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Re: bias frames. I've never used them in DSLR work. I just don't see the point considering the bias is included in the darks for the lights as well as the darks for the flat lights. In my experience, they've just introduced further noise into the final image.
Humayan
Do you subtract them from your flat when you are making a master flat or have you subtracted them from each individual light exposure?
The point here is subtracting them from your flats not your light exposures. If you subtract a dark or use ICNR when taking your flat then you are doing what IU am saying here which is to subtract bias when making a flat master not subtracting bias from a light exposure.
Flats are normalised and then divided into the image, darks are subtracted. So mathematically you can see that if you didn't remove the wrong numbers in the pixel grid caused by bias noise then some numbers that is divided into the image when applying a flat would be falsely too high or too low. The point of a flat is to bring about evenness of illumination of the light. This then would reduce its effectiveness. How much less effective? Perhaps not a lot. Probably depends on how noisy your bias is I suppose. Some cameras are cleaner than others.
However I agree subtracting a bias from an image can cause more noise and unwanted effects as you are subtracting the same thing twice if you subtracted a dark. The use referred to here is subtracting it from the flat images before making a master flat. If you subtracted a dark from the flats then you wouldn't subtract the bias (it'd be much the same anyway - a 3 second dark would look the same as a bias).
I think personally it is also a minor improvement and nothing important in the scheme of things. I have also noticed a slight improvement in noisy images by subtracting a pedestal (100 subtracted from the pixel count assumed to be bias type noise) instead of a bias. Again its subtle but when processing a galaxy image every little bit helps.
I prefer to keep my camera and filters clean and to run the camera as cold as possible to reduce noise so all this callibrating is less important in the fist place.
My TEC180 for example is very evenly illuminated in the first place. So is the AP140. Flats then are almost not needed.
Greg.
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