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Old 30-04-2012, 10:02 AM
bratislav (Bratislav)
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bratislav is offline
 
Join Date: Aug 2006
Location: Europe
Posts: 235
Quote:
Originally Posted by Peter Ward View Post
I understand the modulation transfer function to only be a measure of frequency preservation....hence and not sure what you mean by saying a 50% obstructed telescope is like having a telescope with 1/2 wave of spherical error... hence.... I still don't agree with your analysis

That much spherical error looks bad visually and easily shows up in images. (I've had some clunkers in my time )

You are indeed correct about Roland's spec claims... but I did some extra research prior to getting the scope, and the Honders has spot diagrams well within the diffraction limit across a 60mm field (attached)

I'd be interested to see a ray trace of a 1/2 wave spherical error....but so far I've not manage to track any down.

There is no need to simulate 1/2 wave of spherical, as all modern raytrace programs will be able to easily show the results of 50% of obstruction.

I don't have access to Riccardi's variant used in AP, and can't be bothered to modify my existing designs based on Honders' original ideas, but have another similarly capable design handy (Companar at 12" f/3.3, 50% obstruction). See attached spot diagrams for spot diagrams from zero to 2 degrees off axis (full 70mm circle) and corresponding diffraction affected PSF plots on axis and at 2 degrees off axis (apologies for low quality gifs, don't have access to all my tools at the moment). For PSF diagrams, square dimension is ~18 microns; those diffraction rings are bright enough to swell the calculated 4 micron RMS into 15 micron blur for anything but faintest stars.

I've exchanged many emails with Claas Honders in late 90's and finally managed to convince him to get design into the open, but unfortunately his untimely death put stop to that. Busack now claims to be the earliest inventor of such designs, but without Claas to tell us when did HE come with such ideas, we will never know.
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Last edited by bratislav; 30-04-2012 at 10:42 AM.
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