Dark Horse Observatory
The Effect of ADU Count on Combined Flat Field
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The purpose of Flat Field data reduction is to correct for
optical defects in the system that alter the apparent light delivered to the
CCD and to correct for variations in CCD sensor's response to light. As with
any processing there is the potential to introduce noise. The point of making a
master flat frame is to improve the signal to noise (S/N) ratio. This requires
that one delivers enough signal to the CCD to overcome the read noise and that
a sufficient number are added to average out the read noise that is introduced.
When I was just starting out everything was a mystery especially the concept if image calibration or image reduction. The way and specifics of dark frames (same time and same temperature as your light frames) and bias frames was relatively easy to grasp. However the best settings for flat field images was more difficult to pin down. I heard different average ADU counts, and different number of flat field frames to combine. Some people insisted that sky flats were better and others that light box flats were better. I decided to try and find out for myself, what worked best for my equipment. The following were my attempts to address the 3 questions:
The first variable that I investigated was what the effect of varying the intensity in the flat field.
Clicking on any image will enlarge it for closer inspection.
As you can see, I really needed to clean the optical
path so this is a real test for a flat field. The flat fields worked amazingly
well and the vignetting and dust motes are not visible to me in the final
It was apparent to me that, except for the extremes, there was
very little difference.