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Flat frames, or flat field frames, are just frames that create a flat field of intensity across the pic, so that any imperfections in the pics you take (like dust bunnies etc) that are represented by changes in intensity can be accounted for. One way to make a flat is to take a white T-shirt and stretch it across the front of your scope and then take pics of the T-shirt framed against an evenly illuminated background...like the evening sky at zenith. They have to be of the same length as your light subs (pics).
Bias frames are frames that account for electrical bias in the CCD chips. That's basically an electrical charge that occurs in the chip when no light is striking the surface of the chip. Bias frames can by just snapshots because the bias will be present in the pics no matter the exp' length.
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