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Long fl orthos

Started by dcoyle, 09/07/2006 06:21PM
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Posted 09/07/2006 06:21PM Opening Post
dan coyle
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Why are there no (few) ortho designs above 25mm fl?

What stops working so well? I ask cause as much as I love my 9,12 an 18 orthos, the 25 is not as easy to use, for me.

What are the optical problems of long fl's in orthos?
Thanks
Dan
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Posted 09/07/2006 06:31PM #1
George Kiger
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I don't think optical issues are the reason.

The ortho's strength is a well corrected field, while its weakness is medium-to-narrow field. For low powers the highly corrected field is somewhat wasted, and most people prefer wider apparent fields for their low power eyepieces.
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Posted 09/07/2006 06:33PM #2
William Paolini
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Being no expert on the history of EPs and marketing, if I had to guess it would probably be because of the afov. Why make a 25mm 40afov when you can get 50+ deg with nearly as good correction from the Plossl design? And then it's hard for an Orthos at 40 deg afov to compete against a quality 25mm Erfle with 60 deg afov.

I would hazzard that is has more to do with marketing and user preference (i.e. market forces) rather than any optical issue.
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Posted 09/07/2006 09:05PM #3
Vahe Sahakian
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There probably is a very low demand for long F ratio Orthoscopics, to most Ortho's are eyepieces for planets and to get a reasonable magnification with your typical fast telescopes that are so popular these days there is very limited use for 25mm and longer Orthos.
I use a pair of 34mm ZAO's in a binoviewer which produces 185x with my TEC Mak, enough power to be used as a planetary eyepiece, has a very narrow field but is a superb performer.

Vahe
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Posted 09/07/2006 09:50PM #4
Jon Isaacs
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Why are there no (few) ortho designs above 25mm fl?

What stops working so well? I ask cause as much as I love my 9,12 an 18 orthos, the 25 is not as easy to use, for me.

What are the optical problems of long fl's in orthos?
Thanks
Dan
----

Probably cost, weight and the fact that a Kellner does a reasonable job at this focal length...

jon
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Posted 09/08/2006 12:48AM #5
Larry Thaxton
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I recently picked up a Takahashi 40mm Ortho and other than the need for an adapter, it is an excellent performer and sharp as heck. A 40mm Pentax SMC Kellner suffers the same adapter issue but also yields excellent optical performance in F/8 and above scopes. Both are very nice eyepieces.
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