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2. Reduction of Lamp-Flat Frames


In order to study the behavior of the lamp-flats as a function of temperature, 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. 6 showed the bias levels for all of the lamp-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 ~20 minute time-scales typical for a lamp-flat sequence. Given this, I medianed all 11 bias frames for each flat-test temperature sequence, and then subtracted the medianed bias frame from each of the raw dark and lamp-flat frames.

The next step is to examine the bias-corrected darks, and see if they show behavior consistent with the results of Section 3. In Figure 18 I show a plot of the mean dark current against exposure time similar to Fig. 10, but including data from the dark frames from one of the lamp-flat test sequences. The dark counts for the short exposures fit along the extension of the lines determined for the longer exposures. Similar plots for other temperatures show the dark rates to be consistent with the results of Section 3.

  • Figure 18: Dark rate plotted against exposure time for T = -20 degrees C (squares, dotted line) and T = -10 degrees C (six-pointed stars, dashed line) Additional data at T = -20 degrees C and t=0.83 min show the extension of the dark count to short exposures.

    All other properties of the dark frames are consistent with the results of Section 3, so I medianed each set of darks from a given night, at a given temperature. These combined darks were then subtracted from the individual bias-corrected lamp-flat frames.


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    Updated: 2009 August 20 [pbe]