky value measuring do you use?
  • avilaavila Member, Institute_Staff Posts: 10
    For this problem where there is an offset visible in images with a large offset, I have been using the new matching algorithms in Drizzlepac 2.0. I usually use 'match'.
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