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Optics Design - Schmidt-Newtonian

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#1 Michael Miles

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Posted 11 October 2013 - 11:55 AM

Hi folks:

I was thinking about cannibalizing a C8 or Meade 8" Schmidt-Cas and converting it to a Schmidt- Newtonian. I've got a Canon T1i that I'd like to use at the fast focus of the main mirror. My idea was to remove the secondary, and replace it with a 45-degree Newtonian diagonal. I was also thinking about adding a hyperstar, and got the idea of using the Newtonian configuration because the t1i is too big to put in front of the light path.

Ideas? Oslo files - especially Schmidt-Cas 8" designs. Is it possible to do a cheaper version of the hyperstar? I've got a copy of that Rutten & Van Venrooij book with lots of telescope designs in it somewhere, but I've moved since I bought it, and it's buried in some box somewhere...

Thanks, Michael

#2 Geo31

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Posted 11 October 2013 - 02:03 PM

The problem you're likely to run into, at least with a Celestron (I'm not sure about Meade) is that the final corrections to the optical system are figured into the secondary. You will not likely get as good an image as you would from the SCT config as a result.

That said, there is a nice (although with some cosmetic wear) manual focus Canon 400mm f/2.8 in the refractors section of the classified. That would be around the same focal length, almost as fast, and probably better corrected than anything else you might find.

#3 Michael Miles

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Posted 11 October 2013 - 02:35 PM

Thanks George - I didn't see the canon - could you post a link? Or PM me if it's against the rules to post.

Thanks, Michael

#4 DAVIDG

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Posted 11 October 2013 - 02:42 PM

Mike,
The issue your going to have is coma with a f/2 primary because the corrector is located near the focal length of the primary vs having it located at the radius of curvature and also not having a flat focal plane. Both of these issues are corrected with the lens elements in the Hyperstar.
If you reposition the corrector you can ray trace it's position so coma is equal or slightly smaller then the diameter of pixels in your coma which will make it invisible to the CCD.

- Dave

#5 Geo31

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Posted 12 October 2013 - 07:27 PM

Michael, I assume it's been sold because I'm not seeing now either. Too bad because it was a great deal.

#6 Geo31

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Posted 13 October 2013 - 07:26 AM

Michael, it got moved to the astrophotography section of the classifieds. Alas, it's been sold.

#7 Michael Miles

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Posted 15 October 2013 - 05:04 PM

Thanks George, but I think I'll have more fun messing around with the optics of a C8 anyway since I like to build scopes more than I actually like using them. I mostly just use them to prove that my tools/instruments/techniques are working (not totally, but mostly).

Michael

#8 JohnH

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Posted 15 October 2013 - 05:59 PM

If anyone wants, I have 3 10" plate glass disks I bought indirectly from Bill Cook in Seattle. They would make decent correcter plates.

#9 Michael Miles

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Posted 30 October 2013 - 02:31 PM

OK, I just ordered a used Hyperstar, so let the surgery begin...

Dr. Frankenstein (aka Michael)

#10 Michael Miles

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Posted 09 November 2013 - 04:58 PM

I ran it through online Newt:

http://stellafane.or...b/newt-web.html

and got the following raytrace. The diagonal is 4.1 inches minor axis (offset is almost 0.5 inches!),and the inner field stops reduce the main mirror coverage to about 7.5 inches in diameter for a 55% obstruction. Rutten and van Venrooij mention flat field SCT's having an obstruction of 40%-60% for photographic work. Since that's what I'm planning, I think this will be acceptable. I bought the materials for the spider vane today,and will order the secondary holder and mirror when I get paid at the end of the month. I've figured out a way to mock up the optics without having to disassemble the tube ends. The big thing I'm worried about at this point is the primary baffle and focusing tube - I think they may be too long. I was thinking about removing/replacing the inner baffle tube, but we'll see. I'm concerned about not destroying any of the SCT parts to be able to get a usable SCT after I'm done fiddling if the experiments don't work.

Michael

Getting excited.

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