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Originally Posted by Merlin66
IVO,
Sorry,
what I'm trying to say is that even if you add the contribution of the light from the 1st and 2nd rings ( total star energy = 94%) the apparent saturated star image still seems to exceed the linear diameter of the rings....
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Ok, what sort of excess energy/starlight are you observing?
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The background shot noise level will be sqrt max intensity, which I'm sure would exceed the residual star energy...so no additional signal to expand the image???
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You be must taking really short exposures for the shot noise to drown out the Airy disk! There are many examples of a diffracted Alnitak overwhelming the Horse Head nebula for instance (
http://www.iceinspace.com.au/forum/s...ad.php?t=83886), even though Alnitak was never even projected on the CCD (its light still made it through the aperture of the scope).
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I probably need to use the photometry aperture feature (AA5) to better determine how the summed ADU count in each star image changes. This should be linear (for non- saturated images) - but what about a saturated image?
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You hit the nail on the head - the way your CCD deals with saturation of its wells varies wildly. But it is, alas, almost never truly linear. Saturation is dealt with in various way by the anti-blooming circuits of your CCD (if any). Fitting a Gaussian curve to a saturated star is therefore not very reliable.