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| Joint Astronomy Centre |
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Obtaining a Sky Flat Below we show a series of images taken as sunrise approached on 20010630. D(time) is the time the exposure was taken after the first frame; sunrise occurred at D(time) = 12 minutes. Note also that the telescope was AT ZENITH.At the bottom of the page there is a plot of the count rate per three-second exposure, in which the sharp increase in counts just after sunrise can be clearly seen. Note: these data were taken in NDSTARE with NORMAL Gain. Hi-Gain will give twice as many counts per photon!
And Finally... A recent observer had the following experience while taking a sky-flat at sunrise: "We did our skyflat yesterday morning with the telescope 3 hrs over pointing towards the sunrise and got just the right counts (1000-2000) in 60 sec by starting the sequence right at sunrise (this was by accident as we were planning to do it 12 minutes after sunrise but just as well we didn't; just noticed that your webpage does actually say with telescope at zentih). Tried it just now with telescope 3hrs to west at sunset. We started it a few minutes before sunset but it was too late really (300-1000) counts. With telescope in that position need to start it 5 mins before sunset I reckon." |
