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Here’s how I think of flats and light frames. When calibrating your images, Flats are “divided” into the light frame rather than being subtracted like we do in a dark frame.
Say you have a dust spot in the galaxy (light) frame, and it reduces the intensity of light on the pixels in that small region by say, 50%.
When you take the flat field, the intensity in the dust spot region will be 50% of that in another region that has no dust, or vignetting. That is, a clean non-vignetting region can be said to have an intensity level of unity or 1.
So, when you divide the flat into the light frame, dividing by ½ will “double” the intensity to compensate for the 50% loss caused by the dust spot, whereas dividing by 1 will not make any difference where there is no dust or vignetting.
Likewise, if a flat field region is say x2 as bright due to the effects of a localised hot spot, then dividing the light by the flat (÷2) will bring the intensity back down to unity.
Cheers
Dennis
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