In order to obtain as high a signal-to-noise ratio as possible, we want our lamp-flats to have peak count rates approaching the linearity limit of the detector. We also want the exposure time to be at least a few seconds long. This limit is imposed by the requirement that the time to open and close the shutter be vanishingly small compared to the integration time. Because the shutter time (typically 0.01 second or so) is non-zero, the total exposure time varies from one side of the chip to the other. We want this variation to be small enough to ignore. We also need to determine if the amplitude and pattern of pixel-to-pixel sensitivity variation is a function of temperature.
These considerations led to the following strategy: I obtained sets of lamp-flat, dark and bias frames on four nights from 2004 September through 2005 February. On the first three of these nights, I obtained frames at a set of five temperatures, with temperature increments of 5 degrees C. For each temperature set, I obtained five lamp-flat frames with exposure times of 50 seconds each. I also obtained a set of five dark frames of 50 seconds each, and a total of 11 bias frames at each temperature. The sequence at a given temperature was as follows: 5 bias, 5 flat, 1 bias, 5 dark, 5 bias. The frames on all three of these nights were taken with the wide (100 micron) slit. I used the green LED as a continuum source, and had the gain turned approximately 3/4 up. The intention was to have a peak count-rate of ~20,000 ADU. The range of temperatures encompassed by this set of tests is
Future users are encouraged to obtain lamp-flat sequences at T < -35 degrees C if the conditions present themselves. On 14 February 2005 I obtained a single sequence of lamp-flat, bias and dark frames at T ~ -20 degrees C, using the narrow (25 micron) slit and the green LED. For this sequence the lamp-flat and dark integration times were 10 seconds, and the gain on the LED was turned all the way up.
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