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Folded refractors

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#1 GlenM

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Posted 18 May 2010 - 06:26 PM

I would like to find some information on folded refractors. I thought this was the best place to ask.

When a 6" f/12 Istar lens/cell came up for sale on www.ukastrobuysell.com I couldn't resist.

Having built the 6" f/10 version I certainly don't want to go any longer with the OTA. So I was thinking of folding the light path. I would like to mount this on a Newt style dob mount so I would like the focuser near the top of the tube.

I would very much appreciate your views on the pros and cons of such a project and any other ideas you may have.

Thank You.

#2 Old Will

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Posted 19 May 2010 - 12:03 AM

A three foot tube would certainly be an advantage. Optically it should be fine. You'll need a 3" optical flat to fold it once. 1/4 wave should also be fine. then of course you will need a diagonal mirror for the focuser similar to a newtonian. I suppose the biggest decision would be mounting it in a tube or box. A tube would need to be 8" in diameter to allow the focuser diagonal to not obstruct the light path. A tricky part would be designing and mounting baffles. An advantage of a box would be that you could have it put together with a side left off to allow easier installation of the baffles and collimation. And then use screws to put the side on, not gluing to allow access. I've thought that maybe the collimation could be done with a focuser mounted laser and puff smoke inside to trace the light path. A box would allow easier mounting of the altitude trunnions.

#3 plyscope

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Posted 19 May 2010 - 02:12 AM

Hi Glen

The late Ernie Pfannenschmidt wrote an article in Sky and Telescope some years ago. I have the issue at home and can give you some more details next week when I get back there. He used a D&G 5" f15 lens and very high quality optical flats to maintain the image quality.

Regards

#4 Wes James

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Posted 19 May 2010 - 06:22 AM

Glen-
Here's a good example of the type of box construction Old Will was referring to- Dave Groski's Schupmann. Shows how he traced the optical path on the sides of the box, baffle layouts- everything. The same principles can be applied to a folded refractor. Similar in optical layout concept.

#5 Alan French

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Posted 19 May 2010 - 07:27 AM

Henry Paul advocating folding a refractor "Newtonian style," and it always seemed a reasonable approach to me. It makes for a compact and much more easily mounted instrument.

One disadvantage to folding a refractor is that you are moving the entrance pupil closer to the ground and closer to the observer. I've always felt that having the lens way up in the air, off the ground, and well away from the heat radiating observer was an under-appreciated advantage of a long focal length refractor.

Clear skies, Alan

#6 cloudmagnet

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Posted 19 May 2010 - 08:24 AM

If anyone remembers and/or has a copy of Sam Brown's "All About Telescopes", look on page 129.

#7 Jeff Morgan

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Posted 19 May 2010 - 08:55 AM

Glen,

There have been some very good threads on this topic in the ATM forum within the last year (and further back). I did a project like this and posted my experience and opinions on it. A search through the old postings should turn it up.

Bottom line - it's a fun project, although questionable on an optical performance and economic basis. If you enjoy spending time in the workshop and can come up with an ergonomic arrangement, I would proceed.

#8 GlenM

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Posted 19 May 2010 - 09:24 AM

Thank you very much for the advice and links everyone. I will be doing quite a lot of reading on this.

I know this is a stupid qustion,but is there any way to reduce CA in a system like a folded refractor or do you always need a corrector. I told you it was a stupid question :foreheadslap:

#9 GlenM

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Posted 20 May 2010 - 10:12 AM

I thought not :grin:

#10 plyscope

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Posted 22 May 2010 - 08:27 AM

Hi Glen

The article is in the March 2001 edition of Sky & Telescope on pages 120-123. The Folded Refractor: Quality Optics on a Budget.

Regards

#11 GlenM

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Posted 22 May 2010 - 03:45 PM

Thanks Andy,

I'll see if I have it somwhere.






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