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Short tube refractors and binoviewers?

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#1 peweg9

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Posted 21 April 2021 - 09:26 PM

Although I currently observe with a SW StarTravel 102mm f/4.9, my favorite viewing is through one of several binoculars I own. I'd like to expand my viewing capabilities and am considering a binoviewer for this telescope. Being a short tube refractor I'm concerned this may be an issue, at least from what I've read. I have no experience at all with binoviewers, so I'm hoping other folks who have experience using binoviewers with short tube refractors will chime in with some advice. 

Thanks. 



#2 jprideaux

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Posted 21 April 2021 - 09:59 PM

One quick thing you can check.  With using your diagonal, put your eyepiece in and get focus on something at infinity.  Then see how much your focuser will let you shorten the drawtube moving the eyepiece towards the scope.  This is how much "back-focus" you have to play with.  The binoviewer used with the diagonal will take up some optical length that will need to be made up for with moving your focuser in to achieve focus again.   Some binoviewers will take up as much as 100mm of extra light distance which can be a challenge to make up.  There are a number of tricks to make it work if you don't have enough inward travel distance. 

 

1.  You can simple add a barlow.  Some binoviewers come with a screw-on barlow which  increases the focal-length of the scope and pushes the focal distance out so you can reach focus with no other changes.  The "price" you pay is that your instrument is now at a higher power.   If you like that higher power, then no problem.

 

2.  You can try to make-up for some of the light-path in other ways like purchasing special diagonals with shorter light-path and binoviewers designed to screw directly to the diagonal to save some of that precious distance.  People that do this are trying to give themselves low-power options.  Sometimes they can get a binoviewer to work with these approaches without a barlow or it might help them use a less powerful barlow.

 

3.  Some people have cut their refractor tube to remove some of their tube length so they could reach focus. 

 

4.  Some have used a linear binoviewer which is a different binoviewer design that does not take up any extra light distance so they can be used with scopes that otherwise would have problems.  There are some optical compromises with the linear BV to look into and decide whether they would be important to you.

 

I have a F5.5 refractor and cannot get infinity focus with my conventional binoviewer without a barlow.  I do use it for higher power with taking option 1 above (use a barlow).  I also have a linear binoviewer as well which I mainly use for low power. 

There a lot of posts in this forum about option 2 above as well.  Option 3 is considered drastic by most people but is technically possible.    


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#3 Usquebae

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Posted 22 April 2021 - 12:10 PM

Unless someone here is binoviewing with the same exact scope you have, you will probably need to explore the options listed by jprideaux.  Many BVs come with an OCS/barlow, so you should be able to use your scope with those, but achieving low and wide fields might be a challenge.

 

As for the general concept of BVing with a short refractor, I do it with the Orion 120 F/5 and love the views.  After a focuser upgrade to GSO crayford I can reach focus with Arcturus BV and the 30mm plossls that come with it.  I also use the Orion Linear Binoviewer (which does not alter the light path) with a pair of 15mm Luminos EPs.  Sky-sweeping is really fun with these configurations.


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#4 peweg9

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Posted 22 April 2021 - 03:26 PM

Thank you both. This is precisely the type of information I was looking for. 



#5 Rutilus

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Posted 23 April 2021 - 05:11 AM

One possible way around low power bino-viewing is to use a set of eyepieces from a pair of cheap/broken 

binoculars. I recently made a post in the refractor forum "Bino-viewing without the Barlow" regarding how I 

overcame this problem.  Since making that post I did an experiement by attaching my bino-viewers to the top

of my diagonal and this required the focus mount to be racked out another 30mm. I strongly suspect that my

set-up will work with any refractor telescope of f/5 and above.



#6 Maharg

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Posted 23 April 2021 - 07:08 AM

Along with the poster, I have no experience with binoviewers, keen though I am to enter this field. My two scopes are both short focus refractors: a TeleVue 101is (f/5.4) and an Explore Scientific Essentials 80mm triplet (f/6), which I intend using as my flight-friendly travel scope. Being able to use the same binoviewer on both scopes is an obvious must from a practical, cost point of view, but I am concerned about back-focus issues here. This being so, what brand of binoviewer might be the best option for me, given the two above scopes? I have read a good in-depth review on the Baader Maxbright II, but that's pretty much how far I've dipped my toes into the world of binoviewer brands/models/editions.

 

On Barlows: I'd rather avoid having to use one if possible, especially if I have a pre-purchase choice in the matter. Nevertheless, if forced by circumstances to use one anyway, does this mean (in the case of my Skywatcher 2x Barlow) that it could result in a doubling of magnification when used in conjunction with a binoviewer? If so the narrowing of the field in particular would compromise the great pleasure I have in conducting low-to-medium wide-field power sweeps across The Milky Way etc. . .  which for me is a major draw in wanting to purchase a binoviewer in the first place. So any advice or pointers would be extremely helpful here. Thanks in advance.    



#7 noisejammer

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Posted 23 April 2021 - 09:46 AM

The potential problems with fast scopes are two fold.

 

Firstly, (including a diagonal) most binoviewers require 145-200 mm of inward focuser travel to reach focus. This can be mitigated by installing a Barlow but this comes at the expense of the available field. .

 

Secondly, the fast optical cone is vignetted by the entrance pupil of the binoviewer which can constrain the available field.

 

Your options on fast scope are to either use one of the zero-optical-length binoviewers or install a Barlow. The former typically has a 17mm entrance pupil. With the TV101, this translates to a maximum field of 1.8°. If you go the route of the MB2, you will certainly need a Barlow and your visible field is unlikely to increase meaningfully. Be cautious that a prism is a poor choice for a diagonal at f/5.4. This means that even a relatively short set like the Mk V or MB2 are going to need about 160 mm of focus travel. I don't think the NP101is has this much focus travel available (I found 129mm) so you're almost certainly going to need the 1.7x Glass Path Corrector

 

This is almost as good as it gets - I specified a 115mm f/7 scope specifically for binoviewing and can achieve a true field of  2.0°. (In practice, I get slightly more than 2.2° because I tolerate some vignetting.) On reflection, a shorter focal length would lead to a wider field but the trade off is in image brightness.

 

So that pretty much is the limit of what's possible with binoviewers. If you really want a wider field, a binoscope is the way to go.

 

If you need better info, the first step is to measure how much focus travel you have.


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#8 Maharg

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Posted 25 April 2021 - 11:57 AM

Given that I have the good fortune of owning a Televue refractor, would all the above problems go away if I bought TV's own binoviewer - an expensive option, but one I would certainly consider were it to be doable? 

 

https://www.widescre...at-coupler.html



#9 noisejammer

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Posted 25 April 2021 - 02:39 PM

The short answer is no - a Televue binoviewer will not 'solve' your quest for maximum field of view. The reasons are that 1. the Binovue has quite a long optical path and 2. It doesn't have the widest prisms available.

 

You are limited by the available backfocus of your scope. The best possible field on your scope will be achieved using a Baader Mk V binoviewer, a BBHS T2 diagonal, a quick release and a 1.7x Glass Path Corrector, This will cost a lot but it will get you to between 1.75 and 2 degrees.

 

I strongly suspect that if you substitute any of the parts in this list you won't reach focus.


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#10 jprideaux

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Posted 25 April 2021 - 03:59 PM

Besides reaching focus, there are a number of ergonomic problems to solve with using binoviewers.  

 

If interested in ergonomics and the possibility of using a fork mount, the following thread I started has a lot of pictures of how I am currently using my binoviewers.

 

https://www.cloudyni...cular-shootout/

 

For getting low-power focus, I went the route of using the linear binoviewer.  

But I also have an inexpensive conventional binoviewer as well which I mainly use for high-power.

 

Interestingly, you have to "work" to get low-power with conventional binoviewers and you have to "work" to get high power with the linear binoviewer.    One trick I'm doing with he linear BV (when used with my refractors) is to use a .66 reducer to get better low-power.  


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#11 Maharg

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Posted 26 April 2021 - 02:52 PM

Thank you very much for your informed and extremely helpful remarks - all the more so given that I'm new to this field. One last question: are the issues cited above restricted to short-focus telescopes? In other words, would a more useful (and possibly cheaper) workaround be for me to acquire a longer focus telescope (f/7 & upwards?) and use this in conjunction with a suitable set of binoviewers, even if it does mean a reduction in the field of view? Or is this option dogged with issues as well? Again, thanks in advance.  



#12 noisejammer

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Posted 26 April 2021 - 04:13 PM

The problem is not in the focal length of the scope, it is in the available focus travel and your selection of a binoviewer with the largest prisms possible combined with the shortest optical length.

 

If the NP101is had around 150 mm of available travel, you'd be able to get close to 3°, albeit with some vignetting. If you don't use the Mk V recipe I referred to earlier, you wouldn't do as well.

 

You don't have to go crazy - consider my f/7 FLT110 scope - I replaced the stock focuser with a Moonlite to get more available backfocus so that it would work with my Denk II binoviewers. I sold it before I bought the Mk V but it would be capable of about 2.1°.

 

I mentioned earlier that I achieve about 2.2° - this is using an APM 115mm f/7 scope that I specified with a lot of back focus. If memory serves, it was something like 215mm - enough to use a Baader Clicklock on the focuser and reach focus using the Masuyama 35mm (50°) eyepieces. Less extreme eyepieces like the Tak 30mm LE still give me about 2° but with less vignetting. This was an expensive scope but it is extremely versatile.

 

If you do go the route of a bespoke instrument, drop me a PM and I'll try to assist.


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#13 Kisslija

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Posted 26 April 2021 - 11:03 PM

I have a William Optics binoviewer and use it with an Orion 120mm ST. You can certainly use a barlow, however if you want the widefield views, you can get an optical corrector that provides no magnification from Harry Siebert at Siebert Optics. I have one that threads onto my diagonal and it works great. His website is very confusing and if you're interested in one, it may be best to give him a call.


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#14 jprideaux

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Posted 27 April 2021 - 04:35 AM

I have a William Optics binoviewer and use it with an Orion 120mm ST. You can certainly use a barlow, however if you want the widefield views, you can get an optical corrector that provides no magnification from Harry Siebert at Siebert Optics. I have one that threads onto my diagonal and it works great. His website is very confusing and if you're interested in one, it may be best to give him a call.


I assume you are talking about the diagonal that threads into the binoviewer after removing the nose-piece from the binoviewer to save you around 25 or 30mm back-focus.

Edited by jprideaux, 27 April 2021 - 04:37 AM.


#15 Kisslija

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Posted 28 April 2021 - 09:41 AM

I assume you are talking about the diagonal that threads into the binoviewer after removing the nose-piece from the binoviewer to save you around 25 or 30mm back-focus.

Yes I am.



#16 jimandlaura26

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Posted 29 April 2021 - 12:37 AM

There is at least one exception to all the above, that is a "linear binoviewer" sold by Orion, Lunt and some others who rebrand the same basic hardware. It does not suffer from the back focus challenge noted above, because it uses a dielectric coated beam splitting mirror vice the usual prisms. So no extra optics need to be put in front of it to come to focus either. The method of adjusting interpupillary distance is a linear sliding motion not a bending motion (as in binoculars), which I find simpler to use too.

 

You can find one version here... https://www.telescop...es/p/130300.uts . The embedded video on this Orion page is useful.

 

I originally purchased it for daytime solar H-alpha viewing. But there's always a catch; the beam splitting mirror allows extra reflected daylight in from the sides. So that's very distracting for daytime use. Far less so at night. I have a Denkmeier binoviewer as well, but it is heavier than the lighter Lunt branded linear binoviewer (another advantage). Kind of a toss up which I prefer when all is said and done. I have used the linear binoviewer in my 80mm f/6 and 102mm f/7 app refractors that use 2" diagonals and it performs well in both. Very good for lunar and planetary usage. Eyepieces used are 25mm, 18mm and 12.5mm Orthoscopic (1.25"). Those are a good combination with this bino. Wider FOV modest EPs tend to have artifacts when used in a bino, but the 45-50 degree FOV Orthos perform very well.


Edited by jimandlaura26, 29 April 2021 - 12:48 AM.

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#17 jprideaux

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Posted 29 April 2021 - 05:20 AM

...
the beam splitting mirror allows extra reflected daylight in from the sides. So that's very distracting for daytime use. Far less so at night.
...
I have used the linear binoviewer in my 80mm f/6 and 102mm f/7 app refractors that use 2" diagonals and it performs well in both. Very good for lunar and planetary usage. Eyepieces used are 25mm, 18mm and 12.5mm Orthoscopic (1.25").
...


I’ve found that the bino-bandit helps a lot with reducing the problem of light getting in from the eyepiece-side and ending up reflected in the image with the linears.

I’m looking forward to trying some 18mm Baader orthoscopic eyepieces that I have on order (but unfortunately back ordered). I have had lots of problems with ghost images when using my linears when observing the moon. I’m hoping part of the problem may be that I’m not using the most appropriate eyepieces for the linear. At least with the eyepiece pair that I do have (the Williams optics stock 20mm 66 degree eyepieces that came with the WO binoviewer) I can see ghosting come and go with minute changes in eye position when used with the linear viewing the moon and I find it hard to get the ghosting to go away with both eyes at the same time.

I’m hopeful that the 18mm Baader orthoscopics will be a better match for the linears. So far, when I want to observe the moon with two eyes, I avoid my linears and use instead my WO BV and use the included Barlow to reach focus. I give-up the lower-power view with my WO BV. I typically use my 92mm F5.5 APO refractor.

#18 jimandlaura26

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Posted 30 April 2021 - 01:35 PM

I’ve found that the bino-bandit helps a lot with reducing the problem of light getting in from the eyepiece-side and ending up reflected in the image with the linears.

I’m looking forward to trying some 18mm Baader orthoscopic eyepieces that I have on order (but unfortunately back ordered). I have had lots of problems with ghost images when using my linears when observing the moon. I’m hoping part of the problem may be that I’m not using the most appropriate eyepieces for the linear. At least with the eyepiece pair that I do have (the Williams optics stock 20mm 66 degree eyepieces that came with the WO binoviewer) I can see ghosting come and go with minute changes in eye position when used with the linear viewing the moon and I find it hard to get the ghosting to go away with both eyes at the same time.

I’m hopeful that the 18mm Baader orthoscopics will be a better match for the linears. So far, when I want to observe the moon with two eyes, I avoid my linears and use instead my WO BV and use the included Barlow to reach focus. I give-up the lower-power view with my WO BV. I typically use my 92mm F5.5 APO refractor.

Thanks for the idea concerning bino-bandit. Tried cupping hands and eye guards with some success - kind of annoying to get consistent performance though. Do you have a recommendation concerning bino bandit - which brand looks to be a better solution in your experience?

 

Regarding 18mm Orthos, I have found them to be a sweet-spot for binos (in general)… eye relief, image merging, FOV, etc. The current pair I use are University HDs - which perform very well, but are now only available used. Recently, I obtained Baader (Classic Orthos) 18mm from Agena and Astro Hutech 18mm Abbe Orthos in single eyepieces for Baader Q-Turret use. All perform very well and are in-stock when last checked. There are reviews of them in CN EP discussion section - see especially BillP.


Edited by jimandlaura26, 30 April 2021 - 02:18 PM.


#19 jprideaux

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Posted 30 April 2021 - 10:31 PM

Thanks for the idea concerning bino-bandit. Tried cupping hands and eye guards with some success - kind of annoying to get consistent performance though. Do you have a recommendation concerning bino bandit - which brand looks to be a better solution in your experience?

 

Regarding 18mm Orthos, I have found them to be a sweet-spot for binos (in general)… eye relief, image merging, FOV, etc. The current pair I use are University HDs - which perform very well, but are now only available used. Recently, I obtained Baader (Classic Orthos) 18mm from Agena and Astro Hutech 18mm Abbe Orthos in single eyepieces for Baader Q-Turret use. All perform very well and are in-stock when last checked. There are reviews of them in CN EP discussion section - see especially BillP.

I think all the bino-bandits are pretty much the same.  They are sold from a lot of places and it probably does not from where or which one you get.  They do free your hands up to do things like focus.

 

Thanks for the encouragement that the 18mm orthoscopics are a good fit for binoviewers in general (and probably the linear in particular).  Even though they are close (in magnification) to my existing 20mm, I sometimes switch between binoviewers and it would be nice to leave the 18mm in my linear and the 20mm in the WO BV for faster switch-out.



#20 noisejammer

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Posted 01 May 2021 - 12:54 PM

There is at least one exception to all the above, that is a "linear binoviewer" sold by Orion, Lunt and some others who rebrand the same basic hardware. It does not suffer from the back focus challenge noted above, because it uses a dielectric coated beam splitting mirror vice the usual prisms. So no extra optics need to be put in front of it to come to focus either.

I referred to these (without pointing to a specific label) in this post. The problem is that the entrance pupil of these devices is very small (typically 17mm) which limits the visible field.

 

Stepping aside from the NP101is - it's ill-suited to low power binoviewing - let's compare the visible field when using a Mk V and the Orion (or similar) units. I'll assume a 800 mm focal length that can get to focus without a Barlow

 

Linear binoviewer - max field = 2*atan(17/1600) = 1.22°
 

Mark V - max field = 2*atan(28/1600) = 2.00°

 

In terms of visible area, the Mk V shows off about 2.7x more but to be fair, it's at very considerable cost. If you were using a scope like the NP101is, the Mk V would need a 1.7x GPC causing things change a bit. Bear in mind that the so-called 1.7x is really a 1.5x ...

 

Linear binoviewer - max field = 2*atan(17/1080) = 1.80°

 

Mark V - max field = 2*atan(28/(1.5x1080)) = 1.98° (or 1.75° if the 1.7x GPC really is a 1.7x)

 

So in the specific case of the NP101is, the linear binoviewer makes a lot of sense.




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