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mloffland

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Reged: 09/03/04

Loc: Norman, Oklahoma
Fraunhofer Diffraction - A Primer
      #3308141 - 09/01/09 12:19 PM

Fraunhofer Diffraction - A Primer

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mathteacher
Carpal Tunnel
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Reged: 05/13/07

Loc: Oakland, CA
Re: Fraunhofer Diffraction - A Primer new [Re: mloffland]
      #3312048 - 09/03/09 01:15 AM

Nice article. Sure, Lord mask it is!

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star drop
contra contrail
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Reged: 02/02/08

Loc: Snow Plop, WNY
Re: Fraunhofer Diffraction - A Primer new [Re: mathteacher]
      #3313856 - 09/03/09 10:12 PM

Very good article.
Are the angles of the "Y" 150°, 60°, and 150°?


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bill w
Postmaster
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Reged: 03/26/05

Loc: southern california
Re: Fraunhofer Diffraction - A Primer new [Re: star drop]
      #3316593 - 09/05/09 08:46 AM

great article chris, thanks for posting it

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gezak22
scholastic sledgehammer
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Reged: 08/15/04

Loc: On far side of moon. Send help...
Re: Fraunhofer Diffraction - A Primer new [Re: mloffland]
      #3339733 - 09/17/09 03:32 PM

What dimensions do you recommend for a 100mm f/2 lens. Of course I'll stop it down for photography, but for focusing purposes, I'll leave it at f/2.

Very nice article btw.


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jbalsam
sage
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Reged: 07/06/12

Loc: Darnestown, MD
Re: Fraunhofer Diffraction - A Primer new [Re: gezak22]
      #5644922 - 01/26/13 07:02 PM

Someone pointed me to Chris's article and suggested that I make one of these masks if they do indeed work better than Bahtinov masks. I made one with my laser cutter, and it didn't work nearly as well as the bahtinov mask that I made for my telescope (152mm achromat). I was fairly convinced by the article, but it appears that something is off in the calculations.

Edit: Has anyone else tried making one of these and comparing them to a Bahtinov (or other) focusing mask? I'd like to have some more data points.

Edited by jbalsam (01/26/13 07:08 PM)


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jhayes_tucson
professor emeritus
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Reged: 08/26/12

Loc: Bend, OR
Re: Fraunhofer Diffraction - A Primer new [Re: jbalsam]
      #5673317 - 02/10/13 11:49 PM

"So where is the entrance pupil? For most practical purposes it can be considered as lying immediately in front of the objective, and to have a diameter equal to the objective's clear aperture. In reality it either lies at infinity, or a considerable distance in front of the objective. Because star light entering a telescope from an astronomical object is parallel, the distinction in practice does not matter."

I don't want to get too picky with you guys but I need to make a correction here: the distinction does indeed matter. The entrance pupil is the image of the stop in object space. For a Newtonian (or simple refractor,) the stop is at the objective (the primary mirror surface or the lens) and the entrance pupil lies in the same location--not at infinity. Remember, the chief ray (which defines the maximum field angle) passes through the center of the stop and the pupils (both entrance and exit pupils). Telecentric systems have the stop imaged at infinity so that the chief ray is parallel to the optical axis. Simple telescopes are not telecentric. The exit pupil, on the other hand, is the image of the stop in image space. If you forget about an eyepiece and consider the intensity distribution in the focal plane, these calculations simply look at the effect on PSF for binary apodization of the exit pupil--assuming no wavefront errors.

One other thing to understand about these type of FFT calculation is that the results are only an approximation of what you will actually realize in an optical system. The results are close but there are aliasing and leakage effects (energy leaks from one Fourier period into surrounding periods) that cause small errors. If you know what to look for, you can see a lot of this stuff in these results (mainly because of the low sampling rate.) In order to show what these patterns will do as you add a lot of defocus, you need to use a near-field, Fresnel model. Far field computations can accurately model only a few waves (<5-10 ?) of defocus.

John


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jbalsam
sage
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Reged: 07/06/12

Loc: Darnestown, MD
Re: Fraunhofer Diffraction - A Primer new [Re: jhayes_tucson]
      #5712879 - 03/04/13 04:42 PM

That explanation seems to agree with my experimental results. Thanks for adding to my understanding.

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