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Short tube vs. Long focal length refractors

Started by hastapalla, 07/03/2010 09:24PM
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Posted 07/03/2010 09:24PM Opening Post
robin woolsey
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Hello! I am considering adding another refractor to my collection. I have an 80mm short tube Orion and I want something in the neighborhood of 120mm or larger. I want a good combo scope for planetary and some deep sky observing plus some astrophotography. My question is, do I go for long focal length/higher f/ratios or the short tube short focal length/lower f/ratios? I have budgeted $300 to $500 dollars for an OTA and rings only, no mount or accessories. Any recommendations?
hastapalla!
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Posted 07/03/2010 10:53PM #1
dan hilts
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A refractor in your price range is likely to be an achromat and if a short tube, will show some color error which will hamper good planetary views and photography. A camera has no tolerance for chromatic abberations in a telescope. You can get a 120mm at F8 that is a reasonable compromise and with a minus violet filter the CA can be somewhat controlled. If you can put up a little more cash you might be happier with a 100mm ED for photography but it is unlikely you would find one within your budget.

dan
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Posted 07/03/2010 11:31PM #2
Martin Willes
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Are you sure you don't want to add a diagonal and eyepiece with that 120mm OTA and rings for $300 or would that be asking to much? Seriously I think you'll have a hard time finding a refractor in that size for $300 but who knows. If you do it will most likely be a long focal length. Martin
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"In the beginner's mind there are many possibilities, but in the expert's there are few." Shunryu Suzuki[COLOR="Blue"][/COLOR]
Posted 07/04/2010 12:49AM #3
Jon Isaacs
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Hi

You could buy an ed100 which would be a reasonable compromise for the three tasks, I see these used for around 400.

A short tube 120 f/5 will have significantly more false color than your st80, not good for the planets or photography. The 120 f/8.3 will have less CA OK for the planets, too much for photography.

120 mm refractors for AP cost a few thousand.

Jon
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Posted 07/04/2010 12:50AM #4
Jon Isaacs
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Hi

You could buy an ed100 which would be a reasonable compromise for the three tasks, I see these used for around 400.

A short tube 120 f/5 will have significantly more false color than your st80, not good for the planets or photography. The 120 f/8.3 will have less CA OK for the planets, too much for photography.

120 mm refractors for AP cost a few thousand.

Jon
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Posted 07/04/2010 02:26AM #5
Dale Conrad
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If you add a fringe killer filter to the optical path the f8 instruments will perform pretty well both for visual and photography. Since most deep sky object are not rich in blue, CA is less of a problem for photography, you will still wind up with more blue around stars in the image. The 100ED scopes seen for sale may give you a little better image quality. Just my two cents worth, it's always a battle between the budget and what one would like. grin
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Posted 07/04/2010 04:10AM | Edited 07/04/2010 04:20AM #6
David Simons
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Hi Robin,

Curious why you want a refractor?

Your previous description sounded a lot like a C6 OTA. smile

Getting back on topic, I have a Burgess 127 f8 ( a good one) and although the focuser is marginal, the optics are killer.

http://www.astromart.com/forums/viewpost.asp?forum_post_id=280754&poll_id=&news_id=&page=2
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David Simons
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