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!
|Run||Image||D(time) in minutes||Mean Count (in 3 sec)|
|Sky counts through the F-P with an exposure time of 3 seconds (F-P set for H2 1-0 S(1) imaging; NDStare + Normal readout). The x-axis records the time in minutes; note that Sunrise occurred at D(time) = 12 minutes. The y-axis is the counts measured in 3 seconds. Consequently, a 30 second exposure time would have given 10-times as many counts on the array.|
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.”