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Old 13-12-2011, 08:37 AM
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mithrandir (Andrew)
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Join Date: Jan 2009
Location: Glenhaven
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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?

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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