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Don Reed | all galleries >> Astrophotography >> DSLR Astrophotography - How It's Done > The dark flat field
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The dark flat field

The dark flat field image is optional. If you use dark flat field frames, the bias frames become optional.

If used, the dark flat field frames are acquired at the same ISO and exposure time as the flat field frames, but with no light reaching the camera's sensor. I acquire them just after acquiring the flat field frames, by capping the telescope's optical tube, setting the camera to shutter priority (Tv) mode, and setting the exposure time to be the same as the flat field exposure times. I acquire the same number of dark flat field frames as flat field frames, usually between 10 and 20.

During image processing, the master bias frame is subtracted from each dark flat frame, and then the dark flat frames are median combined to create the master dark flat frame. Then the master bias frame is subtracted from each flat field frame, followed by subtracting the master dark flat frame from each flat field frame. Finally, the bias and dark current-corrected flat field frames are combined and converted to grayscale to produce the master flat frame.

The purpose of the dark flat field is to remove dark current from the flat field frames before they are combined to make the master flat field. But since there should be practically no thermal noise in the short exposures characteristic of these frames, simply subtracting the bias frames from the non-bias-corrected flat field frames should yield essentially the same result as subtracting the bias-corrected master dark flat frame. And since the dark frames also contain the bias signal, subtracting a non-bias-corrected master dark frame from the light frames will remove the bias signal from the light frames.

So it is important to use dark flat frames unless you use bias frames, and vice versa. And it is also perfectly acceptable to use both - I typically do.

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