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JMI Reverse Binoculars RB-16 Questions

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#1 Tom Duncan

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Posted 04 July 2020 - 03:50 PM

Recently got a JMI RB-16 16" f4.5 'reverse binoculars' (big!!!) and have a question:

 

When I have the OTA's aligned as per the instructions and have a star merged in the center of my view the stars outboard from center are doubled and the farther from center they are the wider the separation. Put another way as I sweep the star side to side from center the two images separate and move farther and farther apart from each other. I am using a pair of 19mm Panoptics.

 

Any idea what I've done wrong? I had an RB-66 a while back and it didn't do this. 

 

Thanks in advance for your input. 

 

Tom Duncan

 

PS: Is this the right forum in which to ask this question? 


Edited by Tom Duncan, 04 July 2020 - 03:56 PM.


#2 JoeInMN

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Posted 04 July 2020 - 03:58 PM

Could the two sides have slightly different magnifications?



#3 Richard O'Neill

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Posted 04 July 2020 - 04:25 PM

 I'd recheck the alignment. Good luck.



#4 TOMDEY

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Posted 04 July 2020 - 04:42 PM

Hi, Tom; That's Interesting!

 

I have the same binos and have used them with the same Eyepieces... and do not experience that problem.

 

I commissioned all upgraded mirrors with mine. Normand Fullum made the PMs as a matched set... way more expensive and way better than the JMI stock mirrors.

 

What you describe would be consistent with substantial differential magnification. One confirmation of that is that one side of the field will separate more annoyingly than the other. I can explain what causes that, it anyone is interested. Related to physiological convergence, divergence, and dipvergence requirements, as explained by Bill Cook, in his book Binoculars: Fallacy & Fact.

 

I suppose it could be your eyes... but sounds like either the mirrors or eyepieces (or both) are not identical. You could swap the eyepieces L/R to isolate cause. If swapping them flips the side that is worse --- that would be the eyepieces; if not, it would be the PMs.

 

Here's a picture of mine, with other eyepieces in there, but I also use the TV Panos 24 and 19.   Tom

Attached Thumbnails

  • 14 Toms JMI RB-16 six-motor electronic controls.jpg


#5 PETER DREW

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Posted 04 July 2020 - 05:10 PM

I think it is possibly due to the eyepiece pair being not parallel to the axis of the longitudinal light path. If you could rotate the eyepiece pair one field would rotate anticlockwise and the other one clockwise. A small amount of error can exhibit the appearance you describe despite the central image being merged.l

#6 Tom Duncan

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Posted 04 July 2020 - 05:18 PM

The apparent magnification through the eyepiece is identical on both sides, evaluated by comparing the distances between stars in each eyepiece and what stars were in the field of view. The degree to which the images of the star in the center will separate as it moves off center (laterally, by swinging the scope side to side) is equal both sides of center. That being said I didn't look to see what happens when I move the star up and down in the field.

 

Does the angle of the diagonal have any effect? I was using Vega which was basically directly overhead and had angled the diagonals towards me a bit so I wouldn't have to climb the ladder so far. When I angled them back to their 'normal' position the angle of divergence lessened but didn't completely disappear. 

 

I also wonder if Vega being directly overhead had anything to do with it. 

 

I need to try the scope out again, do some other tests (other EP's for one, an angled view for another) and make better notes.

 

Tom 



#7 TOMDEY

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Posted 04 July 2020 - 05:41 PM

Now that's even more interesting!

 

Trying a different pair of eyepieces would make sense. And also asking someone else to take a look, to see if it's observer-specific, or only the instrument. An interesting thing about the adjustable vergence RB's is that each and every user will automatically/subconsciously auto-adjust to his own personal most-comfortable prism differential, whether the optometrist has measured of flagged any abnormality, or not! It could be that is an affective in what you are experiencing?

 

I know my sister loves her 7x50 binos, even though they are terribly divergent... just happen to match conditional alignment to her personal prism...     Tom



#8 Rich V.

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Posted 04 July 2020 - 05:41 PM

Does the angle of the diagonal have any effect?

 

 

If the diagonals are rotated out of perpendicular towards you relative to the PM's optical axes, the images will rotate in opposite directions; the left side CCW and the right CW.  Only a star in the center would remain stationary.  That field rotation could easily be your problem.

 

Rich


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#9 Tom Duncan

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Posted 04 July 2020 - 06:05 PM

Thanks for that Rich, I thought that might be the case and also thought I had brought them to the correct orientation but I'll add that to the list of things to check for the next test. As I mentioned I lost Vega behind some trees and with the little slot of sky I had there were no other stars to work with. 

 

And Tom, I understand what you are referring to, I once was selling a pair of very nice Zeiss binoculars at a camera show and the customer wanted them badly but he just couldn't see through them, as in just black, he couldn't get them to work with his eyes at all. Friends in the room (and myself of course) had no trouble with them but this guy just couldn't see a thing. Since then whenever I'm offering binoculars at a show I insist they put the ability to see through them easily over whatever brand they might be. 

 

Tom 


Edited by Tom Duncan, 04 July 2020 - 06:08 PM.


#10 TOMDEY

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Posted 04 July 2020 - 10:34 PM

If the diagonals are rotated out of perpendicular towards you relative to the PM's optical axes, the images will rotate in opposite directions; the left side CCW and the right CW.  Only a star in the center would remain stationary.  That field rotation could easily be your problem.

 

Rich

Excellent point! My RBs are fine, but differential field rotation would indeed induce vergence mismatch proportional to field. I'll bet that's what's going on. The JMI RB16 manual gets into that, but not overtly. If that's the problem... it's a super easy fix!    Tom



#11 Tom Duncan

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Posted 24 July 2020 - 12:23 PM

Turns out the issue outlined in my original post was a combination of my eyes, diagonal angle and the tubes being pointed straight up. The mechanicals still need a little bit of attention but it's basically sound and delivers an excellent viewing experience. 

 

Tom 


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