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Prism or Mirror Diagonal in Bino-Ready Refractor

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#1 DSO2

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Posted 10 March 2013 - 01:46 PM

I'd like to use the Baader Maxbright binoviewer with the bino-ready Stellarvue SV110ed refractor. I'm considering using Stellarvue's 2" dielectric mirror diagonal with the telescope's extension removed and no glasspath compensator to have maximum field size (recommended by Stellarvue on its forum). With the extension attached, I can use the mirror diagonal and a glasspath compensator for viewing the moon and planets.

However, I know that Baader recommends use of its prism star diagonal with the Maxbright bino.

Which diagonal do you recommend in both instances (extension removed or attached). I only want to use one diagonal: I don't want to have to switch out a mirror star diagonal for a prism star diagonal depending on whether or not I'm using the telescope in bino-ready mode.

If you recommend the prism star diagonal, should I get the Maxbright 32mm prism star diagonal, or the more expensive Baader/Zeiss prism diagonal?

Thanks!

#2 Mark9473

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Posted 10 March 2013 - 03:05 PM

I am pretty certain that Baader recommend the T2 prism diagonal only for reasons of optical path length, which I assume would not be a concern for you.

#3 Eddgie

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Posted 10 March 2013 - 03:48 PM

There has been over the years an on-going dialog about the benefits of using a prism in refractors vs using a mirror, but I think that this dialog says that in fast refractors, a mirror is a better choice.

And when you think about it, Astro-Physics that sells the Mark V in the US, ships it with a mirror diagonal. If there was some meaningful reason for shipping it with a prism, one would think that they would recommend it for use with their scopes.

But they don't. And my guess is because it doesn't matter.

The primary benefit of the Prism for most of use is the short light path.

You don't need that, so likely no advantage for you.

#4 Tamiji Homma

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Posted 12 March 2013 - 10:24 AM

And when you think about it, Astro-Physics that sells the Mark V in the US, ships it with a mirror diagonal. If there was some meaningful reason for shipping it with a prism, one would think that they would recommend it for use with their scopes.



They (Astro Physics) used to sell Baader T2 Prism Star Diagonal with MarkV. I think they switched to Baader T2 Mirror Diagonal recently (a year or two?). I don't know why but I guess it is due to availability of high quality prism.

I see very little difference between Prism and Mirror diagonal with f/6.3 or slower scope.

I see a little yellow casted view with prism diagonal when scope is f/5.5 or faster.

Tammy

#5 Eddgie

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Posted 12 March 2013 - 12:05 PM

That has been my experience. Only when the refractor is very fast does there seem to be an issue.

Otherwise, if AP thought that there was an issue, my guess is that they would have stayed with the prism.

Alpine Astro still sells the T2-01B (Zeiss prism) and even has them in stock, so APs packaing might reflect a belief that the mirror is a better choice for use with their scopes, but I find it difficult to see a difference between a good prism and a good mirror until the scope gets rather fast (f/7 or faster).






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