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In order to study the behavior of the dark frames as a function of temperature and exposure time, it is first necessary to bias-correct them. The first step in the process is to see if the bias frames fall on the Bias-Temperature relation discussed in Section 2. In Figure 8 I show a Bias-Temperature plot similar to Fig. 1, but including the bias frames from one of the dark-test nights. Similar plots for the other dark-test nights showed the bias levels to be consistent with the Bias-Temperature relation determined in Section 2.
The next step is to look for temporal variations in the bias level at a given detector temperature. A typical dark-test temperature sequence takes 1.5 to 2 hours. Fig. 2 shows an example of the bias level for one temperature sequence from 18 January 2004. As discussed in Section 2, although there is clearly bias-jitter at the +/-1 ADU level, there is no evidence for coherent temporal drift in the bias on 1-2 hour timescales. Given this, I median combined all 13 bias frames for each dark-test temperature sequence, and then subtracted the medianed bias frame from each of the raw dark frames. Figure 9 shows an example dark frame both before and after bias subtraction. Note that the top-left to bottom-right gradient is largely removed, as are several of the bad columns.
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