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Daniel Gruen, 07/23/2012 09:47 AM



From DESSV Plan: Map of bad pixels, defects is stable, as seen from dome flats and median sky flats aka DECam TD-16

Explanation: Bad pixels are pixels whose counts are non-linear in the SB of the sky at their position. The hope is that they are stable over time in the sense that a bad pixel will stay "bad" while no new bad pixels appear. Bad pixels are likely to change in "badness" with thermal cycles (Juan Estrada, private comm.), so some degree of monitoring is required lest they contaminate neighboring pixels.


We require sets of dome flat fields with 4 different flux levels (1) shortest reasonable exposure time, (2) a few times as long as this, (3) half the saturation level and (4) almost at saturation level, to cover non-linearity in the typical low surface-brightness and the almost-saturated regime. They should be bias subtracted. They must be taken at different times of the commissioning procedure to check for stability over time and, subsequently, after certain intervals. They should span the DES filters since one could imagine spectral dependence of non-linearity. Additionally, we require a "known bad pixel" mask for each chip from the DESDM database.

Note that we need sets of dome flats with different exposure times taken without moving the telescope or changing the light source in between, so the gradient stays the same in all images. Small changes in light source luminosity will be corrected. At the lower exposure time limit, we might discover non-linearities due to shutter effects.


for each filter, for each chip:
  • for each day available:
    • for each of the 4 flux levels:
      • calculate median flux of each of the frames
      • multiply each frame with the quotient of desired over median flux level to bring them to the same level
      • median-stack frames from the same chip to generate master flats
    • for each pair of flux levels (1,2), (2,3), (3,4)
      • divide the two master stacks
      • note pixels where the fraction is off the quotient of the desired flux levels (i.e., which show non-linearity) beyond the expected statistical uncertainty of the master flats
      • compare the list of noted pixels to the list of known bad pixels: are there any new ones?
      • save the list for later use
after a few observations separated by some time in between have been made:
for each pair of flux levels (1,2), (2,3), (3,4)
  • compare the number of noted outliers for each date -> is there an increase?
  • compare the bad pixel map created for each date [manually, by blinking the masks] -> is there any visible development?

If we want to monitor the change of these bad pixels over time, we could save the FITS image or simply a list of divided flux levels (2,3) for each date; this will parametrize the non-linearity.