Long fl orthos

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

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.

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.

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

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

jon

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.