## On the Bias Jumps » History » Version 4

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

 1 2 3 1 Eli Rykoff `h1. On the Bias Jumps` 1 Eli Rykoff 1 Eli Rykoff `-Eli Rykoff` 1 Eli Rykoff 1 Eli Rykoff `Something strange is going on with the bias jumps at 2048...` 1 Eli Rykoff 1 Eli Rykoff `I'm taking as an example 180985, CCD 41, somewhat arbitrarily.` 1 Eli Rykoff 1 Eli Rykoff `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:` 1 Eli Rykoff 1 Eli Rykoff 1 Eli Rykoff `|!{width=750px}180985_41_finalcut.jpg!|` 1 Eli Rykoff 3 Eli Rykoff `h2. On the Flat Fields` 3 Eli Rykoff 3 Eli Rykoff `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:` 3 Eli Rykoff 3 Eli Rykoff `https://desar2.cosmology.illinois.edu:7443/DESFiles/desardata/OPS/red/20130725170742_20130217t0227/flatcor/flatcor_i_41.fits` 3 Eli Rykoff 3 Eli Rykoff `*Note this is problem on all the chips; chip 41 is chosen just as a random example of the problem.*` 3 Eli Rykoff 3 Eli Rykoff `And calculated the ratio between that and another (hopefully normal) flat:` 3 Eli Rykoff 3 Eli Rykoff `https://desar2.cosmology.illinois.edu:7443/DESFiles/desardata/OPS/red/20130718144123_20130113t0131/flatcor/flatcor_i_41.fits` 3 Eli Rykoff 3 Eli Rykoff `|!{width=750px}flat_ratio.png!|` 3 Eli Rykoff 3 Eli Rykoff `And the bias shift pops out plain as day.` 3 Eli Rykoff 3 Eli Rykoff `So...where did this come from?` 3 Eli Rykoff 3 Eli Rykoff `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. ` 3 Eli Rykoff 4 Eli Rykoff `|!{width=750px}182427_41_no_so_flat.jpg!|` 3 Eli Rykoff 3 Eli Rykoff `The gradient on this "flat" goes from 16000 - 1600, a factor of 10!` 3 Eli Rykoff 3 Eli Rykoff `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?` 3 Eli Rykoff 3 Eli Rykoff `h2. Original Images: Ignore the horizontal stripe which is an artifact of the sky subtraction` 3 Eli Rykoff 1 Eli Rykoff `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).` 1 Eli Rykoff 1 Eli Rykoff `|!{width=750px}180985_41_raw_obj.jpg!|` 1 Eli Rykoff 1 Eli Rykoff `Similarly, the raw-raw image looks fine (though it's hard to get the scaling right because of the dynamic range).` 1 Eli Rykoff 1 Eli Rykoff `So this is being inserted into the image somewhere along the line...Alex suspects the flat field...` 2 Eli Rykoff 2 Eli Rykoff `Looking at the flat field applied to this image:` 2 Eli Rykoff 2 Eli Rykoff `https://desar2.cosmology.illinois.edu:7443/DESFiles/desardata/OPS/red/20130725170742_20130217t0227/flatcor/flatcor_i_41.fits ` 2 Eli Rykoff 2 Eli Rykoff `And rebinning we see:` 2 Eli Rykoff 2 Eli Rykoff `|!{width=750px}flatcor_41_problem.jpg!|` 2 Eli Rykoff 2 Eli Rykoff `A-ha! A bias jump made its way into the flat field and has now corrupted the downstream images.`