What is a Dark Frame?
A dark frame is an image of the same duration and temperature as the light frame taken with the CCD imager with the lens cap on. This image contains a “map” of the dark signal across the surface of the chip. During processing, the dark frame is subtracted from the light frame using software to remove the dark current signal. This is one of the easiest ways to improve your images. If there is one piece of advice to follow: take and subtract dark frames! Your images will show the improvement. To improve your images, and especially if you are doing photometry, you should take many darks and then average them together. An additional suggestion: Take the dark frame integration just before or after your imaging run so that the temperature conditions of the imager are close to the conditions used for the image itself. If you have a CCD which has thermal control, then you cna take many dark frames at a variety of temperatures: Make a library! I have taken 10 dark frames for each temperature (C
