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Binoviewer quandry

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#1 choran



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Posted 31 January 2013 - 11:40 AM

Thought I'd ask the experts up here the following:
Best bioviewer for Televue 85 and Televue NP101. Looking for great views with a minimum of hassle. These are my only scopes at the moment, but may add a Dob in the next year or so. My research leaves my head spinning. Right now, the only viewer I can say for certain will come to focus in my scopes is the Televue unit.

Thinking of a Binotron, but no guarantee it will focus without mucho fiddling.
Looking for relative simplicity of use, painless diopter adjustment, and wide fields. All comments welcome!

#2 Eddgie



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Posted 31 January 2013 - 01:10 PM

Binoviewer is indeed one of the most difficult buying decisions you can make.

Here is the deal. The Televue Binoviewer will work with your scope because it is shipped with a 2X amplifier, which is basically a Barlow lens.

It is unlikely that your scope will reach focus without this.

A 2x amplifier (OCS as Denkmeier calls them, GPC as Baader calls them) will basically allow any telescope that can reach focus when using a 2" diagonal to reach focus with a binoviewer.

But... At 2x. So, if you use a 32mm Plossl, it will be like using a 16mm eyepeice.

Again, just about everyone sells a 2x barlow for thier binoviewer, so this means that if you are happy with 16mm as your lowest power eyepiece, just about any binoviewer will do (32mm eyepecices x2, or you could use a 40mm Plossl for 20x, but with a small apparent field. Not bad. I use this myself).

Here is the deal though. It may be possible to use less Barlow with a smaller apeture bino and get the same true field.

For example, I used the Baader Maxbright with the Baader T2 diagonal with a 1.5x GPC on my 6" APO. This combination allowed me to reach focus with a few millimeters to spare using a 19mm Panopic.

When I went to the Mark V (about the same light path as the Televue), I could no longer reach focus with the 1.5x GPC, so I would have had to go to a 2x GPC. In other words, I get a bigger aperture bino, but it has to work at higher power!!!! Bummer!
This was possible because the Baader Maxbright/T2 had the shoretest light path . Nothing is shorter.

And the shorter the light path, the less barlow power you need.

What this means is that you can buy a Maxbright/T2, and even though the aperture is not as large as the Televue or Mark V, because you can usually use a lower power Barlow/GPC, you can still get the same size true field, and for a lot less money!!!

And this is what makes binoviewers so frustrating in refractors. Unless you are willing to cut the tube, even using a 1.25" diagonal (which is not a very good way to mount a binoviewer), you still may need a 1.5x or 2x barlow to be able to reach focus.

Here is my advice, and if you are serious about buying a binoviewer, I stronly encourage you to act on it.

Find out the back focus of your scope.

Do this by putting the moon in the field of view, and use a card to project the moon on to the card.

With the focuser fully racked in, measure the distance from the back of the focuser to the card.

This is your back focus.

Now, you can total up the light path from the various units you anticipate using and there, you can figure out how much GPC you will likely need.

Alpine Astro has this info for the Baader produces.

For example, the T2 diagonal has a 38mm light path, and the Baader Maxbright has a 110mm light path. This means that together, you need 148mm of back focus as a minumum, and better would be to add 10mm to that.

Now the Denkmeier has I think a 128mm light path (or something like that) so as you can see, that by itself is 18mm longer than the Maxbright/T2.

The Televue (which I think is about 120mm) will also work with the Baader T2 diagonal, so this configuration would give you about 168mm (10mm added for safety reasons).

As you can see, if your scope reaches focus with the Baader, but not with the other units unless you use a GPC, you could actually get just as wide of a field using the Baader.

You can download the Baader Maxbright manual from Alpine Astro and this will give you an idea of how differet Baader products work, but more importantly, it will help you understand how whatever solution you use will have to be configured (barlow or GPC power).

Televue offers a connector to allow the use of the Baader Prism, and Seibert has their own Binoviewer diagonal steup now.

This is because it is super important to keep the light path as short as possible so that you can use as little GPC as possible.

Unless... you don't care about low power observing. In this case, any binoviewer made will work with just about any refractor that will reach focus when a 2" diagonal is used as long as you use an OCS.

But if you dont' care about having wider fields, then once again, you don't have to worry about the size prism.

I have done the math on half a dozen refractors comparing the Denkmeier/Mark V/Televue to the Maxbright.

In every case, because the Maxbright/T2 offered the shortest light path, it allowed all of the scopes to get as big a true field using the Maxbright/T2 as the bigger aperture binos, because they usually required a higher power barlow to reach focus, negating the advantage of the bigger prism. In other words, if you need an OCS to reach focus, bigger is not at all always better.

For refractors that can't reach focus without some kind of barlow, chances are you will get the widest field out of the Maxbright/T2.

Do your homework.

I went to a Mark V, but no longer use it in my refractor. Once again, I could reach focus using the Maxbright/T2 and 1.5x, but I would need 2x to reach focus with the Mark V.

This would bascially make the focal lenght 2400mm, and guess what... I can get a bigger true field out of my EdgeHD when used with the Mark V!

So, what is the point of binoviewing the 6" APO if I can get a bigger, brighter field out of the EddgeHD?

You don't need barlows for SCTs, so there, the bigger aperture prism is indeed important.

But for refractors, do your homework. The Maxbright/T2 is likely to give you as big a field as a Televue if you have to go one step up in OCS to reach focus.

Let me know if I can help when you have done your homework on back focus.

#3 choran



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Posted 31 January 2013 - 03:47 PM

If you would, give me a little more detail on the moon test, you lost me.
I cannot picture what I need to do but am willing to try! LOL

#4 choran



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Posted 31 January 2013 - 04:13 PM

Re: back focus Do I do this with the diagonal in or out, or does it make any difference? If I leave it in, I assume I move the card until the moon is in focus, then measure from the top of the diagonal. if I do it with the diagonal OUT, I do the same thing, bring the moon into focus with the focuser all the way in, and measure distance from card to rear of focuser?

#5 johnnyha


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Posted 31 January 2013 - 06:16 PM

The Binotron 27 will come to focus at all three power settings in your Televues with a 2" Everbrite diagonal, you just need a part SR to stick inside the diagonal. It's a special barlow that places the lens down inside the nose of the diagonal. Talk to Russ at Denkmeier he can set you up, I'd jump on that new Binotron!

#6 Eddgie



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Posted 01 February 2013 - 09:52 AM

Take the diagonal out.

Point at the moon.

Rack the focuser all the way in.

Put your card at the rear and move it back until the moon is in focus.

The distance from the back of the fully racked in focuser tube to the card is the back focus.

Or do what Johnny suggests and buy a Binotron.

But if you can't afford that, you can get a complete Maxbright/T2 system for about $450.

To get the right GPC though, you will need to know your back focus.

Good luck.

#7 Jim Romanski

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Posted 01 February 2013 - 09:09 PM


I have an Earthwin binoviewer that I used with both the NP101 and the NP127. If you go to the Earthwin site they have information on how to use it with a scope like yours. What I had to do is to unscrew the front piece of the binoviewer which is a lens assembly on a two inch threaded barrel. Then you screw this on to the front of your Televue diagonal (which is threaded). You stick this end into the telescope then put the remaining part of the binoviewer on the other end of the diagonal. Then I was able to use the 3 different power settings. I believe that the denkmeyer works the same way.

I have a pair of 24mm Panoptics and a pair of 10mm Radians that I use in the binoviewer.

I have mixed feeling about binoviewers. I really like big binoculars but I find that binoviewers are a bit of a compromise. You can't really have low power views unless you have a telescope like a TEC 140 whith a removable tail piece that lets you do away with the optical magnifying lens. I enjoy viewing with it and at star parties people really like looking through the NP127 and binoviewer. I find the loss of light in 4" scope to be pretty significant. You still get great views of planets and the moon but after that it's only bright DSO that are worth looking at.

#8 rguasto


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Posted 01 February 2013 - 09:30 PM

You still get great views of planets and the moon but after that it's only bright DSO that are worth looking at.

Couldn't agree more

#9 choran



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Posted 01 February 2013 - 11:55 PM

Thanks for the advice all! It is greatly appreciated. I'll continue my research. Waiting for the clouds to part so I can try the moon test, for informational purposes if nothing else.
When i was trying out scopes, I looked through a televue binoviewer through a televue scope, using 24 panoptics, and I was pretty happy with the view. My wife liked it a bunch more than single eyed viewing. I am aware of the limitations, though. Will mull it all over. Would be something to see, I'll bet, in your 17 inch dob, Jim! That one has light to spare, I'll bet! Would love to have a look through one of those!!

#10 BCNGreyCat


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Posted 02 February 2013 - 01:57 PM

I have a TV85 and a TV102. When using the Baader mark V binoviewer, I cannot reach focus with the 1.25x GPC. The 1.7x GPC can barely reach focus when the focuser draw tube all the way moved in for day light observing (e.g. birding). But 1.7x GPC is not reaching focus for infinity distance neither. You will need the 2.6x GPC for stars.

So I think you need either the Baader Mark V plus 2.6x GPC, or the Televue Bino Viewer with 2.0x amplifier. The Televue bino should definitely work with TV85/NP101

I don't have the Denk bino. But I guess you will need a 2x amplifier/barlow to reach focus as well.

#11 choran



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Posted 02 February 2013 - 10:36 PM

Than you as well for your very kind advice.

#12 Stephen S

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Posted 03 February 2013 - 01:02 AM

I have a Siebert Black Night Binoviewer with a 1.3x, 2x and 2.7x OCA. The set up works great with my NP101. The instructions on he Siebert website are incredibly helpful. Once I followed the instructions, everything fell into place.

Just got this,set up recently and have not had a ton of time to fiddle with it. That said, everything has worked seamlessly thus far. Best of luck.

#13 urassner


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Posted 03 February 2013 - 10:48 AM

I have a Denk II system with power switch and a TV Genesis SDF, which I think has very similar specs as the NP-101. I use the OCS from Denkmeier to reach focus. As mentioned above, you screw the OCS into the threaded front end of the diagonal to adjust for the additional back focus required. I love the power switch setup, which gives you 3 magnifications with one set of eyepieces. Denkmeier also offers a higher power OCS which gives you higher magnification, but did require an extension tubing before the diagonal to come into focus. Denkmeier has a nice glossary document (https://www.denkmeie...df/Glossary.pdf) that puts the required backfocus by the Denk system at 114.5 mm + some additional to accommodate for the different eyepieces. So, depending how much back focus your scope has it would need to be shortened to allow for the Denk system, the diagonal and some extra to allow for different eyepieces to come to focus. Depending on your focuser, you would need an extension if you were to use it without the binoviewer setup.

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