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Webster Telescopes Forum • View topic - binoviewer questions
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 Post subject: binoviewer questions
PostPosted: Tue Jul 06, 2010 7:26 pm 
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Location: England
Hello,
This is my first post, waiting in the line for a D18/20 and keeping busy with a 10" dob (currently in pieces).....

Has anyone measured the typical focusser position for a classic binoviewer eyepiece on the D-series scopes eg 24pan, 19pan or 16nagler. So the focusser in-travel left when such an eyepiece is tested in mono mode (minus paracorr). Backfocus requirement varies considerably between binoviewer setups, one of which requires the focusser to be racked-in approx 45mm vs, mono viewing (borderline for many dobs).

Question 2: I thought that to avoid clipping an f/4 light cone, a binoviewer optical corrector needs to be 2" format with a wide field lens. So imagine a 1.25" optical corrector unit protruding down the focusser and towards the secondary mirror - this will intercept only part of a steep light cone if it hits significantly before the focal plane. But in practise ..... do you get dimming (i.e. not much use for galaxy hunting) or vignetting from that ?
Jon


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 Post subject: Re: binoviewer questions
PostPosted: Wed Jul 07, 2010 2:39 pm 
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The only binos we have tested in house have had 2" nose pieces (specifically the Denk II ).

Other brands of binos (often with 1.25" nose pieces) tend to not come to focus at all in a Newtonian telescope. They often need 4" or more of infocus, and I don't know of any commercial Newt that can provide that. You can use a barlow so they come to focus, but then the bino has 2x magnification, and that limits the usable eyepieces, objects and FOV.

No Paracorr is needed with the Denk binos, it has a built in corrector.


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 Post subject: Re: binoviewer questions
PostPosted: Wed Jul 07, 2010 7:26 pm 
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thanks Jim. I was curious how much backfocus you design into the scopes for binoviewer use. To get the DenkII to focus, do you need to extend the Newtonian spacer tube that fits between the PowerX Switch and OCS ? The Zeiss/ Baader MkV bino needs approx 1.5 cm more backfocus than that (i.e. Denk without extending the Newtonian spacer)


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 Post subject: Re: binoviewer questions
PostPosted: Sun Jul 11, 2010 6:55 pm 
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Hi, just to persist a bit on binoviewers and backfocus if I may. It is great you test your scopes with DenkIIs. I have one of these with the powerswitch and 2" OCS. Do you test with the Denk "middle Newtonian spacer" expanded ?? If you fully expand this hardly any extra focusser in-travel is needed, so most makes of Newtonian scopes should work. If you dont expand it, about 1.5" extra backfocus is needed.
I hear alot of praise for the Baader MkV binoviewer as possibly being the closest to perfection bino that exists, and am thinking of getting one, now the euro is weak. It was originally manufactured by Zeiss, before they lost interest in amateur astronomy. There is a drawback though- the backfocus requirement is more than the Denk, but not hugely so (unlike the binos mentioned in Jim's response). Perhaps 1" more max. If you design the Webster scopes so the DenkII only just comes into focus after expanding the Denk middle spacer, then MkV is a no-go. It wont focus.

Related to this, someone on the web commented that the truss length on your scopes is adjustable- but I cant see how that could be so. In the event of just needing a tad more backfocus, this would do it.

Thanks guys,
jon


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 Post subject: Re: binoviewer questions
PostPosted: Mon Jul 12, 2010 12:33 pm 
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In our fast scopes (like a f/3.6), the Newtonian spacer does not have to be extended on the Denk IIs .

I've never seen the Baader in person, but with the thousands of users on this forum, maybe someone will comment (or better yet, send us one to test).

Yes, our scope hardware gives you some room to adjust the length of the truss poles, so you could just lower the UTA an inch if you need to.


If you are certain on the MkVs, send them to us and we will simply optimize your scope to use them.

BTW: usually, the problem with Newts and Binoviewers is needing more INFOCUS (needing to be closer to the primary) than backfocus (needing to be farther from the primary). If the problem was backfocus, one could just buy a drawtube extender and the problem would be solved. I suspect we are talking about the same thing, but are just using different terminology on different sides of the Atlantic.


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 Post subject: Re: binoviewer questions
PostPosted: Mon Jul 12, 2010 6:28 pm 
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thanks Jim, firstly you are correct I was using the wrong terminology; we talk about the same thing ! By the sounds of it, your C series scopes might be fine for the MkV, possibly needing that extra adjustment on the truss length at worst. Thanks for offering to optimise my scope build if I post a MkV to you. That would be "the" sure-fire solution - we should talk when the build start is approaching for my D18/20 if that's OK.
Jon


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