Posted 18 May 2010 - 06:26 PM
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.
Posted 19 May 2010 - 12:03 AM
Posted 19 May 2010 - 02:12 AM
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.
Posted 19 May 2010 - 06:22 AM
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.
Posted 19 May 2010 - 07:27 AM
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
Posted 19 May 2010 - 08:24 AM
Posted 19 May 2010 - 08:55 AM
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.
Posted 19 May 2010 - 09:24 AM
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
Posted 22 May 2010 - 08:27 AM
The article is in the March 2001 edition of Sky & Telescope on pages 120-123. The Folded Refractor: Quality Optics on a Budget.