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On the Bias Jumps » History » Version 4

Eli Rykoff, 11/01/2013 03:48 PM

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h1. On the Bias Jumps
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-Eli Rykoff
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Something strange is going on with the bias jumps at 2048...
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I'm taking as an example 180985, CCD 41, somewhat arbitrarily.
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I use swarp to subtract the sky and rebin to a 2" scale to highlight the sky levels.  This is the finalcut image which shows the problem:
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|!{width=750px}180985_41_finalcut.jpg!|
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h2. On the Flat Fields
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Below if you squint you can just see the bias problem in the flat.  But there has to be a better way to visualize this.  And there is!  I took the offending flat:
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https://desar2.cosmology.illinois.edu:7443/DESFiles/desardata/OPS/red/20130725170742_20130217t0227/flatcor/flatcor_i_41.fits
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*Note this is problem on all the chips; chip 41 is chosen just as a random example of the problem.*
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And calculated the ratio between that and another (hopefully normal) flat:
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https://desar2.cosmology.illinois.edu:7443/DESFiles/desardata/OPS/red/20130718144123_20130113t0131/flatcor/flatcor_i_41.fits
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|!{width=750px}flat_ratio.png!|
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And the bias shift pops out plain as day.
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So...where did this come from?
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I looked at the header of the t0227 offending flat and downloaded all 91 of the input images.  Visual inspection shows a slew of them (00182422 - 00182431 inclusive) are not healthy images.  
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|!{width=750px}182427_41_no_so_flat.jpg!|
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The gradient on this "flat" goes from 16000 - 1600, a factor of 10!
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But... are 10% of the images being bad in a superflat enough to cause this sort of problem?  And why does this show up as a bias shift in the middle?
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h2. Original Images: Ignore the horizontal stripe which is an artifact of the sky subtraction
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But if we go back to the raw_obj image (same scaling) which has had DECam_crosstalk run on it then it looks fine!  (Well, it could use a good flat-fielding).
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|!{width=750px}180985_41_raw_obj.jpg!|
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Similarly, the raw-raw image looks fine (though it's hard to get the scaling right because of the dynamic range).
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So this is being inserted into the image somewhere along the line...Alex suspects the flat field...
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Looking at the flat field applied to this image:
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https://desar2.cosmology.illinois.edu:7443/DESFiles/desardata/OPS/red/20130725170742_20130217t0227/flatcor/flatcor_i_41.fits 
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And rebinning we see:
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|!{width=750px}flatcor_41_problem.jpg!|
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A-ha!  A bias jump made its way into the flat field and has now corrupted the downstream images.