Quote:
Originally Posted by janoskiss
With a perfect optics and detector stars should look like Airy disks. If you use a telescope other than a refractor, they should look like the equivalent of an Airy disk for your scope (in more precise technical terms: the square magnitude of the Fourier transform of the aperture function of your scope).
But with practical detectors (CCDs etc) the signal will bleed to neighbouring pixels. That's both good and bad: good because in the image brighter stars will look brighter; bad because the extreme brightness of stars is only captured by bloating their imprint on the pixel array. It's a dynamic range issue and can to some degree be tamed with HDR techniques.
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Quote:
Originally Posted by RickS
Get a FSQ-106, Ray! Your Airy disk in red would be about 3 arc sec.
Or, move your gear to SRO  Our Ceravolo 300 at SRO should have an Airy disk a little over 1 arc sec and the seeing is often better than that.
Cheers,
Rick. |
OK, agree that there is a diffraction contribution, but I still think that atmospheric seeing overwhelmingly determines the star profiles most of the time.
just resurrected an old Excel model to check (it isn't perfect, but I tested it against the measured PSF of my old 200mm scope and think it works well enough - would gladly be corrected if anyone has better). The attached image shows the star profiles from three scopes (FSQ106, 250mmNewtonian with 0.3 central obstruction and 400mm with 0.35 CO). Three atmospheric seeing conditions were considered (0.1arcsec - perfect, 1arcsec - exceptional and 3arcsec - ordinary). The model is the standard 2xBessel with convolution of a Gaussian to represent seeing (should really have used Moffat, but Gaussian is close enough for this task). Note that the y-axis has a log scale.
Even in 1 arcsec seeing, you don't see an Airy disk structure and the scopes all have significant regions where the star shape is primarily determined by the seeing and not by diffraction. In 3 arcsec, diffraction does not get a look-in for determining the shapes of most stars - it's all down to seeing for 3-4 orders of magnitude from the peak (ie after stretching, all but the brightest stars will have profiles dominated by seeing). The seeing-dominated part of the profiles is quite steep, so I would expect stars in that brightness zone to have fairly tight boundaries
All of these scopes produce star profiles with skirts (due to diffraction) and there will also be some scattering (not considered). The larger scopes will give dimmer skirts around the bright stars, but with all, the increase in star size with bright stars or long exposures is due to this skirt structure and not any charge overflow or blooming in the sensor (most sensors have potential barriers and overflow drains specifically to prevent this from happening).
The skirts are brighter on the FSQ - this accounts for the big cotton balls these scopes produce around very bright stars. This is not a fault and can look very attractive - it is just what happens with smaller apertures. The larger scopes produce dimmer skirts, so stars from these will only grow very large when the exposure is long enough and the stretching is sufficient to bring the dim skirts into the scene dynamic range.
well anyway, that's how I see it and would welcome any corrections/comment. regards Ray