In order to obtain as high a signal-to-noise as possible, we want our sky-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, for reasons discussed in Section 4.1.1. This turns out not to be a limitation for our system, as discussed below. Scattered sunlight can potentially cause illumination irregularities, so one cannot begin observing until after the sun is below the effective horizon. The level of the effective horizon changes seasonally due to the variation in leaf cover of the trees. Even in winter, the presence of trees along the western horizon means that one can begin twilight sky-flats somewhat before nominal sunset.
These considerations led to the following strategy: I obtained sets of sky-flat, lamp-flat, dark and bias frames on four nights from 2005 April through 2005 July. On the first three of these nights, I used the narrow (25 micron) slit. On the last, I used the wide (100 micron) slit. Given the short time available for twilight observations, data were obtained at a single detector temperature on each night. The temperature information is given in Table 2. Given the poor level of temperature control for our system, one will often be forced to obtained sky-flats at system temperatures substantially higher than one will obtain later in the night. Intercomparison of frames obtained at different temperatures on different nights indicates that this is not a serious problem.
On each night I obtained data as follows: 5 bias, as many sky-flats as possible, 1 bias, 5 lamp-flats, 1-3 bias, 3 darks, 4 bias. The strategy for obtaining sky-flats is to begin as soon as the sun is well behind the trees, and continue to take longer and longer sky-flats until the sky is too dark to usefully continue. One needs to obtain lamp-flats as well, following the procedure discussed in Section 4.1.4, above. If one exposes a CCD to very high count-levels, one is often left with some residual charge on the detector after read-out. This charge will be read out over the next few exposures, and can lead to ghost images. These can be especially problematic for low-exposure frames such as darks. I noticed just such ghost problems during the course of these tests, and determined that the best means of dealing with them is to obtain, but not save, a few short (~10 second) darks after the lamp-flats. One must then obtain a set of darks with exposure time equal to that of the longest sky-flat (and at the same detector temperature!).
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