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Originally Posted by mithrandir
FWHM is commonly used as a measure of focusing and choosing stars for stacking.
Is the real point in that FWHM becomes unreliable once the central pixels come close to saturation?
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Comparing FWHM between different stars/frames becomes unreliable. Gaussian curve fitting (of which the FW at HM is a by product) can still be used to help with determining sub-pixel accurate location though (as used for stacking purposes) because in that case you're just interested in the location of the predicted peak of the Gaussian curve, not its width or the magnitude at its maximum.
Quote:
Once you have a circle of saturated pixels FWHM has to be at least twice the diameter of the circle of saturated pixels.
So the moral of the story would appear to me - and to Craig Stark from the Nebulosity doco - do not use saturated stars to do any sort of measurement of focus, or alignment for stacking.
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I don't see anything wrong with using them for stacking/alignment, but Craig may have reasons I'm not aware of?