Dark Frames

Dark Frames

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Dark Frames

I take my dark frames at the beginning of each CCD session. Some camera operating manuals say you can re-use dark frames and re-calibrate them before subtracting them from your image. I have found that although it might work in principle, in practise it does not work or if it does it does not work as well as taking one prior to taking the first image..

How do you take a dark frame.
The dark frame is taken by covering the CCD chip, usually by covering the end of the telescope so no light can reach the CCD chip at all. On some cameras there is a shutter and some have an electronic shutter. My ST7 has a shutter and my Meade 216xt has an electronic shutter. I cover the end of the telescope with both cameras as the ST7's shutter is not totally light proof and the 216XT has no shutter at all. The thing to remember is to take the dark frame at the same temperature and for the same time period as the astronomical image.

What does a dark frame do.
At a given temperature and for a given exposure time a CCD chip will accumulate a certain amount of noise that is not related to an image. This noise is not random. That is if you repeat the same exposure under the same circumstances (temperature & time) the noise can be repeated. This means that if we take a dark frame and then take an image at the same temperature and for the same time as the dark frame the unwanted noise that is accumulated and recorded during the exposure on the dark frame will be on the image as well. So if we subtract the dark frame from the image we can remove all the unwanted noise.

IMAGE (including unwanted noise) - DARK FRAME (the image of the unwanted noise) = IMAGE (without the unwanted noise).

Although I said the noise was repeatable it is not quite. There is a small amount of variation between exposures. This can best be accounted for by taking several dark frames (I usually use four) and averaging them together before subtracting them from the image.