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Old 24-11-2013, 11:10 AM
clive milne
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clive milne is offline
Join Date: May 2011
Location: Freo WA
Posts: 1,443
Originally Posted by Peter Ward View Post
You conveniently ignore the physical size of the airy disk with this approach.

Airy disk size (assuming perfect optics) is purely a function of F-ratio.

At F8 the light is being spread over 10 microns. At F3.8 this contracts to 4.7 microns.

Not withstanding sampling, assuming the same aperture, you are concentrating the same flux into fewer pixels....
Yep... but you would have to increase the system focal ratio to around f24 (with a typical ccd ) before you are sampling the image at a level where diffraction becomes a sensible limit... this is why you can throw 50 percent of the collected energy into the diffraction rings without suffering any loss of detail at f4.... whether that is in the form of large central obstruction or a moderate amount of sa.... it really doesn't matter as long as the distribution curve is the same.

The design parameters of the hst are an interesting case in point.... when the goal is ultimate deep sky resolution, f24 (you would assume) is ideal coupled with a small secondary obstruction (15%) that doesn't throw a high percentage of its light out of the airy disk. The design parameters of the hst however, don't include an atmospheric component that would otherwise impose a different limit and therefore a different optical configuration optimised for those specific conditions.

Last edited by clive milne; 24-11-2013 at 12:01 PM.
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