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Are short imaging scopes better for binoviewing?

Started by nmunk, 06/15/2010 09:28PM
Posted 06/15/2010 09:28PM Opening Post
Hello all,

I am told that some people shorten their refractor OTAs to achieve focus...

Would a short imaging APO OTA like the Televue 102iis or the Takahashi FSQ85 be a good choice for binoviewing with a 45 degree erecting prism?

I believe both are designed to bring the focuser closer to the objective than in the standard models for use with long imaging "trains" and come w/ extensions for visual observing.

It would be great to be able to binoview w/ a 45 degree erecting prism w/o having to use a 2x barlow for a wider FOV.

Thanks & clear skies,

Posted 06/16/2010 12:42AM #1
I know some people who have a Televue 102is and a Burgess 24. I am not sure if they have a 45 degree diagonal, but I will ask them to try to focus their Burgess and see how much travel they have left using an 1 1/4" diagonal. The Burgess is one of the Chinese binoviewers with a 101mm light path.

I have always been a fan of scope with enough in-travel that they do not need an OCS/OCA or barlow to focus. I have a Meade SN-8 and a Meade SN-10 that I installed low profile moonlite focusers on to achieve this.

My TAK FS-128 can focus one of the short light path Chinese binoviewers without needing any help. I have even gotten my Denk and my Siebert Blacknight to focus in it with the right eye-pieces. My Nephew's Televue proved a bit too much (130mm light path) for even it to focus. This was also done with a 1 1/4 Lumibrite 90 degree diagonal.

The 210mm refractor that I am building will have a shortened tube capable of focusing my Siebert 2" binoviewers. It will also have a screw on extension made for the length of the 2" binoviewer light path for normal use.

Don Durbin
Posted 06/17/2010 04:16AM #2
I was wrong my friends have the Televue 101is f/5.6.

We could not reach focus on the moon using just a 1 1/4" diagonal and a Burgess 24 (101mm light path) binoviewer. It looked to me like we needed another 10mm or 15mm of in-travel. We had to use an OCA.

Don Durbin
Posted 09/12/2010 09:15PM #3
At the far end are scopes with a full 8" - 9" of infocus, permitting a big 2" diagonal and a binoviewer with a 4" optical path length. I think my TEC is like that, and I know my TMB 100/8 was like that.

Below that you can go with the shorter prism diagonals. I don't know anything abut 45 deg prisms or their path lengths. The Baader 2" prism has a shorter path length than full diagonals do.

Posted 09/13/2010 05:35AM #4
Binoviewer is going to need about 5.5 to 6 inches of back focus -- that
is the distance from the focuser at its inner most point to the focal
plane. Maybe 7 to 8 inches if you use a 90 degree diagonal (1.25 inch
or T type). Thats a lot of backfocus even for an imaging scope.
I don't think you can assume that a scope designed for imaging will
necessarily have enough backfocus for binoviewing. You really need
to check your binoviewer and then evaluate the scope specs.

There are lots of tricks to making binos work. Some eyepieces need
less backfocus -- that will help. A prism diagonal will help
(vs. a mirror model). You can pickup a few mm of backfocus by adding
clear filters or glass in the optical path. Weak Barlows help
-- 1.2, 1.4, 1.6x, etc. -- doesn't have to be 2x.
Posted 09/22/2010 03:12PM #5
Thanks for your help everybody.

It turned out with more research that indeed the 102iis(not the FSQ85 however) was designed with 1x binoviewing in mind. There are several other OTAs out there now with this as part of thier design which does involve a shorter tube length to accommodate the longer path length created by the binoviewer.

I was able to reach focus in my new 102iis(which I also find great for imaging) with all my EPs in a celestron binoviewer in a televue 60deg erecting prism which is reversed left to right but I can live with that.

Thanks again & clear skies,