Airy Disk Radius

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Airy Disk Radius

Postby Rkonrad » Thu Jan 25, 2018 3:45 am

I"m trying to understand what this means - sorry I'm not getting it from the help function.

I have found that when doing an isolate under Life, with or without mask, that if I raise this value, the DSO is less glowing/blurred. When I want to retain more detail I up this value. I wouldn't mind knowing how this works. Thanks!

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Re: Airy Disk Radius

Postby admin » Fri Mar 16, 2018 6:22 am

Hi Richard,

Apologies for the late reply - a number of posts slipped through the cracks again.

The large-scale energy model ("the blurry image") is synthesised by convolving the light in your image with a point spread function that mimics that light's (re)acquisition through a perfect lens with a circular aperture.
That point spread function looks like an Airy disk.

The resulting light (e.g. the "blurry image") is then added back into the image in different ways, providing you a tool that lets you re-apply the light in your image in different ways.

The Airy disk radius essentially specifies how quickly the rings in the airy disk point spread function repeat and fall off; slow (small diameter, more "blur", showing individual rings and a big bulging center) or fast (large diameter, less "blur", showing diffraction patterns).

The "right" Airy disk radius is the radius that suits the field of view best. For wide fields, a larger radius is appropriate. For extreme closeups/magnifications a small radius is more appropriate.

The effect is best shown by using the following settings; Saturation 0%, Glow Threshold 0%, Output Glow Only Yes.
Then compare the subsequently stretched (by using for example AutoDev of Develop) output of different settings for Airy Disk Radius. With small radii, you will notice distinct rings around point lights (such as stars). With larger radii, you will notice diffraction patterns around point lights.

The used algorithm has things in common with Synth. However, whereas Synth is much deeper in functionality (because it focuses exclusively on remodelling the stellar profiles), the Life module uses a simpler model that assumes a uniform, "ideal" refractor-type setup (since it does not focus on the stellar profiles and very much needs to keep them intact).

The way the light is re-synthesised by modelling a real Airy disk is possibly overkill (A simple Gaussian blur comes close to achieving the same thing), but I believe the retention of the subtleties of the diffraction patterns is important.

I hope this helps at all!
Ivo Jager
StarTools creator and astronomy enthusiast
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