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What effect does a binoviewer have on brightness?

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#1 CrazyPanda

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Posted 09 August 2017 - 08:55 AM

I would assume that since you're splitting one source of light into two, you're reducing the amount of light each eye gets by half. 

That is, if a mono eyepiece is producing a 4mm exit pupil, a binoviewer is the equivalent of a 2.83mm exit pupil in each eye? 

 

Is that correct? Are there any other brightness effects a binoviewer has? 

 

The reason I ask is because I'm planning a telescope purchase around what my preference is for a minimum exit pupil in a given eye. 



#2 Eddgie

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Posted 09 August 2017 - 09:35 AM

That is correct.

 

What this does not factor in though is a function of the brain called "Summation" and you can read about that in the posts that Edz made on binoviewing.

 

To summarize his posts, while the single eye only gets the 50% of the brightness that the eye would get in monocular vision, when the brain gets the two signals, it adds them together in such a way that the brightness is perceived as brighter than each eye alone see.   

 

This means that when binoviewing, a target will appear about as bright as it would be in a telescope of about 70% of the aperture used at the same exit pupil.  (or something like this... You can get the exact details in the article on summation). 

 

 

Also, the surface brightness of extended objects can be made brighter simply by using slightly longer focal length eyepieces..  In other words, rather than using a 10mm pair of eyepieces, which would give a dimmer view than a single 10mm eyepiece, one could use two 13mm eyepeices and get about the same brightness, though at a slightly reduced scale.  Now most people report that things look bigger when using both eyes, so this reduced scale actually does not seem to be a problem in normal use.

 

Your math is correct though, but you are only seeing part of the equation. To see the rest of the equation, you may want to read Edz's post on binocular summation:  https://www.cloudyni...nd-information/


Edited by Eddgie, 09 August 2017 - 09:36 AM.

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#3 Eddgie

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Posted 09 August 2017 - 09:39 AM

And while binocular summation does raise the brightness by a factor of something like 40% for extended objects, it does little for limiting magnitude so you absolutely do loose some faint stars in clusters when using a binoviewr, but not as many as one might think.  We are talking only a .2 or .3 magnitude difference so on brighter clusters, almost no damage is done.   On faint clusters though, you do miss some stars. 

 

And don't crucify me if the numbers I used are not exact.  I am not going to go re-read the paper to correct myself.  I will leave that to you or other forum members that want to learn more about binocular summation. I think my numbers are about right, but I may be off some. 


Edited by Eddgie, 09 August 2017 - 09:41 AM.


#4 REC

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Posted 09 August 2017 - 10:08 AM

Good question and reply's by Ed. For planets and the moon, it doesn't appear to be an issue as they are bright to begin with. Actually, I get a much better view of Jupiter with a BV because it cuts down on the glare. I can easily see more detail in the bands ect. 

 

I usually start with a pair of 20mm EP in the BV and go from there. When I observe bright DSO, I'll switch out to a pair of 26mm SP that have a brighter exit pupil.



#5 MrJones

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Posted 09 August 2017 - 10:35 AM

Splitting the light into two beams does not by itself change the exit pupil. It only reduces brightness at each eye. Brightness is not the same as exit pupil.

 

That being said most binoviewers will have optics that change the magnification and along with OCS, Barlows and whatever you will almost never have 1:1 exit pupil into BV vs. out. My Denk II supposedly have 15% magnification by themselves which seems accurate, so they change the exit pupil by 15%.



#6 Eddgie

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Posted 09 August 2017 - 10:45 AM

Splitting the light into two beams does not by itself change the exit pupil. It only reduces brightness at each eye. Brightness is not the same as exit pupil.

 

That being said most binoviewers will have optics that change the magnification and along with OCS, Barlows and whatever you will almost never have 1:1 exit pupil into BV vs. out. My Denk II supposedly have 15% magnification by themselves which seems accurate, so they change the exit pupil by 15%.

 

The OP did not at all suggest that it changes the exit pupil.  He said that "if a mono eyepiece is producing a 4mm exit pupil, a binoviewer is the equivalent of a 2.83mm exit pupil..." (Emphasis added by me) and he is correct in that statement.  The exit pupil is not changed, but the brightness is reduced by this factor and that is what the OP said.

 

And I have used many configurations that worked at 1x.   While it is true that in most cases, people are using some amount of amplification, that is a just an equipment selection issue and many people that convert exclusively to binoviewing often go on to buy scopes that are binoviewer friendly, though your comment is indeed accurate that most people today are not working at 1x.

 

It is important to be clear to the OP though that 1x is indeed possible and many people use 1x. 


Edited by Eddgie, 09 August 2017 - 10:45 AM.


#7 CrazyPanda

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Posted 09 August 2017 - 11:54 AM

Thanks very much for the information and resources Eddgie. That will help me determine what I want to get for a scope.

 

According to this:

 


for 100mm scope with binoviewer
first scope splits light, therefore (100 sqrd / 2) = 10000/2 = 5000 sqmm
therefore each eye receives only sqrt 5000 or 70.7mm aperture of light
then binocular vision takes place
Sqrt [(70.7)sqrd + (70.7)sqrd ] = Sqrt 10000 = 100.
Then, 100/70.7 = gain factor, = 1.41
(The Binocular Vision Gain factor will always be 1.41)
The gain factor is applied to the information reaching the dominant eye. Assume in this case it is getting the full 70.7 or 5,000sqmm of light. The gain is then 5,000 x 1.41 = 7000sqmm. That represents an aperture of Sqrt 7,000 = 84mm.

 

 

If I were to plug in the numbers for a 16" (406mm) scope, I would arrive at roughly 406 / sqrt(1.41) = 342mm or monoview brightness equivalent of 13.5" (but with all of the other visual benefits such as increased perception of contrast and detail). Given my primary interest in using a binoviewer is for planets, and knowing what I know about the average seeing conditions where I live, even with binoviewing, I will hit the magnification limit for my area long before I hit my minimum brightness preference.

 

I was debating going up to 20" instead of 16" just to make sure I can get maximum brightness when using a binoviewer at the magnification Im usually limited to, but it seems I don't have to. 



#8 Eddgie

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Posted 09 August 2017 - 07:41 PM

I routinely used binoviewers at 440x on planets with my 12" dob but I felt that the maximum contrast [festoon detail on Jupiter as an example] was usually found at 350x or so. I have had amazing views with this setup.

Yes, 16" will run out of seeing before brightness on most nights.

#9 GlennLeDrew

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Posted 09 August 2017 - 07:51 PM

Given that a mono scope with BV supplies light to each eye such that the equivalent aperture for each eye is 0.707, and that image summation in the brain increases a bino aperture to the mono scope equivalent of 1.189, we see that 0.707 * 1.189 = 0.84.

 

We can say, then, that a mono scope *with* BV delivers the image brightness of a mono scope by itself having an aperture of 0.84 that of the BV'd scope.

 

The tactic of using a lower magnification/larger exit pupil in order to regain lost brightness when bino viewing may not pay dividends. Yes, the image is brighter and hence visual system noise makes a smaller contribution. But the reduced image scale makes for reduced resolving power. It might well turn out that no disadvantage attends the retention of the dimmer image because the visual system could extract no less information from the larger image in spite of worse visual system noise.




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