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Binoviewer for close-focus on spotting scope?

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#1 jprideaux

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Posted 26 August 2019 - 06:49 AM

I realize that refractor nitrogen filled terrestrial spotting scopes won’t reach focus at infinity with using binoviewers (unless perhaps the linear offering) but I was wondering about reaching focus on close objects? One of my interests is viewing insects and spiders at high magnification at close focus. I also like using 2 eyes. I was wondering if it would be worth buying a binoviewer on something like a Celestron Regal ED and see if it would focus at around 10 feet. Would there be vignetting problems? Or perhaps a short astronomical grade refractor with binoviewer for close focus.

#2 Eddgie

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Posted 26 August 2019 - 07:43 AM

First, I am assuming that this model can use and reach focus with a standard 1.25" eyepiece like a Plossl.

 

Most spotting scope zoom eyepieces have very small field stops and one would assume that the scopes themselves do not have a rear aperture large enough to support a full 27mm field stop, so that may be the limit for vignetting.

 

If though, the scope has a 27mm rear aperture, then a binoviewer with a 27mm aperture should produce an unvignetted view.

 

As long as the scope can reach focus at 10 feet, then the only real issue that you would need some kind of Barlow or Optical corrector to be able to reach focus, and typically, this means that you are going to wind up with powers of 2x or higher.

 

If you went with a binoviewer that uses a 1.25" nose, it might be possible to use either the 2x or 2.6x barlow that comes with these, but there is no guarantee you would reach focus with the 2x, though for close focus, you might be able to do so with a 2.6x or 3x.

 

If you went with the Televue 2.5x powermate, this would likely reach focus and with a binoviewer, it would give about 2.2x. 

 

If you go with a T2 ready binoviewer, you can use the Televue Binoviewer 2x adapter.  This makes a binoviewer more or less par-focal with a regular eyepiece. (This is my most preferred solution for making sure you can reach focus in just about any scope with almost 100% assurance with nothing additional to buy.  It was also designed to correct for the spherochromatism that happens when the light cone passes through the prisms in the binocular, so gives the best possible contrast.) 

 

So, I don't see any reason you can't do it but the vignetting may have more to do with the scope than the binoviewer, but the good news is that the inexpensive binoviewers generally have small apertures and if the scope has a small rear aperture, then one of these small units would probably be a good match. 

 

This is an inexpensive binoviewer that has had the 1.25" nose swapped for an RAF Camera machined adapter, which costs about $30, so that it can work with T2 components.   The Televue Binoviewer 2x amplifier has a T2 mounting ring and is shown mounted to the BV in this picture.  The reason I recommend this as probably the best all-around solution is because this makes the binoviewer more or less Par-focal with a standard 1.25" eyepiece, so this means that a binoviewer set up this way will reach focus in the vast majority of telescopes with nothing else to buy.   With this, I can pretty much guarantee that you will reach focus but having not used it in a spotting scope, I can't tell you anything about if and how much vignetting you would see, but again, that may be more a function of the rear aperture of the spotting scope than the binoviewer.  I have used it in my Lunt 80mm ED and my Lunt LS80THa (both f/7 ED refractors) and again it reaches focus with no problem using even 2" diagonal in the Lunt 80ED. 

 

BV with TV.jpg


Edited by Eddgie, 26 August 2019 - 07:50 AM.

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#3 Jon Isaacs

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Posted 26 August 2019 - 08:34 AM

Ed:

 

My guess is that the OP is hoping to avoid an optical corrector by being close enough that the focal plane in pushed back far enough that the corrector is unnecessary.

 

Spotting scopes generally have a focal length of 480 mm.

 

I calculate that at 10 feet, the focal plane is push back about 90 mm, at 8 feet it's pushed back about 118 mm.

 

I don't know what the typical path length of a binoviewer is but if it's around 4 inches, a corrector may be unnecessary.

 

Jon


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#4 jprideaux

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Posted 26 August 2019 - 12:20 PM

Yes, I was thinking that because close-focus pushes out the focal-plane, then perhaps an optical corrector (like a barlow or other) may not be needed for the special case of focusing on something close.  The binoviewer may actually allow the spotting scope to focus closer than it otherwise would be able to (at the expense of loosing infinity focus).  For this close-focus application, the binoviewer may actually be considered a focus benefit instead of a hindrance on getting the desired focus.  Of course, for infinity focus, one needs to make up for the extra focus distance the binoviewer will take up by adding a barlow or other optical corrector.   

 

I will probably need to just do the experiment of purchasing an inexpensive binoviewer and spotting scope and then if that works, then decide whether to upgrade the equipment with more expensive items.  I was mainly wanting to see if anyone else has already "done this experiment"  smile.gif


Edited by jprideaux, 26 August 2019 - 12:21 PM.

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#5 Eddgie

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Posted 26 August 2019 - 01:26 PM

The shortest light path binoviewers I know of are 96mm of light path.

 

If you have this much back focus, then you should be OK.



#6 Eddgie

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Posted 26 August 2019 - 02:07 PM

Jon, 

I am confused by this because while I know that an astronomical telescope has to be racked out to reach focus, it would appear that spotting scopes do this internally.

 

Otherwise, if the OP used a standard eyepiece, he would have to use extension tubes to reach focus for close focus if it behaved like a standard astronomy telescope.

 

So, I guess I am just confused as to how he can go from infinity to 10 feet without having to use an extension tube for the eyepiece and this would say that the focusing is done by internal elements and the focal plane does not move rearward with changes in distance so that would mean that he would be unable to push the focal plane back for a binoviewer. 

 

Maybe I am wrong though, but I have never seen the need for extension tubes on spotting scopes when used at close distances so I don't understand how the focal plane could be moved rearward for a BV. 



#7 jprideaux

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Posted 26 August 2019 - 06:07 PM

All I know is that with a pair of binoculars that I have (similar to two spotting scopes), that I can see the eyepieces moving in and out as I change focus.  This movement is after the prisms.  Out as I focus closer, in as I focus farther way.

 

Consider D as the focus travel difference between the object at infinity versus the focus on an object at a much closer distance.  The thin lens equation tells us that D will be larger as the focal length increases.  Thus we see specialty binoculars that can focus from infinity to real close all have very short focal lengths (so the focus travel D doesn't have to be very long and can be accommodated with standard focus travel) and scopes with long focal lengths (I'm talking refractors here) tend to give-up on close focus and stick to focusing around infinity.  

 

Extension tubes move the eyepieces farther away from the main objective.  I was wondering if binoviewers work a little like extension tubes with effectively moving the eyepieces farther away from the main objective.  And therefore, if you want infinity focus, you need to make up for that "being farther way" by adding an optical corrector to increase the focal length to get the image focus to also be farther way from the main objective.  If you want to focus on a close object, then perhaps the binoviewer will by itself effectively move the eyepieces farther way from the main objective to coincide better with where the image would happen to focus (due to the thin lens equation) when focusing on a close object. 

 

It would kind-of be like the act of focusing on a close object acts like an "Optical corrector" for the binoviewer.

If you then wanted to focus at infinity, you would then need to add in a true optical corrector to change the focal-length.

 

Anyway, that is my speculation.  Please educate me if I'm wrong...

 

Saying things a little differently, spotting scopes tend to have short focal length main objectives which results in a short focus range.  I was wondering if a larger spotting scope that did not have a very good close-focus could take a binoviewer (which causes the eyepieces to effectively be farther way) and then try to focus on an object that was closer than what the spotting scope would normally be able to focus on.  I was wondering of the binoviewer would allow the spotting scope to reach focus on that object that was previously too close.


Edited by jprideaux, 26 August 2019 - 06:19 PM.


#8 jprideaux

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Posted 26 August 2019 - 07:04 PM

If it is the case that spotting scopes have an internal focusing mechanism unlike my binoculars which makes the ideas talked about here not relevant, then I could also consider getting a smallish APO refractor and try that with a binoviewer for close focus.  I've been wanting to get a small APO anyway.  I would need to get used to insects and spiders being upside-down (or left-right reversed) depending on whether or not diagonal is used (and what kind).  Lots to think about.



#9 Jon Isaacs

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Posted 26 August 2019 - 08:36 PM

Jon,

I am confused by this because while I know that an astronomical telescope has to be racked out to reach focus, it would appear that spotting scopes do this internally.

 

 

Ed:

 

This the way I think about it. 

 

Leave the scope focused at an object at infinity.  If I now point the scope at an object 10 feet away, the focal plane moves back about 97 mm.  This is a result of the 1/f = 1/f1 + 1/f2 equation for a lens with a 480 mm focal length.

 

The binoviewer is the extension tube.

 

Further focusing can be done with the internal focuser.

 

This much should work.

 

I think the problem is that the spotting scope prisms and optics are undersized for fully illuminating a focal plane 97 mm or more behind the current location.  They're barely adequate as is.

 

So an astro scope would be a better way to go. If I had a binoviewer, id try it but but it seems feasible.

 

Jon




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