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Very useful post. That's exactly what I was thinking about: using a focal reducer/corrector for visual to eliminate field curvature and coma and make a standard SCT almost like an EdgeHD for visual.


Uncle Rod,what do you think about/have experienced with these 2 reducers/correctors? Are they better than Celestron 0.63x for visual?


http://www.teleskop-express.de/shop/product_info.php/language/en/info/p3291_2--Korrektor-und-Reducer-0-8x-fuer-Schmidt-Cassegrains.html

http://starizona.com/acb/Starizona-SCT-Corrector-P3230C0.aspx


Both reducers/corectors pretend to eliminate field curvature and coma for standard SCTs.

I use the Alan Gee for visual most of the time with a Baader T2 diagonal with 35mm clear aperture which accepts even 2" eyepieces.

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I still find the original Celestron 6.3 r/c to be the most efficacious for _me_.

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Sorry, Uncle Rod; I know only enough to see that I do not understand. Please could you clarify? I think you're saying that, with a focal reducer that weighs very little, one can then achieve the desired result with a smaller, shorter, less expensive eyepiece.

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Long focal length eyepieces are often a pain to use, many having too much eyerelief, like the old TV 55mm Plossl. But the bottom line is that the image with the r/c at the same magnication will always look better because of the field flattening.

NO. The Crayford may put the eyepiece far enough back that it will not achieve focus with the r/c.

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Very useful post. That's exactly what I was thinking about: using a focal reducer/corrector for visual to eliminate field curvature and coma and make a standard SCT almost like an EdgeHD for visual.


Uncle Rod,what do you think about/have experienced with these 2 reducers/correctors? Are they better than Celestron 0.63x for visual?


http://www.teleskop-express.de/shop/product_info.php/language/en/info/p3291_2--Korrektor-und-Reducer-0-8x-fuer-Schmidt-Cassegrains.html

http://starizona.com/acb/Starizona-SCT-Corrector-P3230C0.aspx


Both reducers/corectors pretend to eliminate field curvature and coma for standard SCTs.

I use the Alan Gee for visual most of the time with a Baader T2 diagonal with 35mm clear aperture which accepts even 2" eyepieces.

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I still find the original Celestron 6.3 r/c to be the most efficacious for _me_.

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I suppose you haven't had the occasion to try those 2 reducers yet. True?

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I've been using the Celestron .63 for some time now with my C8... Markarian's Chain was particularly pleasant with the 17mm Nagler type 4.

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...in order that a focal reducer not cause bad vignetting, it should have a clear aperture *at least* 50% larger than the field stop diameter, with double or more being better. The actual minimum size depends on the degree of reduction desired, and to some extent the reducer's own focal length. The greater the reduction, and the longer the FR focal length, the larger the FR lens diameter must be.

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The Starizona? No. I have used the other one, or one very similar to it.

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For those who say you can not use 2" eyepieces with a focal reducer, just study this diagonal and see that you'll be able to use eyepieces up to 2" 30mm SWA: http://www.alpineastro.com/Star_Diagonals/t-2_diagonal_configs/t2_diagonal_configs.htm. That's what I'll do soon.

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Given the large apertures of these diagonals, it is possible to configure these diagonals for use with 2" eyepieces (see 2" configuration below).  Some vignetting may be noticeable on the largest true-field 2" eyepieces, though some users have even used 30mm Naglers with good results. 

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I see.

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To be honest, you should ask someone else. For visual use the r/c works great, but frankly, I don't spend my time worrying about the field edge, either. I am pretty happy with the Denkmeier Starsweeper, too, and it does not claim to be a flattener.

For imaging? The Celestron f/6.3 r/c was designed by a man who knew/knows what he is doing, and it worked great in the 25mm film days. It is even better with the APS sized chips I use today. I mean _Jim Riffle_ what more of a pedigree to you need?

hat's my answer that's worked for nearly twenty years for me. I've tried quite a few different reducers, and keep coming back Jim Riffle's creation.

Edited by rmollise (04/24/12 04:06 PM)

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