Hi,

From time to time a question comes up about the need to take dark frames for flat fields. As long as you are operating the camera cool enough, -20C for most modern cameras, there will not be a significant build up of dark current to impact the flat frame at all for flat field exposures of up to 30 sec. or so.

The arithmetic behind this is discussed in the newly added calibration section of the Flat Field application note available here:
http://www.hiddenloft.darkhorizons.org/notes/ccdap.htm

Hi John
Well, with my first Meridian Flip under by belt....I have had a mental mind block....having taken sky flats, do I have to flip and mirror my them when calibrating images taken after a meridian flip.....it doesnt make sense to me because I never compute the angle of the camera when it takes the sky flat, so I guess the orientation of the object does not matter at all right?
cheers
Martin