Quote:
Originally Posted by Merlin66
Ray,
help me here....
Neglecting vignetting/ shadowing/ fringing (the usual target for flats...)
Why would dividing by what I see as an "evenly illuminated but random pixel distribution" image improve the SNR????
What is it actually doing to the signal?
For AP I can see the background "graininess" caused by the low SNR.....
but I can't see how dividing by basically a "constant" is going to lift the target signal above the background and effectively add to the SNR???
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Hi Ken - hope this makes sense.
Ignoring read and dark noise, there are 2 other major noise components - the shot noise from the sky and the fixed pattern variations in pixel sensitivity that modulate the sky and target signal and give you additional noise (fixed pattern noise, FPN). You can get rid of the FPN by using flats, which compensate for the variations in pixel sensitivity - but the flats have to be perfect to completely remove FPN. The analysis shows how much flat signal you need to get the flats close enough to perfect - and thereby to ensure that the noise introduced by the flats is submerged under the sky shot noise and not a significant component of the total noise.
Put another way, evenly illuminated flats are not constant at the pixel scale - they have embedded graininess due to the pixel-level fixed pattern variability in the sensor sensitivity. The lights have exactly the same graininess, so you can use the flats to compensate for the fixed pattern graininess in the lights.
You have a Sony chip with low inherent FP variation, so this is all a bit academic for your system unless you do very long exposures or have bright sky. However, the majority of cameras out there have Kodak sensors and they need flats, since they have significantly higher FP variation.