2013-02-22

The following images show the vertical median profile (along pixel columns) of OTA45 and OTA46 after a series of detrend steps have been applied. Three exposures were chosen for this, one from M31 (the middle pairs), and two from 3PI (the top and bottom pairs). The first image shows the raw overscan subtracted image along with the dark corrected image. The bottom image continues from there, adding the flat and pattern corrections. The important points to notice:

• The raw data shows a cell-to-cell "scoop" shape that is caused by the dark current.
• Upon dark correction, the "scoops" are largely removed from each cell, although OTA46 (x > 5000) now has a downturn (anti-scoop?) shape for the M31 exposure. I suspect that this is due to the presence of a video cell on OTA46 in this exposure, resulting in a different dark current relative to the dark model. The bottom exposure also has a video cell in OTA46, and I think there's some evidence of a similar overcorrection in that OTA.
• After applying the flat field correction, the cells of the individual OTAs match each other well. Further adding the pattern.continuity correction seems to do little but shift the OTA background to be slightly more flat.
• The bumps in the M31/OTA45/x=4700 and bottom/OTA46/x=5900 are from bright features on those exposures that are not fully removed by the median filtering, and should not be confused with issues with detrending.
• Manually extrapolating the M31 dark corrected edge value to something not "anti-scooped", and then applying the expected flat field correction seems to provide a value that would be consistent with no offset between OTAs (measured OTA46 edge value ~178, flat field corrected value ~151, "correct OTA46 value" ~185 => expected correct flat field value ~157. measured OTA45 edge value ~160).