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Celestron 500mm F3.6 ("Comet Catcher")
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PostPosted: Thu Dec 20, 2012 9:21 am    Post subject: Reply with quote
sammo wrote:
There is a spherical primary mirror if the telescope has a schmidt corrector plate. If you have a parabolic mirror it's much easier to use a comma corrector only, doesn't make sense to have a big heavy expensive corrector in the front.

The Comet-Catcher might be an exception here. It's much shorter than other Schmidt-Newtons and even Makstuovs I've seen with that specifications - I think that's the reason that they need some "double"-correction.
The lens is only ~15x48cm while for example Lichtenknecker 500/3.5 is 18x70cm
I'm not 100% sure though. I read it somewhere that it has a parabolized and not a spheric mirror.


PostPosted: Thu Dec 20, 2012 10:51 am    Post subject: Reply with quote
All these telescopes are much shorter than they really need to be for maximum correction.

The *correct* distance of the Schmidt plate to the main mirror is two times the distance between the main mirror and focal point (the focal point is roughly at the point where the secondary mirror is mounted).

http://en.wikipedia.org/wiki/Schmidt_corrector_plate

http://www.telescope-optics.net/Schmidt-camera.htm

But in reality, the Schmidt plate is mounted roughly at the focal point of the main mirror. For perfect correction, the Schmidt plate would need to be *twice as far away* from the main mirror! You can imagine how much bigger and heavier the scopes would be if they made them the correct way.

That's why in most Schmidt Cassegrain or Schmidt Newton telescopes the correction is not really perfect.

The main optical error that remains in a Schmidt telescope is field curvature. I assume that is what we see in the photographic results from the Comet Catcher as well. Thomas


PostPosted: Tue Feb 26, 2013 8:48 pm    Post subject: Reply with quote
Little update
It's usable up to lets say ~3 meters for close-ups with tubes, at 1-2 meter it gets very soft. Unlike other mirror lenses I know it has a slighty less distracting bokeh, I guess due the very large aperture.
With Pentax Q (5.5x crop) instead of NEX (1.5x crop) results get slightly disappointing, similar, maybe a little better than these results by Jesito http://forum.mflenses.com/moonshots-with-the-pentax-q-t56951.html. It's crealy made for brightness and for very high resolutions.
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