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BV pupil question for optics experts

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#1 jprideaux

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Posted 19 July 2021 - 06:22 PM

It has been claimed from time to time that the image in each half of the linear BV can be brighter than the image in each half of a conventional BV.  Obviously the light in either type of BV is split between the two sides.  It is also the case that the conventional BV has two half-intensity full-circles where as the linear BV has two full-intensity half-circles for the exit pupil.  I can think of one way in which the linear BV can produce an image that gets around twice the photons.  Please correct any mis-statements of what I say below to try to explain how this could occur.

 

Consider the black circles below as the telescope/eyepiece exit-pupil and the red circles as the entrance pupil of the eye.  I'm looking at a case where the eye is not dilating wide enough to take full advantage of the telescope/eyepiece exit pupil.  (forgive any non-standard nomenclature).

 

pupils.jpg

 

Relative to the single eyepiece, half the light would be going into each eye for the case of the conventional BV.

But for the case of linear BV, if the same-size entrance pupil of each eye could be positioned to be in the full intensity illuminated half of the half-pupil, then each eye is getting the full intensity light - thus brighter.  Another way of thinking about it would be that in this case more light is wasted (not getting into the eye) for the case of the conventional BV as compared to the linear BV.

 

It could be that the tables could be turned with different pupil sizes relative to the telescope/eyepiece pupil.

 

I have both a linear BV as well as a Williams Optics conventional BV but I don't have the necessary eyepieces to get the exact same magnification in each set-up to do my own experiment.  An experiment could be done with a manual camera but I'm not yet set-up for that.

 

So please correct any flaws in my reasoning above.  I'm curious.

 

(Note that one sees the full image in each eye of the linear when the eye is positioned at the correct distance (like looking through a non-circular keyhole).


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#2 MikeTahtib

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Posted 19 July 2021 - 07:20 PM

I've been wondering about this for a while.  I was wondering if there's a way to get full brightness from a binoviewer, or if you need a binoscope.  It seems to me that the light gets cut in half regardless of exit pupil, although I've never heard of a linear binoviewer.  It sounds like each eyepiece looks at only half of the mirror?  If so, then I think that's a way of getting a binoscope of a much smaller aperture, although I don't really understand how it works, with the parabolic mirror still focusing to one point.  If your eyepiece isn't there, I'm not quite sure what is happening.  Unfortunately, I don' t have much to offer the conversation, but I do want to read what others say, and to learn more about this linear binoviewer.



#3 jimhoward999

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Posted 19 July 2021 - 07:21 PM

It seems like your logic is perfect.  But I question if there are any practical cases where the telescope exit pupil is considerably larger than the dilated pupil of the observer.  In binoculars yes, but in a telescope with a BV?  The magnification would need to be really low.    Even in a old observer the dilated pupil would be 5mm or more for most (although not all) people.

 

I thought that one way a linear BV can be brighter is that splitting the aperture can be more efficient than dividing the channels with beamsplitters. 

 

A second way I think is that the divided pupil is smaller in the linear BV.   The pupil is then more likely to be fully captured by the larger eye pupil.


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#4 David I

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Posted 19 July 2021 - 07:45 PM

Maybe I am wrong here but if the Williams Optics conventional BV without using an OCS does not change magnification from cyclops view , then you can use the same eyepiece for all three set ups. Your measuring the light so it does not have to be in focus , just be out of focus as much as the Williams Optics set up. Maybe you can use a cell phone on manual exposure setting to record images. I think Jim is right , it would need to be a very bright view to have that extra benefit with the linear.


Edited by David I, 19 July 2021 - 07:46 PM.


#5 eyespy

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Posted 19 July 2021 - 07:54 PM

Hi David,

 

A conventional binoviewer used in a catadioptric telescope without an OCS will roughly double the magnification due to the change in mirror/front element separation.  If used in a refractor that will reach focus without the OCA, then there will be no increase in magnification.

 

I believe that there are approximately five additional lens elements in each side of the Linear binoviewer in order for it to be able to reach focus without an OCS.

 

Doug.....


Edited by eyespy, 19 July 2021 - 07:58 PM.


#6 David I

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Posted 19 July 2021 - 08:08 PM

Ok Doug  that's good, then jprideaux does not have a magnification issue unless the linear is changing the magnification from the cyclops view. 

jprideaux, please give us more details of your experimental protocol.



#7 jprideaux

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Posted 19 July 2021 - 10:17 PM

One thing I could try is to use the Williams Optics BV with my refractor without using my diagonal - that is just straight through.
It can reach focus that way without any OCA. Then I could hook up an iPhone using nightCap with a set exposure for a certain object. Then repeat the experiment with the linear with the same eyepieces on the same subject and see if there are any exposure differences between the images. That may not really show what the experience would be with the human eye, though.
It could be (as suggested earlier) that what I suggested in the OP would only happen for daylight low-power viewing with eye-pupil really small (or looking at the moon at night). Since I have both BVs, I will eventually try some-kind of meaningful experiment. If anyone has any ideas as for what might be meaningful, please share. I could also just look at things myself through the BVs and report back but there may be too much uncontrolled to trust the results. For example by the time I replaces one BVer with the other and switched eyepieces, my eye’s pupils might have changed.
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#8 David I

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Posted 20 July 2021 - 06:56 AM

I agree it's hard to get good data from your constant changing eye pupil. Go straight through with the binos and use a lower set exposure starting with the linear so it is a bit under exposed, then the resulting darker exposure from the conventional bino should be evident.

#9 David I

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Posted 20 July 2021 - 07:02 AM

And don't forget to take the single eyepiece exposure. Thank you!

#10 jprideaux

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Posted 20 July 2021 - 12:09 PM

And don't forget to take the single eyepiece exposure. Thank you!

I can't promise when I can get to it but I will set-up a test to do all three.

 

1.  single eyepiece with (no diagonal, using extension tube)

2.  same eyepiece with WO BV (no diagonal)

3.  same eyepiece with linear (no diagonal, using extension tube).

 

To control the lighting, I'll probably do a set-up indoors at around 25 feet away.

Once I get the process down good, I might then repeat the experiment for an astronomical object at infinity - perhaps the moon.

 

Being thus far only a visual astronomer, I will be limited in this experiment with using my iPhone connected afocally to the eyepiece (with a bracket) and I will need to first master how to use the nightCap app to lock the exposure so the same exposure is used for all three pictures.  Hopefully that is not hard. One other variable is how far the iphone lens is from the eyepiece.  I'll try to be consistent between all three pictures.  For the BV pictures, I will, of course, put on an eyepiece cap on the eyepiece not being used.

 

If you (or anyone) thinks of some other good experimental protocols, let me know.  Thanks!



#11 jprideaux

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Posted 20 July 2021 - 11:47 PM

I did a little indoor experiment on focusing my AT92 refractor on a drawing about 21 feet away.  

All pictures were taken with my iphone held to the same 18mm Baader orthoscopic eyepiece by a smartphone-eyepiece adapter.  

Each picture was taken using the app nightCap where the exposure was locked at a 60th of a second.  

Since it was a fairly close focus situation which pushed the focus out, I used the following extension tubes with the set-up.

 

singleEyepiece - 50mm extension tube + 80 mm extension tube - telescope.

Williams Optics BV - 50 mm extension tube - telescope

Linear BV - 50mm extension tube + 80 mm extension tube - telescope

 

In all three images, the bracket that clamped onto the eyepiece did not let the camera get all the way to the ideal closeness to the eyepiece but I made it off by the same amount for each picture.  I think this contributed in each case to more field curvature than what would have been if I could have gotten the camera closer to the eyepiece.

 

The following are the three pictures.  The only editing was to re-size them and add the lettering at the top.

 

singleEyepiece_small.jpg

 

linear_small.jpg

 

WO_small.jpg

 

I find the results with the pictures a bit suspect in that although I had the exposure locked at 1/60th of a second, perhaps there are things going on with taking an iphone picture that I don't understand.  I would have expected to see a difference between at least the single eyepiece and the WO BV.  Since I did not, I probably cannot trust these results.  

 

I then loaded a light-meter app on my iphone and centered, focused, and then switched to the light-meter app with each combination and played around with the smart-phone-eyepiece adapter for the maximum lux value:

 

Single Eyepiece:   104 lux

Linear BV  34  lux

WO BV      33 lux

 

I don't know if I trust these values either.  I will repeat the experiment later on with a uniform white wall so I hopefully don't get so much variance with the camera/eyepiece positioning for each individual set-up.  At any rate, the results above don't show a lot of difference between the Linear and the WO BV.  


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#12 noisejammer

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Posted 21 July 2021 - 06:06 AM

When using a binoviewer, (about) half the energy is going down each of the optical paths. 100 lux turns into 50. Transmission at each air-glass and glass-air interface is never perfect - even excellent coatings only offer about 99.5% transmission, so let's say the binoviewers are using coatings that allow 98-99% transmission.

 

I count 3 air-glass and 3 glass-air interfaces, so that's a total transmission of between 0.986 and 0.996 . 50 lux turns into something between 44 & 47.

 

Then there's transmission. Most optical glasses exhibit transmission of 0.95 or better (for 5 mm) in the visual range. If we assume the binoviewers use BK7, the optical path length will be something like 35mm of glass so that the 44-47 lux is suddenly multiplied by 0.957 = 31 - 33 lux.

 

In other words, you measured pretty much what you might expect. Ain't nature a pain...


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#13 eyespy

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Posted 21 July 2021 - 01:56 PM

I think that the brain does a pretty good job of taking those two 33+/- Lux levels and when combined we ‘see’ a bino image that seems to be brighter than the sum of its parts.  Sometimes, nature can be great too !!

 

Doug.....



#14 jprideaux

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Posted 21 July 2021 - 04:41 PM

When using a binoviewer, (about) half the energy is going down each of the optical paths. 100 lux turns into 50. Transmission at each air-glass and glass-air interface is never perfect - even excellent coatings only offer about 99.5% transmission, so let's say the binoviewers are using coatings that allow 98-99% transmission.

 

I count 3 air-glass and 3 glass-air interfaces, so that's a total transmission of between 0.986 and 0.996 . 50 lux turns into something between 44 & 47.

 

Then there's transmission. Most optical glasses exhibit transmission of 0.95 or better (for 5 mm) in the visual range. If we assume the binoviewers use BK7, the optical path length will be something like 35mm of glass so that the 44-47 lux is suddenly multiplied by 0.957 = 31 - 33 lux.

 

In other words, you measured pretty much what you might expect. Ain't nature a pain...

Thanks for offering some reasoning concerning my lux numbers in that they may be realistic after-all.  Perhaps I trust them a bit more now.  I'll also trust them a bit more after I repeat the experiment with a bit different set-up.

 

My main goal here was to see if the Linear produces a brighter image as compared to the WO BV.  So far, at least in the experimental protocol I did, they seem to produce about the same brightness and both dimmer than the single eyepiece (which is of course the conventional wisdom).  If there is ever anything like what I proposed in my original post, I did not reproduce that situation with my experiment. 

 

And, of course, my experiment with the camera will not address any "summing" that the brain can do...  Yes, we get that "for free" regardless of the type of BV we use. 



#15 kroum

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Posted 22 July 2021 - 12:27 AM

In addition to shutter speed, cameras also have ISO (gain), and some even aperture control.  I’m not sure if the iPhone has aperture control, but it for sure has ISO, so you have to get an app that lets you have full manual control of exposure, not just shutter speed.

When you control just the shutter speed, the automatic exposure compensates by bumping up the gain or opening the aperture further if possible, and the final image appears to have the same brightness.



#16 noisejammer

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Posted 22 July 2021 - 04:01 AM

.. My main goal here was to see if the Linear produces a brighter image as compared to the WO BV.  So far, at least in the experimental protocol I did, they seem to produce about the same brightness and both dimmer than the single eyepiece (which is of course the conventional wisdom). ...

I think the so called Linear bv has far more air-glass and glass-air interfaces. On the other hand, the WO offering has more glass to transit. That said, I expect the Linear bv's coatings were optimised so that the more complex system appeared to be no dimmer than the relatively inexpensive offerings.
 



#17 jprideaux

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Posted 22 July 2021 - 06:23 AM

In addition to shutter speed, cameras also have ISO (gain), and some even aperture control. I’m not sure if the iPhone has aperture control, but it for sure has ISO, so you have to get an app that lets you have full manual control of exposure, not just shutter speed.
When you control just the shutter speed, the automatic exposure compensates by bumping up the gain or opening the aperture further if possible, and the final image appears to have the same brightness.


Yes, the nightCap app I was using lets you lock down “exposure” (whatever they mean by that) but only temporarily the ISO. I’ve found that the ISO setting manually set is not retained after the phone goes into screen-savor mode and then the screen turned back on. I did not realize this when I took the pictures so the ISO was most likely different between them - thus producing the same apparent brightness in the images. So, yes, don’t draw any conclusions from those 3 pictures. I’ll try that picture-taking experiment again once I’ve mastered controlling those settings in the nightcap app.

#18 faackanders2

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Posted 25 July 2021 - 06:06 PM

It seems like your logic is perfect.  But I question if there are any practical cases where the telescope exit pupil is considerably larger than the dilated pupil of the observer.  In binoculars yes, but in a telescope with a BV?  The magnification would need to be really low.    Even in a old observer the dilated pupil would be 5mm or more for most (although not all) people.

 

I thought that one way a linear BV can be brighter is that splitting the aperture can be more efficient than dividing the channels with beamsplitters. 

 

A second way I think is that the divided pupil is smaller in the linear BV.   The pupil is then more likely to be fully captured by the larger eye pupil.

Almost all binoviewers only accept 1.25" eyepieces and require OCS/barlows increasing power to focus.  So exit pupil of binoviewers would be higher power and smaller that with the same eyepiece in a telescope.

My widest binoviewer eyepiece is 24mm 68AFOV panoptics.




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