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Thanks Peter. Found a little something from Mr Ron Wodasky on this, which answers my Q:
FWHM is what you would need to be concerned with if image sharpness is what you are looking at.
Half-flux diameter is used on out-of-focus star images to aid in focusing away from best focus. It is a good measure for determining exactly how far from focus you are, and thus is very useful for a software product like FocuxMax. It's not very useful for human eyes, however, since we are mostly concerned with knowing when we are at good focus.
FWHM is useful for that - you want the smallest FWHM you can get if you are focusing yourself, or if you are evaluating seeing conditions (better seeing = smaller FWHM). More specifically:
Half flux diameter is the distance where half of the total light flux is inside that diameter, and half is outside.
FWHM is the diameter of a star image at half the height of the maximum brightness level.
Last edited by Logieberra; 29-09-2013 at 11:13 AM.
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