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Old 05-02-2014, 08:57 PM
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Geoff45 (Geoff)
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Star edges--hard or soft?

What do people think about the way stars should look in astroimages? Should they be tight circles with hard edges or should they have a bright centre which fades away with a soft edge into the background?
Geoff
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Old 06-02-2014, 11:28 AM
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gregbradley
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Soft edges. Hard edges look shocking. If I overdo deconvolution then sometimes I select the stars and run a small blur to soften them up. But generally its a processing error to make stars have hard edges. Usually its caused by oversharpening, overuse of deconvolution. Decon works better on oversampled data than undersampled data.

Its all part of the airy disc theory. In telescopes that are well made you should get the bulk of the energy in the first airy disc and a little in the secondary ring.

I guess the fact its called an airy disc implies a soft edge.

Greg.
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Old 13-02-2014, 08:42 AM
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Shiraz (Ray)
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Good question Geoff.

The way I see it is that stars are actually hard points of light with no size, but the atmosphere (and to a lesser extent your scope/mount) blurs them into fuzzy balls with nearly Gaussian profiles and with size depending on brightness. The basic question is - do you want them to look like they really are or as they look after being distorted by the atmosphere?

Deconvolution attempts to get stars back to what they really look like, but it cannot restore them entirely to points due to saturation and noise. You end up with smaller hard edge spots rather than unresolved points (and they are likely to be saturated). In my view, this is just as valid a representation as soft edged spots, but convention is that soft-edged, unsaturated fuzzy balls are “right” (possibly a carry-over from the days when everyone used refractors). As Greg points out, it is fairly easy to tweak deconvolved stars to look how you want and hard edged spots can be softened with a simple Gaussian blur to look “natural”. You can overlay the star colour from the original image should that be important to you. Of course, you will be deliberately getting further away from reality, but the result will look more conventional and colourful. Your choice. regards Ray

Last edited by Shiraz; 13-02-2014 at 06:53 PM.
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Old 13-02-2014, 10:58 AM
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strongmanmike (Michael)
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I agree with Ray.

In the end I think if one avoids the small sharp spot in the middle of a larger fuzz ball or diffraction spikes that narrow as they get closer to the star and provided the very edge of the bright disc of the star is not razor sharp you are looking ok ...but every image is different aaaand although I do try I certainly know I don't always get it 100% right

Mike
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