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2. Reduction of Sky-Flat Frames


In order to study the behavior of the sky-flats as a function of temperature and exposure, it is first necessary to bias- and dark-correct them. The procedure for bias correction is identical to that outlined in Section 3.2, on the reduction of dark frames. Plots similar to Fig. 8 showed the bias levels for all of the sky-flat tests to be consistent with the Bias-Temperature relation determined in Section 2. Plots similar to Fig. 2 revealed no evidence for coherent temporal drift in the bias on the ~1-hour time-scales typical for a twilight flat sequence. Given this, I median-combined all bias frames for each sky-flat test sequence, and then subtracted the medianed bias frame from each of the raw dark, lamp-flat and sky-flat frames.

The next step is to dark-correct the sky- and lamp-flat frames. The procedure for dark correction is identical to that outlined in Section 4.1.2, on the reduction of lamp-flat frames. Plots similar to Fig. 10 showed the dark levels for all of the sky-flat tests to be consistent with the Dark-current—Temperature relation determined in Section 3. As the properties of the dark frames are consistent with the results of Section 3, I median-combined each set of darks from a given night. These darks were then scaled to the exposure times of the bias-corrected lamp- and sky-flat frames, and subtracted from them. I then reduced the lamp-flats, and extracted the small spatial-scale information from them, as outlined in Section 4.1.3 above. The resulting map of small-scale variations will be combined with the illumination-correction frame to be generated from the reduced sky-flats.


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Updated: 2009 August 20 [pbe]