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hyperstar for celestron edge hd 8 inch

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#1 Binsky3434

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Posted 05 November 2014 - 05:25 PM

Hello, i recently purchased a celestron edge hd 8 inch telescope on a cgem mount. I have had no complaints at all and im interested in the f/2 otional system but am unsure of purchasing it for a few reason. The first reason is i have a Canon 60Da ( which is great ) for a camera that i would think has to large of a surface area in front of the main objective. I was wondering if it is to big and how the obstruction would effect the image. The price of the Hyperstar is in the range of $900, which is a lot to spend only to find out that it is not suitable for the set up you have. Any comments and tips would be greatly appreciated.



#2 jhayes_tucson

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Posted 05 November 2014 - 06:49 PM

First, the Hyperstar is a fantastic way to grow the capabilities of your scope.   You will block a big part of the aperture with a 60Da, but I bet that it will still work pretty well.  The big problem will be the diffraction spikes.  Because of the small aperture, you are going to have a hard time masking the sharp edges.  Another option is to look into a OSC CCD with a round form factor.  ATIK makes a few nice looking cameras that would work well on a C8.  Lastly, I'd suggest that you call Dean at Starizona to discuss it with him.  His site lists the Astrovid Stellacam, Atik 4-Series, Mallincam, and Starlight Xpress (11mm or smaller) cameras as being 8" Hyperstar compatible.  He may have some additional advice.  He is also sure to point out that the current Hyperstar on the ISS is an 9.25" Edge mated to a Canon 7D.  That camera is a little bit bigger than the 60Da.  Good luck with it...

 

John



#3 Mike I. Jones

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Posted 06 November 2014 - 07:19 PM

Oh man, I just couldn't leave this unresponded - I had wondered the same thing myself. 

 

Here's what the 60Da does to an 8" f/10 SCT, modeled as a perfect lens since I don't have the exact prescriptions for the SCT or Hyperstar.  The camera outline is accurate to better than a millimeter, modeled as a user-defined obscuration in Zemax.  Notice that it's centered on the camera optical axis, which is offset from the camera body as indicated.  I made a guess at the 2.6" central obstruction diameter with the camera not in the path, but it doesn't change the answer much.  The main hits to having the 60Da in front of the 8" scope are (1) increased light blockage amounting to about 1 stop, and (2) increased diffraction scattering that drops the MTF values down quite a bit. 

 

This doesn't say don't do it, just don't expect image contrast and resolution to be as good as without the camera in the path.

 

Great question.

Mike

 

Attached Thumbnails

  • Canon 60Da MTF PSF and pupil with and without.jpg


#4 freestar8n

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Posted 07 November 2014 - 08:07 AM

I wouldn't use MTF here because deep sky imaging is far from diffraction limited in the first place. The real problem is more obvious - and that is how the camera completely defeats the purpose of the "fast" system the OP wants. The OP wants "f/2" - but with all that aperture blocked, the effective T-stop including transmission loss over a full 8" aperture may be T/4 or something. It amounts to a large and heavy system with the effective aperture and performance of a small and inexpensive refractor.

I would either spend the money on the hyperstar - along with a small camera as John suggests - or just get a good prime telephoto lens such as the canon 135 f/2. It is a very nice lens that would work for astro and normal daytime work. If you already have a modified camera - a lens like that would have many uses, whereas I don't think that camera is practical on an 8" hyperstar.

In fact - I didn't even think it was possible to use a dslr on 8" hyperstar without a custom adapter? Are you sure there is an adapter that would even work?

Another camera to consider is the ASI-120m or qhy5l-ii. They are small and inexpensive, and have small, 3.75um pixels that at least have a chance of capturing a small star spot without blocking light. Also, there are probably small sx cameras available used that would work pretty well - but with larger pixels.

Frank

#5 Tucker512

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Posted 07 November 2014 - 07:10 PM

Aside from the obstruction, a standard DSLR won't work with the 8" HyperStar because there is not enough backfocus.  It works fine on a 9.25" or larger scope, where the extra obstruction is less of any issue anyway.  A small CCD camera is certainly the ideal solution.

 

Scott



#6 Michael Miles

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Posted 12 November 2014 - 01:06 AM

Aside from the obstruction, a standard DSLR won't work with the 8" HyperStar because there is not enough backfocus.  It works fine on a 9.25" or larger scope, where the extra obstruction is less of any issue anyway.  A small CCD camera is certainly the ideal solution.

 

Scott

 

I use my Canon EOS-M on a standard C8, and there's enough back focus.  You'll have to get a custom camera adapter from Starizona to fit the EOS-M.  It's also a much smaller camera and has less obstruction (maybe Mike Jones might be inspired to do the diffraction calculation with this camera :) ).  I don't know how much the obstruction is versus a round camera, but it is much lighter since I don't have to haul a computer out to take pictures.  Here's my post with my custom adapters (you can use Starizona's version instead):

 

http://www.cloudynig...kenscope/page-4

 

Keep in mind that I'm still going through the process of getting the best collimation (when you look at the astrophotos) since I completely took the scope apart for my custom experiments.

 

Michael


Edited by Michael Miles, 12 November 2014 - 01:08 AM.



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