With darks and dark flat frames, each pixel consists of the following:
LightPixel = Bias + Dark + LightSignal
FlatPixel = Bias + DarkFlat + FlatSignal
DarkPixel = Bias + Dark
DarkFlatPixel = Bias + DarkFlat
Calibration involves the following:
Calibrated Light Pixel = (LightPixel - DarkPixel) / (FlatPixel - DarkFlatPixel)
= (Bias + Dark + LightSignal - Bias - Dark) / (Bias + DarkFlat + FlatSignal - Bias - DarkFlat)
= LightSignal / FlatSignal
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