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Why not FLP scopes?

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Old Tuesday 8th January 2019, 20:41   #1
lucznik
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Why not FLP scopes?

Its been awhile since Ive been on so; I apologize if this has been addressed but:

Why dont more birdwatchers use Folded Light Path (FLP) spotting scopes such as the Leupold Golden Rings, The Minox MD60 (or 80) Z, or perhaps the more budget friendly Bushnell Excursion or Legend T, or the Sightmark Latitude scopes?

These seem to offer eye relief that is way better than what is offered by conventional prismatic scopes which is very attractive to practically-blind suckers like me. They also use mirrors to reflect, rather than prisms to refract, light so; they should be less prone to problems with chromatic aberration. Nevertheless, they dont ever seem to make it on anyones list of high-quality/high performance options to suggest to prospective buyers.

Am I missing something about these scopes? Is there something inherently wrong with them that make them optically less-awesome than their prismatic siblings?
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Old Wednesday 9th January 2019, 12:29   #2
jring
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Hi,

a metal first surface mirror will eat up around 10% of the light plus it's tends to degrade with time... the degradation can be slowed by SiO2 deposition, but the light loss is there. Light loss can be lowered by using dielectric mirrors (for smaller flat mirrors at least) but those are expensive to make...

Also all the folded light path scopes mentioned actually use lenses. You could use a catadiopric scope like a Schmidt-Cassegrain or Maksutov-Cassegrain of course...

Joachim
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Old Wednesday 9th January 2019, 15:56   #3
henry link
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Hi lucznik,

Keep in mind that the two flat mirrors used for folding don't act as an erecting system. Most of these scopes seem to use the same lens erecting system/zoom eyepiece combination found in rifle scopes, but I think I recall seeing a Russian one that used a Schmidt prism for 45 image erecting. Just as in rifle scopes the result usually is very long eye relief combined with a very narrow AFOV of around 45 across the entire zoom range. The Minox and Leupold scopes appear to be identical, probably made by Kamakura Koki and I can't find any mention of ED glass. I haven't seen one of these, but I doubt that they are competitive with modern ED scopes of conventional design.

I think the most interesting one of these in recent years was the 72mm Zeiss/Hensoldt. Its AFOV was wider than usual at a little under 60, but it also didn't mention ED glass and it cost over $4000.

Henry

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Old Wednesday 9th January 2019, 16:12   #4
Binastro
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I haven't seen or used these more expensive folded refractor scopes, but the Yukon 6x-100x100 folded refractor that I have is optically good, but Yukon vary a lot in quality. It is very light weight and plastic and I doubt it would survive a drop.
I also have the smaller Yukon folded refractor spotting scopes but these are rather poor quality.

The folded refractor scopes may have f/6 objectives rather than f/5 so may perform better but have smaller fields.

I have a very good 30x50 Yukon folded refractor binocular, but the coatings are poor on most Yukon optics.

There was an 8 inch aperture folded refractor doing the rounds for many years in the U.K.

Wall's 30 inch aperture refractor had a folded path but was still huge.

There is also a British 6 inch folded refractor binocular someone made.

They may be difficult to align properly in big sizes, but there is a lot to be said for folded refractors.
There are also long focus folded path mirror scopes of various types, some exotic.

Some of the large aerial cameras have very long focal length folded optics up to about 34 inch aperture.
I cannot remember if the 144 inch focal length f/8 camera had folded optics or not. About 19 inch aperture and using 28 inch square film. Now used for measuring asteroid positions to 1/50th arcsecond accuracy. Maybe designed by Baker.
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