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diffraction limited bvers

Started by cysmarc, 04/23/2007 06:48AM
Posted 04/23/2007 06:48AM Opening Post
Hi,
I've read somewhere (don't remember where) That no binocs (prisms) are diffraction limited. Is this true? and if so why is everyone who is into bving worried about the telescope being diffraction limited when the bv will throw it off? Better yet could the ptv of a bv be say 1/3 overcorrected be matched with a scope 1/3 wave undercorrected to get a null scope?
Clear Skies

Posted 04/23/2007 09:34AM | Edited 04/23/2007 02:06PM #1
There can be two types of issues -- quality of the prisms,
and spherical aberration in the prisms. Both will generally
be very sensitive to the F-ratio of the light beam going
through the prisms. Perhaps the best way to improve the
optical performance is to place a Barlow lens in front of
the bino (i.e. between the OTA and bino) and use moderate
focal length eyepieces, rather than short focal length
eyepieces.

You could devise a scheme to under-correct the telescope
objective to match the bino. But this then makes it
difficult to use without the bino.
Posted 04/23/2007 10:48AM #2
>>I've read somewhere (don't remember where) That no binocs (prisms) are diffraction limited. Is this true?>>

You can also read somewhere that the sky is really yellow and the earth does not turn on its axis - such is the internet.

Most Bviewers were adapted from microscopes. Microscopes have long F-ratio objectives, which means that the light beams are extremely narrow compared to those of a telescope objective. Narrow light beams means the prisms do not need to be very accurate.

The Baader MarkV Binoviewer was designed by Zeiss specifically to be used with their fluorite triplet refractors, which have focal ratios from F6 to F10. The prisms are all very accurate and the binoviewer is totally diffraction limited at F6. The viewer has a built-in a compensating lens which I designed for them to be used with all non-Zeiss lenses (the Zeiss lenses were corrected specifically for use with prism diagonals and prism binoviewers). This compensating lens can be removed for wide field low power observing, but should be attached for high power planetary (it adds 1.25x to the image). The downside is the expense of such a viewer - it just cannot be made cheaply and have high quality optical surfaces.

To get diffraction limited performance out of less expensive binoviewers with fast scopes, you would probably want to use a Barlow ahead of the viewer. In any case, it depends on how observant you are. I have an older model Binoviewer originally sold by Celestron that shows quite noticeable optical defects (astigmatism and color error), and yet when I demonstrated this viewer to amateurs at telescope meets, most cannot see the defects. This tells me that most people are not that sensitive to optical errors that are quite gross and obvious to myself. They are enjoying the view through the bino, and that is most important.

Rolando
Posted 04/24/2007 07:07AM #3
Itz, my guess is that my Stellarvue BV1's have prisms polished to 1/4 wave at best, certainly not to the quality of my SV102ABV's Russian made objective. The BV1 also has standard coatings. I feel that using two eyes instead of one can help compensate for any defects. I just made an observing post in the Stellaruve forum here, and as you can see I was able to resolve the tiny crater Collins, 1.2 miles in diameter, with my SV102ABV and binoviewers.

David E