44 replies to this topic

[Princess Leah]

When I use my 100/f5 or 80ED at home viewing in the city with a 25mm or 30mm UFF eyepiece I often have to stop down the telescope to 65-70mm to reduce the exit pupil (and darken the sky).

 

I could use a shorter focal length EP, but I love the wide views.

 

I don't seem to have the problem with my 15x70 or 20x80 binos.

Even though the exit pupils are large the sky doesn't seem as bright. 

 

My conclusion is that with all the porro prisms/reflections/ vignetting in the Binoculars, the exit pupil of binoculars is not comparable with a astro refractor.

There is more light throughput in a refractor? Is this accurate?

 

I was wondering if binoviewers would effectively act as an aperture stop, while giving me the benefit of two eyes.

 

Many thanks

Leah

[Eddgie]

When you use a binoviewer you are only getting 50% of the light in each eye that you would get with a regular eyepiece using one eye. This is because the binoviewer has a beam splitter that splits the light into two identical but dimmer cones, with each cone serving one eye. Binocular summation does kind of stack the two images in your brain, but the result is that your brain perceives an image that is about as bright as it would be if you were using a single eyepiece in a telescope that was 70% as big as the scope with the binoviewer.

 

What this means is that when you use the binoviewer, the image is not perceived as being  brighter than it would be for 56mm aperture and hence, the sky appears darker. You also lose about .3 magnitueds.

 

You do not alter the contrast ratio and in fact, using both eyes does slightly improve your contrast performance, though to offset the loss in brightness, you would want to use about 10% -15% lower power than you would with one eye. 

Edited by Eddgie, 03 January 2024 - 11:54 AM.

[RAKing]

I have a similar problem when I am looking at the moon or Jupiter.  The view is so bright I have to add a neutral density filter - or I will lose all of my "night vision".

 

Regular binoviewers split the light into two beams and the light passes through more glass before it gets to your eye.  But my brain stitches these two beams back together in my brain to give me a single, composite view, so I am not sure how much light is actually lost. 

 

I actually love the better view through my BV.  I am much more relaxed using two eyes, I can study the object much longer with less fatigue, and I am happy to trade any small light loss for the nicer view.

 

Eddgie's numbers are as good as any.

 

Ron

[Princess Leah]

Both great replies. Many thanks for your time.

 

The only bit I don't understand is Eddgie mentioned 56mm aperture?

 

 

 

What this means is that when you use the binoviewer, the image is not perceived as being  brighter than it would be for 56mm aperture and hence, the sky appears darker. You also lose about .3 magnitueds.

 

 

Also for the sake of argument if a standard pair of binoculars light grasping potential is 1.4x aperture.

This equates to 2X light capture.

For example a 15x70 is effectively 15x100.

Does this also apply to binoviewers?

If so are we not back at the square one/same amount of light?

[Eddgie]

The 56mm is 70% of the 80mm and this is the aperture that would give the same brightness when using one eye as the 80mm does when using two eyes with a binoviewer.

 

No, does not apply to binoviewers. 

[Princess Leah]

Understood, thanks!

 

Any idea why it doesn't apply to Binoviewers?

[RAKing]

Binoculars are like two separate telescopes, connected together.  They have erecting prisms to give you the correct image, but two separate objective lenses.

 

A binoviewer has a single opening in the front, so all the light enters there, then gets split to the separate eyepiece paths.

 

Ron

[Princess Leah]

Binoculars are like two separate telescopes, connected together.  They have erecting prisms to give you the correct image, but two separate objective lenses.

 

A binoviewer has a single opening in the front, so all the light enters there, then gets split to the separate eyepiece paths.

 

Ron

Thanks for explaining Rob.

 

I'm not suggesting light can be increased.

However in theory light shouldn't be reduced.

That's if the 1.4 theory of two eyes instead of one is correct.

 

The light input is halved in value when it splits.

But in the brain each half is increased to create a 2X increase in light overall.

 

Therefore no light is lost.?

I agree there is a rabbit out the hutch somewhere in my thinking!

 

Correction.

 

My theory would only work if the light was halved. In reality due to the prisms the light would be less than half when it splits.

[Eddgie]

 

 

The light input is halved in value when it splits.

But in the brain each half is increased to create a 2X increase in light overall.

 

 

This is not really the way it works.

 

If you want to learn more about binocular summation, I recommend this link. This is about the best information you will find on the topic.

 

https://www.cloudyni...sion-summation/

[Jon Isaacs]

This thread seems to be about binoculars rather than binoviewers.. 

 

"I don't seem to have the problem with my 15x70 or 20x80 binos.

Even though the exit pupils are large the sky doesn't seem as bright. "

 

Jon

[Princess Leah]

This is not really the way it works.

 

If you want to learn more about binocular summation, I recommend this link. This is about the best information you will find on the topic.

 

https://www.cloudyni...sion-summation/

Thanks for this link.

[noisejammer]

This is not really the way it works.

 

If you want to learn more about binocular summation, I recommend this link. This is about the best information you will find on the topic.

 

https://www.cloudyni...sion-summation/

Thanks for the link, but I'm really puzzled by some of EdZ's reasoning. It doesn't seem to be consistent throughout but I'll re-read it in the morning.

 

As an example, he wrote

Suppose you were to take an 8" scope and use a binoviewer. Take the 8" area = 64 sq in. Thru the binoviewer the light gets divided in half to each eye. Therefore, 32 sq in to each eye. Now you can apply the binocular vision factor to the amount of light that each eye receives. So 32 x 1.4 = 44.8. Sq rt 44.8 = 6.7". So an 8" scope with a binoviewer provides the effective light as in a 6.7" scope to one eye. The 8" scope with binoviewer is nearly equal to the effective light gathering of the two 6" scopes.

For those who find the math suspect, EdZ has ignored the pi/4 correction between the area of squares & circles. It disappears from the ratio so the conclusion is still the same.

 

Put differently, it is stating that when using a binoviewer with equally good eyes, you perceive more than you would if you dumped all the light into one eye. In other words, there is processing gain in addition to the (0.5 + 0.5) direct addition.

 

This is consistent with my own observations. - using a binoviewer I have detected the G & perhaps the H1 star in the Trapezium. Both were invisible to monocular vision. (TOA150 / Tak diagonal / Denk II) Sorry, I don't recall the magnification but it was probably around 220x because my shortest eyepieces are 5mm. I once worked out that this translated to ~6 photons per second reaching each retina. They couldn't be seen but they could be detected, suggesting that the brain has some sort of coincidence detection circuitry.

 

But ... I'm not sure this is what the OP was really asking.

[noisejammer]

When I use my 100/f5 or 80ED at home viewing in the city with a 25mm or 30mm UFF eyepiece I often have to stop down the telescope to 65-70mm to reduce the exit pupil (and darken the sky).

I could use a shorter focal length EP, but I love the wide views.

I don't seem to have the problem with my 15x70 or 20x80 binos.

Even though the exit pupils are large the sky doesn't seem as bright.

My conclusion is that with all the porro prisms/reflections/ vignetting in the Binoculars, the exit pupil of binoculars is not comparable with a astro refractor.

 

Q1 ...

 

There is more light throughput in a refractor? Is this accurate?

On the whole, refractors do transmit more light than reflectors. Like all things, it is not as simple as a yes / no.

 

Assume 98.5% transmission per surface, a good diagonal and negligible absorption. A refractor will transmit about (0.985)⁵ = 93% of the incident light. Typically, a reflector has 90% reflectivity, then it transmits 81% of the light but some of the other 19% gets scattered which degrades the contrast. If it has high reflectivity coatings (95%-ish), it will transmit about 90% of the light and less will be scattered.

 

Then you need to consider the aperture and the central obstruction from a secondary. As a rule of thumb, a refractor will about equal a reflector that has an aperture of 150%. So a 80 mm refractor is about equal to a 120 mm reflector. More or less. The image quality of the refractor will be usually be better. The refractor will be far more expensive.

 

Q2 ...

 

I was wondering if binoviewers would effectively act as an aperture stop, while giving me the benefit of two eyes.

Hmm .. I think the effect you are seeing is related to the diameter of the exit pupil.

 

With your 15x70 bino's, the EP is 70 mm /15 = 4.7 mm while with your 20x80 bino's it is 4 mm.

 

Your 100 f/5 has an exit pupil of 5 mm with your 25 mm eyepiece and 6 mm with your 30 mm.

 

The apparent brightness of the sky goes with the square of the exit pupil, so that a 100 f/5 + 30 mm will make the sky appear 36/16 = 2,25x brighter than the 20x80 binos. That's 0.9 magnitudes brighter or nearly 2/3 of the visible stars being squashed.

 

Stopping your 100 f/5 down to 65 mm returns its exit pupil to about 4 mm so that you should see about the same number of stars as your 20x80 bino's. My explanation of it being related to exit pupil seems consistent.

 

So what to do ... I'll do the sums for your 100 f/5.

 

Let's assume my understanding of the discussion in EdZ's article (linked to by Eddgie) is basically right. Binoviewers send half the light each way and using two eyes makes things appear brighter. The effective aperture of your scope will be about 100 x 6.7 /8  = 84 mm.

 

1. If you use large prism binoviewers and you can reach focus without a Barlow or corrector, you can get away with 30 or 32 mm Plossl eyepieces. Your magnification will be the same but the true field will be a lot smaller - say 3 degrees instead of 4. The effective exit pupil will then be 5.2 mm. The sky will be about 0.6 magnitudes brighter.

 

2. If you use large prism binoviewers and you can reach focus without a Barlow or corrector, you can get away with 24 mm UFF eyepieces. Your magnification will be 21x instead of 16x but the true field will be the same as a 32 mm Plossl - say 3 degrees. The effective exit pupil will then be 4 mm so the sky will have similar brightness at slightly higher magnification.

 

3. If you switched to (say) an AT 20mm 100 deg eyepiece instead of a binoviewer, the magnification would be 25x and you would have a 4mm exit pupil. So that's good. You'd also have a true field of about 4 degrees. That's good too. The primary issue is that you might get a lot of coma at the edge of your field. You might want to check the Eyepieces forum for this.

Caveat - a large prism binoviewer will cost you at least $800 (used Denk II + Baader T2 diagonal + adapter + nosepiece) and you need at least 180 mm of available inward focus travel. You will also need two eyepieces ... so going for wide fields with binoviewers easily puts you on the wrong side of a kilodollar. It would be prudent to test your system at a club meeting.

Also, the so-called 'linear binoviewers' vignette badly with longer focal length eyepieces. They're easier to use but they won't get you wide fields.

 

[ABQJeff]

Posting a statement I made in another thread, in my experience/for my eyes, binocular summation increases brightness by Sqrt (2) (~1.4) and binoviewers reduce the effective aperture by 1/Sqrt (Sqrt (2)) (~0.84). Thus a pair of 102mm telescopes used together as a binoscope are roughly equivalent to a 170mm refractor with binoviewers (for brightness, for resolution the 170mm refractor is still better as that is aperture dependent).

Edit: thus in line with Eddgie saying to use about 15-20% less power with binoviewers to get same brightness. What helps is that the brain will make binoviewed objects look bigger so using the lower power won’t actually decrease apparent size all that much.

Edited by ABQJeff, 05 January 2024 - 08:22 PM.

[Princess Leah]

Q1 ...

 

On the whole, refractors do transmit more light than reflectors. Like all things, it is not as simple as a yes / no.

 

Assume 98.5% transmission per surface, a good diagonal and negligible absorption. A refractor will transmit about (0.985)⁵ = 93% of the incident light. Typically, a reflector has 90% reflectivity, then it transmits 81% of the light but some of the other 19% gets scattered which degrades the contrast. If it has high reflectivity coatings (95%-ish), it will transmit about 90% of the light and less will be scattered.

 

Then you need to consider the aperture and the central obstruction from a secondary. As a rule of thumb, a refractor will about equal a reflector that has an aperture of 150%. So a 80 mm refractor is about equal to a 120 mm reflector. More or less. The image quality of the refractor will be usually be better. The refractor will be far more expensive.

 

Q2 ...

 

Hmm .. I think the effect you are seeing is related to the diameter of the exit pupil.

 

With your 15x70 bino's, the EP is 70 mm /15 = 4.7 mm while with your 20x80 bino's it is 4 mm.

 

Your 100 f/5 has an exit pupil of 5 mm with your 25 mm eyepiece and 6 mm with your 30 mm.

 

The apparent brightness of the sky goes with the square of the exit pupil, so that a 100 f/5 + 30 mm will make the sky appear 36/16 = 2,25x brighter than the 20x80 binos. That's 0.9 magnitudes brighter or nearly 2/3 of the visible stars being squashed.

 

Stopping your 100 f/5 down to 65 mm returns its exit pupil to about 4 mm so that you should see about the same number of stars as your 20x80 bino's. My explanation of it being related to exit pupil seems consistent.

 

So what to do ... I'll do the sums for your 100 f/5.

 

Let's assume my understanding of the discussion in EdZ's article (linked to by Eddgie) is basically right. Binoviewers send half the light each way and using two eyes makes things appear brighter. The effective aperture of your scope will be about 100 x 6.7 /8  = 84 mm.

 

1. If you use large prism binoviewers and you can reach focus without a Barlow or corrector, you can get away with 30 or 32 mm Plossl eyepieces. Your magnification will be the same but the true field will be a lot smaller - say 3 degrees instead of 4. The effective exit pupil will then be 5.2 mm. The sky will be about 0.6 magnitudes brighter.

 

2. If you use large prism binoviewers and you can reach focus without a Barlow or corrector, you can get away with 24 mm UFF eyepieces. Your magnification will be 21x instead of 16x but the true field will be the same as a 32 mm Plossl - say 3 degrees. The effective exit pupil will then be 4 mm so the sky will have similar brightness at slightly higher magnification.

 

3. If you switched to (say) an AT 20mm 100 deg eyepiece instead of a binoviewer, the magnification would be 25x and you would have a 4mm exit pupil. So that's good. You'd also have a true field of about 4 degrees. That's good too. The primary issue is that you might get a lot of coma at the edge of your field. You might want to check the Eyepieces forum for this.

Caveat - a large prism binoviewer will cost you at least $800 (used Denk II + Baader T2 diagonal + adapter + nosepiece) and you need at least 180 mm of available inward focus travel. You will also need two eyepieces ... so going for wide fields with binoviewers easily puts you on the wrong side of a kilodollar. It would be prudent to test your system at a club meeting.

Also, the so-called 'linear binoviewers' vignette badly with longer focal length eyepieces. They're easier to use but they won't get you wide fields.

 

A great reply. But we've gone 'off piste!'  

I was comparing binos to astro refractors and considering why a 4mm exit pupil in binoculars is less impactful to sky brightness than a 4mm exit pupil in an astro refractor.

I put this down to the binoculars acting like a spotting scope ie light is lost through the multitude of porro prisms.

So skies are darker in spotting scopes, binoculars and perhaps even binoviewers as there is less light throughput.

 

As I was already stopping down my telescope for wide views using the 30mm uff- I  was thinking that the reduction of light from using binoviewers might in effect do that for me - all be it with the same exit pupil size.

 

But I agree a 20mm ultra wide eyepiece is the way forward!

[Princess Leah]

One other point,

 

You state:-

 

'This is consistent with my own observations. - using a binoviewer I have detected the G & perhaps the H1 star in the Trapezium. Both were invisible to monocular vision. (TOA150 / Tak diagonal / Denk II) Sorry, I don't recall the magnification but it was probably around 220x because my shortest eyepieces are 5mm. I once worked out that this translated to ~6 photons per second reaching each retina. They couldn't be seen but they could be detected, suggesting that the brain has some sort of coincidence detection circuitry.'

 

We're you using the TOA150 with the binoviewers?

Edited by Princess Leah, 06 January 2024 - 08:51 AM.

[noisejammer]

One other point,

 

You state:-

 

'This is consistent with my own observations. - using a binoviewer I have detected the G & perhaps the H1 star in the Trapezium. Both were invisible to monocular vision. (TOA150 / Tak diagonal / Denk II) Sorry, I don't recall the magnification but it was probably around 220x because my shortest eyepieces are 5mm. I once worked out that this translated to ~6 photons per second reaching each retina. They couldn't be seen but they could be detected, suggesting that the brain has some sort of coincidence detection circuitry.'

 

We're you using the TOA150 with the binoviewers?

Yep, "TOA150 / Tak diagonal / Denk II".

[noisejammer]

A great reply. But we've gone 'off piste!'

I was comparing binos to astro refractors and considering why a 4mm exit pupil in binoculars is less impactful to sky brightness than a 4mm exit pupil in an astro refractor.

I put this down to the binoculars acting like a spotting scope ie light is lost through the multitude of porro prisms.

So skies are darker in spotting scopes, binoculars and perhaps even binoviewers as there is less light throughput.

 

As I was already stopping down my telescope for wide views using the 30mm uff- I  was thinking that the reduction of light from using binoviewers might in effect do that for me - all be it with the same exit pupil size.

 

But I agree a 20mm ultra wide eyepiece is the way forward!

Hmm - I evidently misunderstood your question.

 

There is negligible absorption of light through optical glass. It's common for the prisms to be made of BaK4 - if interested, you can look up its transmission for yourself.

A regular porro-prism set of binoculars will usually have a cemented doublet as the objective. This implies 6 air-glass interfaces compared with 4 for an ED doublet. This should barely be noticeable except the scopes coating will be better. And then there's the eyepiece - that could be anything.

 

There isn't enough info to decide what happens with your 80mm refractor. Assuming it's f/7, the exit pupils will be 3,6 & 4.2 mm respectively. The sky should be darker with the refractor - if it's not, it may point a finger at the coatings on your binoculars.

 

In any event. the 20mm UW would play better with your refractor than with a f/5 Newtonian.
 

[Princess Leah]

I thought there was no doubt the porro prisms/ half pentaprism in a spotting scope reduce the brightness.

 

Surely it can't just be the focussing lens and the waterproofing membrane. Can it?

That's why I assumed it would be the same with porro binos.

 

I have about twelve pairs of porro binos, mostly opticron and Pentax, my background is birdwatching/nature watching.

 

All are very good quality.

 

Every refractor I have used from cheap synta to my 80 f7ED works better (usually) with an exit pupil of about 3mm to 3.5mm max in the city for wide views 

However my porro binos work fine with a 4mm exit pupil.

Perhaps my eyes are just being fooled with the stars appearing brighter in the binos and therefore making the sky appear darker.

 

I think most people would agree astro refractors are more sensitive to light pollution/bright skies than binoculars.

I could be wrong though.

[faackanders2]

Thanks for explaining Rob.

 

I'm not suggesting light can be increased.

However in theory light shouldn't be reduced.

That's if the 1.4 theory of two eyes instead of one is correct.

 

The light input is halved in value when it splits.

But in the brain each half is increased to create a 2X increase in light overall.

 

Therefore no light is lost.?

I agree there is a rabbit out the hutch somewhere in my thinking!

 

Correction.

 

My theory would only work if the light was halved. In reality due to the prisms the light would be less than half when it splits.

When looking through a telescope all the light that enters the eyepiece goes into the single eye.

When looking through the the same telescope with  binoviewers half the light goes to each eye, so the brain does receive the same total of light (excluding inefficiency losses) but it is half to each eye.

 

To see larger or dimmer objects requires single eyepiece, if you can't see it all in binoviewers.

 

P.S.  Binoculars and binocular telescopes have two seperate apperture scopes and all light from each aperture enters each eye, but the brain get the sum of two aperture (one from each eye).

[Princess Leah]

binoviewers reduce the effective aperture by 1/Sqrt (Sqrt (2)) (~0.84). 

Edit: thus in line with Eddgie saying to use about 15-20% less power with binoviewers to get same brightness. What helps is that the brain will make binoviewed objects look bigger so using the lower power won’t actually decrease apparent size all that much.

0.84 seems much less a reduction than what Eddgie suggests above?

Did you mean 0.74?

[ABQJeff]

0.84 seems much less a reduction than what Eddgie suggests above?

Did you mean 0.74?

If binocular brightness gain goes as Sqrt (2) then it is 0.84.  If Binocular brightness gain is say 0.9 of this due to light loss in the binoviewer prisms, then you get 0.76.

 

In truth there are no exact numbers as it really depends on the individual and the equipment.  Obviously using two eyes thru two equal Apertures A will get no more than A effective = 2*A and no leas than A effective = A.  using binoviewers will at most allow A effective = A and at least A effective = 0.5 * A.  Between those simple limits is debatable and varies per person.  Test yourself on your equipment to see where you fall.  It’s fun!

 

Jeff

[Princess Leah]

What I mean is 0.84 reduction is much less than what Eddgie suggests?

Agreed?

[Princess Leah]

I think Eddgie is suggesting a figure of about 0.7. (56mm from a 80mm refractor was his example).

[Hwunkzeep]

I spent $500+ for an Orion linear binoviewer (I risk sounding like a shill here).

 

It delivers semi-circular exit pupils at 100% brightness, yet somehow you can look through either eyepiece and still see a full image (I still don't quite understand how it does this).

 

Believe me, two retinas (your eyes) beats ANY monoviewing mode in terms of brightness.

 

That said, binoviewing is a bit fussy.  It takes a bit more work.  But using both your eyes blows away anything you can do in mono... (in terms of a backyard telescope, in VISUAL)...