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It is common to obtain sky-flats during twilight at either the beginning or the end of an observing night. This creates two problems that we must consider. First, the count-rate will be a steep function of time relative to sunset. Thus the required exposure time for good signal to noise will increase as (evening) twilight progresses. Second, the lack of reliable detector temperature control with our system means that sky-flats are very likely to be obtained at higher detector temperatures than data frames obtained later in the evening. I discuss each of these issues in turn, below.
Fig. 29 shows the variation in peak count-rate as a function of time relative to sunset. Concentrating on the data for the narrow slit, we see that the three sets of observations are in generally good agreement with one another. The only serious discrepancy is with the first few points obtained on 08 April 2005. This was the first sky-flat test I conducted, and the initial set of exposures were very short (hence the large error-bars). As a result, these frames do not have a very high signal-to-noise ratio. I thus take the remaining data to be a better guide to establishing an observing strategy.
One can begin taking sky-flats as soon as the Sun is below the local effective horizon. This will be before nominal sunset, and the timing will vary with the seasons. To determine what exposure times to adopt, observers should use Fig. 29 to estimate the peak count-rate at the mid-exposure time, and see how long it would take at this count-rate to reach 20,000 counts. Try not to exceed that limit. Continue to obtain sky-flats until the sky is sufficiently dark that exposure times become prohibitive. As I discuss below, it will be unwise to take sky-flats with exposure times longer than ~10 minutes.
The data shown in Figs. 24 and 29, and Table 2 indicate that the sensitivity of the detector does not appear to be a strong function of detector temperature. This means that we should be able to determine illumination-correction maps from sky-flats obtained at detector temperatures higher than will be used for the data frames taken later in the night. But if one obtains sky-flats at a different detector temperature than one uses for data frames, one must also obtain dark and bias frames at the detector temperature appropriate to the sky-flats as well as at the detector temperature appropriate to the data frames. This is at the heart of my comment above about keeping the sky-flat exposure times short. If one has a maximum sky-flat exposure time of 10 minutes, one must then devote 30 minutes to obtaining a set of darks at the same detector temperature as the sky-flats.
Given these constraints, the following set of guidelines should allow observers using our system to obtain sky-flat frames adequate for most purposes:
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