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Binocular limiting magnitude


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Hi everyone,

I would just like to ask if I'm fully understanding this correctly.

With a set of 15x70 binoculars, the objectives are of course 70mm. When I calculate what apparent magnitude these should be for - mag: 11.93 - is that accurate? I used a telescope calculator. But I'm unsure whether or not the two lenses change or increase the amount it can see, or is it just mainly a comfort thing?

Many thanks

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6 minutes ago, AstroRuz said:

Hi everyone,

I would just like to ask if I'm fully understanding this correctly.

With a set of 15x70 binoculars, the objectives are of course 70mm. When I calculate what apparent magnitude these should be for - mag: 11.93 - is that accurate? I used a telescope calculator. But I'm unsure whether or not the two lenses change or increase the amount it can see, or is it just mainly a comfort thing?

Many thanks

They increase amount of what you can see - and even more than aperture alone would suggest.

It would stand to reason that 70mm binos are equivalent to 99mm telescope aperture (two 70mm openings have same surface as one 99mm circular opening), but in reality 70mm binos "see deeper". It has to do with our brain more than anything else. Brain has all sorts of interesting "features" - one of them is noise suppression. We never see photon noise, although we see levels of light at which that should be obvious (some sources say we can detect 7 photons / second flux).

Interesting feature of this noise suppression is that it is related to whether we see things with one or two eyes. It works better when we observe with both eyes - rationale being, if signal is coming from two sources it must be true rather than random noise. This means that threshold stars when using monocular vision will be often discarded as noise, but not as often when using binocular vision - we see deeper with two eyes.

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5 minutes ago, vlaiv said:

They increase amount of what you can see - and even more than aperture alone would suggest.

It would stand to reason that 70mm binos are equivalent to 99mm telescope aperture (two 70mm openings have same surface as one 99mm circular opening), but in reality 70mm binos "see deeper". It has to do with our brain more than anything else. Brain has all sorts of interesting "features" - one of them is noise suppression. We never see photon noise, although we see levels of light at which that should be obvious (some sources say we can detect 7 photons / second flux).

Interesting feature of this noise suppression is that it is related to whether we see things with one or two eyes. It works better when we observe with both eyes - rationale being, if signal is coming from two sources it must be true rather than random noise. This means that threshold stars when using monocular vision will be often discarded as noise, but not as often when using binocular vision - we see deeper with two eyes.

Thanks Vlaiv!

So I'm understanding it as; even though they aren't operating as a 99mm equivalent telescope. Being able to use both eyes actually increases what we see because our brain isn't discarding low level light fluctuations. Making the image to our eyes apparently brighter and nicer to look at? Some stars will be more obvious than monocular as when both eyes see it, the brain basically says "yep that's real, that's there"?

Built in noise reduction in our brain. Who'd have thought it!

Edited by AstroRuz
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22 minutes ago, AstroRuz said:

Thanks Vlaiv!

So I'm understanding it as; even though they aren't operating as a 99mm equivalent telescope. Being able to use both eyes actually increases what we see because our brain isn't discarding low level light fluctuations. Making the image to our eyes apparently brighter and nicer to look at? Some stars will be more obvious than monocular as when both eyes see it, the brain basically says "yep that's real, that's there"?

Built in noise reduction in our brain. Who'd have thought it!

Yep, that sums it pretty nicely.

I was also surprised to learn about noise reduction - but it sort of makes sense when you compare imaging and visual. In images we always have noise and it is obvious - but in visual you never see noise. Things do pop in and out of view when of threshold brightness - yet, we never see "speckles of noise" on those objects - brain just smooths things out for us.

I only once saw noise visually and it was very strange experience. I was playing with Ha filters in particular conditions. It was summer time so outside was bright sunny day and I was inside my house - pretty shaded to keep the heat out. I held Ha filter against my eye and looked outside at sunlit landscape.

Image looked like watching it on old CRT TV set with poor reception - very noisy with constant little flashes in deep red color. I guess that brain "turned off noise reduction" because of amount of ambient light - and Ha filter blocked enough of light for noise to become obvious.

Another good comparison is view thru night vision device where noise is also visible.

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With a set of 15x70 binoculars, the objectives are of course 70mm

You'd have though so, wouldn't you? But they often aren't. The most common 15x70s - UP BA1s (branded as Celstron Skymaster, Revelation, etc) have an effective aperture of 62mm - there is a fixed iris at the entrance to the prism housing that cuts it down, so as to reduce aberrations and sharpen the image, at the cost of image brightness.

The binocular noise reduction (aka "physiological summation") is analogous to "stacking" in astro imaging. A combination of physiological and statistical summation gives you a factor of about 1.4x; this is borne out in practice

It's very difficult to state what the limiting magnitude of any binocular (or telescope for that matter) will be, because there are so many variables:
* The sky conditions
* The observer's eyes
* The star's colour
* The transmission factor of the instrument
* Direct or averted vision
* The magnification used (increasing magnification fort any given aperture darkens sky background, hence increasing contrast and making fainter stars detectable)

A lot of work was done on this in the early 20th Century; it's neatly summarised in Sidgwick's Amateur Astronomer's Handbook (1954). At one extreme, WH Steavenson was able to observed a mag 11.9 star with a 1.7" (43mm) refactor; based on that, the crude calculation for a single 70mm glass would give you a LM of around 12.8, and for two of them (ie 70mm binoculars) about 13.7. We should be so lucky! 🙂

 

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