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How Binoculars and Telescopes Work

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Old Wednesday 11th September 2019, 20:56   #1
Tringa45
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How Binoculars and Telescopes Work

This may seem obvious to many, but I'll explain my reasons for posting later.

A telescope or binocular can be regarded as a "light-funnel" in which light is collected in a large diameter objective and channelled into a small exit pupil.
The magnification is the ratio of objective diameter to exit pupil diameter, so a 10x50 binocular would have a 5 mm exit pupil.

Used terrestrially this same 10x binocular provides a 10x angular magnification of an object in height and width and a solid angle gain of one hundredfold.
If the binocular were well designed and manufactured and tripod mounted it would potentially be able to relay one hundred times as much information of that object as the naked eye. We have an enormous light gain but no gain in brightness because the light is spread across that hundredfold solid angle and the brightness of the image would in fact suffer a little due to transmission losses.

Now binoculars and telescopes can reveal stars that are not perceptible with the naked eye. I'm not a hobby astronomer but recommend having a look at the night sky with binoculars even in an urban environment. Stars in our galaxy can be tens or even thousands of light years distant, so are essentially dimensionless light sources. Our 10x50 binocular with its 1963 sq. mm objectives collects parallel light rays from a star and delivers them to the oculars, where they emerge again parallel from 19,6 sq. mm exit pupils. We consequently have the potential of increasing the brightness 100x but with losses due to absorption (transmission losses) and discrepancies in eye pupil and exit pupil size.

I have held a long written discussion with another birdforum member of undoubted competence in some areas, who saw a contradiction in the reduced brightness of terrestrial objects through a binocular and the increased brightness of stars. I think this can be explained though by the fact that the binocular cannot bring the point light source of the star to a point focus but rather to a finite sized Airy disc. Viewed through the oculars this is a diffused image even if it appears as a point source to our eyes.

The topic is open to discussion.

John
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Old Wednesday 11th September 2019, 22:08   #2
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Caveat:I've done a fair amount of astronomical observing over the years, but will claim no deep competence with regard to the optical mechanics and 'rules'(?) of this discussion.


An infinitely small point source is more a theoretical description or concept, perhaps not a physically perceivable reality with our visual system....with or without optical aid. The airy disc assumption makes sense. Even if we see 'points of light', they probably take up some space on our retina.

Obviously a 1x glass optical system with an objective the same size as our own pupils could only reduce transmission. However as soon as we increase the size of the objective, and add magnification, something else occurs. We are collecting more light, and magnifying whats in the field of view, even though light transmission through the system is not 100%

How many 'pixels' on our retina need to be stimulated before the perception of light is triggered? Is it area based as well as based on the strength of the source? The OP's question I think pertains more to our own visual system than binoculars or telescopes. The scale of the visible universe may also be playing a role... ;-)

Consider also the source. A star generates light, just like our sun does. Suppose you point a 10x50 pair of binoculars at a reflected highlight of the sun on a glass telephone insulator, or at a welding torch 1/4 mile away? Could you damage your eye? whereas a welding torch viewed without binoculars at that distance would not? Collecting more light than the eye can normally, and focusing it on the retina in a small area can have consequences!

Extended objects, whether cosmic or terrestrial are exempt from this discussion I take it..

Interesting question, btw

-Bill

Last edited by wdc : Wednesday 11th September 2019 at 22:12.
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Old Wednesday 11th September 2019, 22:13   #3
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A 20x50 binocular will commonly show stars almost twice as faint as a 10x50. Even hand held.
This is thought to be because the background sky brightness is decreased and contrast increased. Stars are about the same.

With a telescope fainter and fainter stars are seen as the magnification is increased.
Limited eventually by I think by atmospheric turbulence.

So called theories that I have seen just don't work.
Empirically fainter and fainter stars are seen up to quite high magnifications.

Just saying that a 10x50 binocular shows 100 times fainter stars is not accurate.

But this discussion can go on for ever with different views from theorists and those who actually look through telescopes and binoculars at stars.

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Old Wednesday 11th September 2019, 22:39   #4
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Regarding looking at reflections of the Sun etc. or distant welding torches with telescopes.
Safety first. If in doubt, don't do it.

This whole topic takes a lifetime of experience.
Even well known astronomers have got things completely wrong, because basically they didn't know their stuff, but thought they did.
I had a well regarded paper rejected and had to send it to the U.S.A. where it was instantly taken up.
I have recently found out that my rejector has upset numerous people in the past and was eventually basically sacked.

I read about someone who advised people to look at the Sun for health reasons, and other demented souls advised the same to prevent actors such as Sir Roger Moore from squinting in movies. The actors suffered permanent eye damage.

So basically, don't look at the Sun or welders torches, but look at as many other stars as you want, unless perhaps a magnitude minus 12 supernova.

Also at as many extended objects at night as you want.

I spent years surveying the whole visible sky for deep sky objects from town with a 5 inch refractor.
Why, I don't know, Something to do I suppose.
Eventually, all I got was insomnia. (:

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Old Wednesday 11th September 2019, 22:58   #5
Mark9473
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Quote:
Originally Posted by Tringa45 View Post
This may seem obvious to many, but I'll explain my reasons for posting later.

Our 10x50 binocular with its 1963 sq. mm objectives collects parallel light rays from a star and delivers them to the oculars, where they emerge again parallel from 19,6 sq. mm exit pupils. We consequently have the potential of increasing the brightness 100x but with losses due to absorption (transmission losses) and discrepancies in eye pupil and exit pupil size.
The reality is closer to 50x fainter stars, which is difficult to explain by just transmission losses (I'm assuming the eye pupils are near the exit pupil).
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Old Wednesday 11th September 2019, 23:37   #6
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The last paragraph about the Airy disc doesn't make any sense - I would actually dare to say it makes negative sense because it purportedly contorts a simple issue with an irrelevant effect.

Thanks to the pupil problem, a telescope should never increase the observed surface brightness of an extended object, as it spreads the extra light over extra viewing angle. This doesn't matter for any object that is at a given magnification percieved by eye as a point, because there no increase in viewing angle happened. That's almost all there is to the fact that telescopes are so good at showing faint stars - the only other thing is that larger magnification (with exit pupil smaller than eye) dims the background, making this even more prominent.

Now you can see the backwards argument presented here - any effect that makes a star bigger than a point makes its observation worse, not better. That's why there are often diminishing returns with large telescopes because the atmospheric turbulence blurs the star - and also why from a certain magnification, the Airy disc works against you.

However there is an interesting thing that is still somewhat poorly understood - even though a telescope has no right to make extended objects brighter, it makes them easier to see, as evidenced by the endless riches of galaxies and nebulae visible with a telescope. Obviously, for objects that are so small that they would have been point-like to naked eye, this gain is clear, but it happens even for objects that are already extended - it is simply easier for our eye to see a very faint haze if said haze is really large. This has to do with the fact that what we see is very much not a faithful recording of the image created by the eye on the retina, but the result of huge post processing. I have repeatedly indulged in sessions of "retinal torture" trying to see faint extended nebulae through sizes of telescopes and it is often quite hard to be sure what exactly it is I am seeing, but this is definitely one of the greatest things to be doing under dark skies.
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Old Thursday 12th September 2019, 06:59   #7
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I’m afraid I’m the one that triggered the argument. I mentioned that while I understood telescopes and binoculars can make things not only visibly bigger, and perceptually brighter, I had a problem with the claims by some (actually many) that a passive optical device could somehow amplify, funnel or concentrate light into the eye. My limited understanding of the laws of physics say that is nor possible, and am not persuaded that stars, by nature of their small angular diameter can be any exception to that rule. That leaves me, or us, with a problem of how to explain why?

I’m no astronomer, but I have spent quite a bit of time trying to understand why terrestrial objects become more visible with magnification, and believe some of the same science applies to viewing stars. But first a bit of a ramble.

There now seems seems to be a growing acknowledgement that that binoculars and telescopes cannot make extended objects like galaxies (clusters of stars) brighter, only bigger, but few seem to concede it applies to individual stars. I’m convinced the same science applies. Of course stars are actually huge but their distance means their angular size is small. I believe Betelgeuse is amongst the biggest at about 0.05 arcseconds. The receptive angle of a single receptor in the fovea of the eye is about 20 arcseconds. If the light path was totally unperturbed, it’s image on the retina would only occupy 1/160000th of that of a single receptor, and that is for a visibly big star.

That’s the theory, but what really happens? If I step outside my back door and just look at the night sky, to me at least they appear to have different sizes. That means that between the atmosphere and aberrations of the eye the image has expanded to trigger more than a single receptor. The theoretical maximum acuity of the eye is approximately twice the diameter of the foveal receptors or about 40 arcseconds. With a fully dilated nocturnal pupil it is much, much worse. The studies I’ve seen suggests 240 arcseconds would be a very good result for bright stars. That means the retinal image of the star has now expanded to almost 6 million times the area. Very dim objects will need to be orders of magnitude bigger still to be resolved. As far as the retina is concerned stars don't look dimensionless, but their apparent size will depend on some combination of their original angular size and luminance, and be influenced by magnification.

To jump to another element in the story I want to give a brief mention to the eye's photosensitivity. The dynamic range of the eye is variously reported to be in the billion to trillion fold range, but as digital photographers will most likely be aware, that range needs to mostly be cut into 8 bit or 256 fold chunks to match the dynamic range of the eye for any particular scene. The eye adjusts to the center weighted luminance of the view by adjusting not only pupil diameter but also the sensitivity of the different receptor and by switching from low sensitivity cones to high sensitivity rods. The pertinent point is what happens to the light outside that range. The eye adjusts approximately to the average scene luminance. I know the sensitivity profile is not linear but for the sake of simplicity anything in the view 125x the average light level or more will be maxed out, and progress into glare. Anything 125x lower than average will appear black. Dim detail is lost and may be removed as system noise by visual image processing. Very small dim objects will be filtered from the view you think you see. The average light level and then dynamic range of the light within a scene make a big difference, but I'll let others discuss astro magnitudes. Magnification, by narrowing the view alters the visual, dynamics of a particular scene, and with it, the ability to see dim objects.

I’ll stop for now as I’ve probably already bored everyone to death, but there is a lot, lot more to the story if anyone is interested. Altogether it convinces me that the key to understanding space object visibility is the eye and magnification alone, not some mysterious light grasping effect.

David

Last edited by typo : Thursday 12th September 2019 at 15:50.
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Old Thursday 12th September 2019, 09:03   #8
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Hypothesis, individual opinion, theory.
None of this actually matters.

What one has to do is to get some skill at actually observing stars.

Then get an accurate star chart of visual magnitudes, say from magnitude 3 to 11 or 12.
I used one in Perseus, but I can't find it at the moment.

One goes to a reasonably dark place.
One takes 20 minutes to get dark adapted.
Then one sees what the faintest stars are with unaided eyes or distance glasses.
The eyes must be rested and relaxed.
One records all relevant details. Location, observers name, transparency, seeing, time in UT etc. etc.

Then one dose the same with the chosen binocular and then one sees the actual difference between binocular and unaided eyes.

This is repeated as many times as necessary until the results more or less agree.

That's it.
No hypothesis, personal opinion or theory.

The only problems are that some observers are not able to judge star magnitudes and different observers respond differently, so that the visual magnitudes may not be very accurate for them.

Photographic magnitudes are completely different.

When it comes to extended objects or comets, things are much more complicated.

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Old Thursday 12th September 2019, 09:23   #9
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With regards to limiting magnitudes.
Mount Wilson observers with a 60 inch scope I think or maybe a 100 inch reported a limit of magnitude 16 or so.

However, a very respected variable star observer gets to magnitude 16 or nearly 17 with a 16 inch scope from a city.
A very very good observer got to magnitude 19 with the Hawaii 24 inch scope high on the mountain. Probably using oxygen.

Imagers nowadays get to magnitude 20, even 21 with 14 inch scopes. They are very skilled.

Visual observers must make sure the stars they use are not variable when listing magnitudes.

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Old Thursday 12th September 2019, 12:54   #10
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I am trying to resolve an apparent contradiction, but some of this is getting very OT.

@ Mark & Binastro, I did write "potential" and qualified that with transmission losses and dilated pupils. I think too that the uneven light distribution in the Airy disc would play a significant role in reducing that potential.

Perhaps we can agree that a telescope cannot increase the surface brightness of an object. If that object were evenly illuminated, the illumination of its image as seen through the telescope would always be lower. Perhaps we can also agree that stars are essentially dimensionless light sources. No refractor ever built would be capable of resolving 0,05", let alone through the earth's atmosphere, even under ideal seeing conditions.

@ opisska, The image of a dimensionless star is focussed by a telescope objective to a diffused image (Airy disc). This is then viewed through the ocular (effectively a short focal length loupe) and will still probably be perceived by the viewer as a point source despite the distribution of light in the image. If you disagree with this, please supply your reasons instead of dismissing it as nonsense.

Lastly I would mention that many astronomical telescope manufacturers quote "Light gathering power" in their specifications. This is the squared quotient of the objective diameter and a dilated 7 mm eye pupil.

John
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Old Thursday 12th September 2019, 13:10   #11
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This is an interesting discussion you guys are having. Like David, I have been wondering about the very real perceptual observation that through binoculars and telescopes, dimmer stars look visibly smaller than brighter stars.

I could be wrong about this, but one explanation that presents itself comes from our brains not producing still photos as it were of whatever it is we are looking at, but rather processing a millisecond-to-millisecond flow of retinal information that is constantly changing. A fully developed Airy disk diffraction pattern consists of a rather large number of individual photons having taken their paths through the optical system and landing on their probability-determined receptors/film/sensor. If many fewer photons are received, not that many of them will have landed on the first diffraction ring let alone the further ones

A couple of days ago there was a story about some vision related neuroscience research published by Finnish researchers on mice that apparently alter their behaviour based on detecting just a few photons of light:
https://www.cell.com/neuron/fulltext...all%3Dtrue#%20

If human vision works anything like this, isn't it conceivable that the light intensity our eyes receive from the faintest stars we can detect is simply so low as to not being able to form anything like an Airy disk diffraction pattern on our retina?

- Kimmo
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Old Thursday 12th September 2019, 13:31   #12
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It is correct that the surface brightness in a telescope image is always less than with unaided eyes.
Unless one has an image intensifier.

I think that there is an insect that actually has an image intensifier in its eye system.

It has long been surmised that the best human observers can detect two photons of light.
One doesn't need mice except for experiments.

Skilled observers wait for very faint stars to appear. It can take twenty minutes of careful watching to see the faint star three or four times, which is what I take to be a confirmed sighting.

Using averted vision properly, which is a skill and an art, the eye/s can integrate up to 6 seconds for extended sources.

B.
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Old Thursday 12th September 2019, 13:58   #13
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The Light gathering power relating telescope aperture to a 7mm pupil is wrong.
However it is important to maintain this fiction, which has been used for decades, to provide consistency.

The manufacturers also don't tell you that their optical instruments vignet and the real aperture is often smaller than quoted.

The BAA Handbook used to quote a formula for limiting magnitude of 2+5log to base 10 D.
This was restated adding that the constant 2 could be replaced by a value between 3 and 4, particularly when higher magnifications are used.
This was as a result of my work, although I suggested a change including a formula for magnification, but they simplified it.
The Handbook no longer has any formula.

I have been involved with human vision in practice for over sixty years.

The fact that the eye/ brain system takes up a large part of our brain to sort it out means that such an enormous subject cannot be simplified into one or two pages or even one or two learned books.

B.
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Old Thursday 12th September 2019, 15:42   #14
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Kimmo,

The way the brain acquires an image and presents the information to form a mental picture, does not occur in quite the way most people would imagine. In a fairly wooly sense, for photopic vision you might describe the "video" frame rate varying between about 30 frames per second low resolution image and about 2.5 fps higher resolution. Mentally you may not be aware of the difference as what you think you see is actually a hybrid of new info, interlaced with old and remembered info.

At low light levels the receptors change from photopic cones to relatively low density rods, with a consequential drop in acuity. At very low light levels those rods neuronally reorganised into clusters, increasing the probability of photon detection, but reducing acuity significantly further still. The integration time for signal acquisition in substantially prologed, but I don't recall now by how much.

I believe single photon detection by the human eye has been convincingly demonstrated... statistically at least. The cone cells have a relatively low density so a photon is quite likely to miss a receptor all together, and if it does interact the electronic signal will not be propagated consistently by a single event. The neuronal clustering of the receptors increases the probability of multiple photon interactions and signal propagation. Forget Airy discs. Nearing that limit the resulting acuity is very low indeed. You would be lucky to spot a doorway from 4 ft away. .....sorry Kimmo, that's 1.22m. I've been contemplating whether we might be back on imperial by the end of the year.

David

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Old Thursday 12th September 2019, 23:42   #15
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This has caused much discussion and much confusion in dedicated astronomy forums. The laws of physics (optical engineering?) constrain and inform what we perceive, but the resulting experience of "seeing something" is more complex. I agree with Binastro that seeing is a skill that is developed. I have had a very skilled comet observer find something I could not see in my telescope. Once he found it, he taught me how to see it by telling me exactly where in the field to concentrate my attention, how to avert my vision, and then he lightly rapped the telescope tube to cause a slight motion.

As Binastro said, collecting data related to experience, in itself a challenging task, is absolutely necessary. Then we can see which explanations make sense and are consistent with all or most of the data. There is an extensive data set collected during World War II (Blackwell, R.H., Contrast Thresholds of the Human Eye, Journal of the Optical Society of America, v36, p624-643, 1946). I believe the goal was to determine requirements for binoculars and/or other optical devices for military use. They constructed a test theater and collected data from their test subjects to determine what they could and could not detect under different lighting conditions with emphasis on low light conditions. They varied the size of the object, the contrast (difference in surface brightness between object and background), and the background surface brightness.

The general conclusions were:
  1. Detectability depends on object size, contrast between object and background (percentage difference between object surface brightness and background surface brightness), and background surface brightness
  2. At a given background surface brightness there is a critical size, below which the detectability of the object depends only on the product of object surface brightness and object surface area. The critical size varies from below 1 arc minute for bright background to many tens of arc minutes for dark background.
  3. At a given background surface brightness, and with the same contrast, a larger object is easier to detect if it is larger than the critical size
  4. At a given background surface brightness, and a given size above the critical size, an object with greater contrast (more surface brightness relative to background) is easier to detect

In a book called The Visual Astronomy of the Deep Sky published in 1990, Roger Clark concluded from Blackwell's data and his personal observing experience that there was an optimum magnification (really a broad range of magnification) for studying extended objects at night (nebulae, galaxies, etc.) that depended on the surface brightness of the object, the size of the object, and the background brightness of the sky. There is a good overview with links to discussion (much nit picking and BB stacking) here.

My summary of Clark's analysis is that for a given aperture:
  • When you magnify you decrease surface brightness and increase object area the same amount so the product of the two is unchanged.
  • The background is always larger than the critical size so its surface brightness always decreases when you magnify and its area is limited by the field stop so you get less light from the background as you magnify.
  • Below the critical size mentioned in point 2 above, the magnified sky background is darker and the product of object surface brightness and object surface area are the same so detectability is better. That is a description of fainter point sources becoming detectable with increasing magnification.
  • Above the critical size, both the object and the background are darkening (decrease in surface brightness) at the same rate with magnification, so this leaves you with the same contrast and a darker view of your object, but the increase in size may well help you to detect the object and/or see more details.
  • Using the Blackwell data Clark balanced the improved detectability with increasing size against the worsened detectability due to darker view for extended objects and calculated an optimal magnification for a 600+ objects.
.
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Old Friday 13th September 2019, 05:49   #16
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I expect most here are familiar with Holger's paper on the work of Berek which also considers the Leinhos and Kohler's work as well as Blackwell's. These were all terrestrial studies, probably aimed at a military audience. Undoubtedly many of the core findings are relevant to Astronomy, as Clark and others have found, but I do get a bit uncomfortable at times when things are extrapolated beyond the original parameters and purpose of the original studies.

I've attached a copy of Blackwell paper.

David
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Old Monday 16th September 2019, 06:38   #17
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I had no intention of provoking this thread, but the topic is a good one, and it's a shame we haven't got further in addressing the title "How Binoculars and Telescopes Work". It's something that's been taxing me since I joined the forum, and has been the cause of heated discussions here and elsewhere. Not for the first time, it was my objection to a popular idea that a binocular or telescope could somehow amplify the light that caused the disagreement. I maintain that's impossibe, and the answer lies elsewhere.

My core premise is very simple. A camera lens has no functionality on it's own. A film or sensor is integral to how the device works. Binoculars and telescopes are no different. A sensor, in this case the eye, is required for functionality. You cannot claim to understand how a binocular or telescope works without understanding how the eye works. I'm sure others will disagree.

David
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Old Monday 16th September 2019, 18:52   #18
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David,

There seems to be a notion that, with the ability of a binocular or telescope to show a light source (star) brighter, there would have to be a net energy gain as in the illusion of perpetual motion. This is not so.

Before the introduction of electronically amplified hearing aids the hard-of-hearing often used ear trumpets. The sound waves (anologue to electromagnetic radiation) impinging on a large horn mouth (objective) were channelled into a small orifice in the ear (exit pupil).

Beethoven had one of these.

I am sure that it effected a genuine increase in sound pressure level at Beethoven's eardrum and that this had nothing to do with any peculiarities of his ear/brain interface .

John
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Old Monday 16th September 2019, 19:49   #19
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I thought I would follow up my last post with the assertion that all binoculars or telescopes "amplify" light, and that by the square of the magnification, reduced only by a factor of the percentage transmission and, of course, any vignetting in the instrument.

All objects, which can be viewed with binoculars are light emitters, whether they are light sources themselves or are reflecting sunlight or artificial light.

The light from an object falling on the objectives of a 10x binocular would be concentrated into exit pupils 1/10th. the diameter and 1/100th. the area. However an object subtending 1 at the eye would appear to subtend 10 through the binocular, which is a hundredfold increase in the solid angle.

The light gain is distributed over a larger apparent angle and can consequently not bring about an increase in surface brightness. There is in fact a reduction due to transmissin losses.

John
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Old Monday 16th September 2019, 22:07   #20
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Holgers paper?
http://www.holgermerlitz.de/bino_per...erformance.pdf

Peter
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Old Monday 16th September 2019, 22:10   #21
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No one has mentioned that the atmospheric turbulence typically smears stars across an arcsecond or more (depending on local heat sources, jet stream etc),”. Above about 8” and you start getting affected by atmospheric turbulence and the resolution doesn’t improve with aperture (Freid parameter), unless you have an expensive laser powered adaptive optics setup.

For seeing the most stars in a field of view the following might be useful:

https://www.cloudynights.com/article...tar-counts-r88

Don’t forget he power of binocular vision, enabling fainter little stars to be seen using a solidly mounted binoculars… close one eye and they vanish.
Binastro, II have always used 2.7+5log10(D)…. From Norton’s. Interesting the first number isn’t so carved in Stone! What was your suggested formula?
Unless you are good at counting I would avoid intensifiers, that can show vast numbers of stars in otherwise blank skies (using suitable filters), reminding you of the impact of light pollution. Intensifiers are like other imaging devices though and respond best when paired to very fast optics.

Collecting data on human optical perception is usually hindered by very small sample size as it’s hard to persuade sane people to spend hours in dark boxes looking at dim flickering lights. The standard human observer for colour perception is only based on a few dozen subjects and so not necessarily very representative of the spread actually found in society.

Interesting thread!

Peter
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Old Monday 16th September 2019, 22:17   #22
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I don't think that my 317mm Dall Kirkham at 1,000x magnification, which I have actually used, increased the light by 1,000,000 times.
Above 700x was empty magnification.
But at 600x, which was fine on Jupiter's moons, I don't think the light was increased by 600x squared.

It is more complicated than that.

As to theories etc. It is the theorists who probably feel that they have the answers.
Well, for me, it is actual observations, results and careful records that count.

I don't think that these two camps will ever agree.

It reminds me of the discourse about how many angels can stand on a pinhead.
Some say 24. Some say an infinite number and so on.
I think possibly one.

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B.
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Old Monday 16th September 2019, 22:31   #23
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Hi Peter,
Post 21.

Norton's formula is good.
I did try to find my formula that I gave about twenty years ago, but I haven't got the strength to look through my papers to find it. I can't remember it, but it had a factor similar to Norton's plus an adjustment for magnification.

8 inch maximum aperture is too pessimistic for best resolution.
There is I think a French 1 metre scope high on a mountain that does well, and I think that a 24 inch scope on a mountain, such as the one in Hawaii is fully capable of fine resolution.

The moments of highest resolution may be brief but good observers are able to take advantage of these.
Big Bear Lake 1.6m solar scope is very good, but does have adaptive optics. The Sun is of course difficult with all the heat.

Regards,
B.
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Old Monday 16th September 2019, 23:49   #24
Tringa45
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Quote:
Originally Posted by Binastro View Post
I don't think that my 317mm Dall Kirkham at 1,000x magnification, which I have actually used, increased the light by 1,000,000 times.
Above 700x was empty magnification.
But at 600x, which was fine on Jupiter's moons, I don't think the light was increased by 600x squared.
In post#1 I used the term potential.
Discrepancies between exit pupil and eye pupil (and secondary mirrors) will reduce that potential and exit pupils of 0,3 mm and 0,5 mm in the example above will drastically reduce that potential.

I was trying to present my interpretation of the function of telescopes and binoculars and to leave out the perceptual aspects. Driving skills are not relevant to the function of an internal combustion engine.

John
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Old Tuesday 17th September 2019, 06:28   #25
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I think it might be useful to step back a moment and consider what brightness actually means. It is a visual perception. It is an individual's personal evaluation of the relative luminosity of a target as detected by the retina and processed by the brain. An individual's physiology, experience and skills will vary, and so will the perception. It will, also change with magnification, as tens of millions binocular and telescope owners may well have noticed.

We have already mentioned in this thread studies by Berek, Leinhos and Khler, Blackwell and Holger's analysis, with relation to the terrestrial use of binoculars in low light. They have all produced tables, graphs and formulas that relate visual perception to light level and instrument design. Make no mistake, these studies primarily describe the functionality of the eye.

I know the astronomers here can offer a host of other names which have done much the same thing. Magnitude is possibly the best known. It is simply standardised scale of apparent relative brightness. A scale of visual perception.

Brightness is not the same as photometry, which the actual measured light level, though I'm as guilty as anyone in using it interchangeably at times. A binocular or telescope cannot increase surface emitted or reflected light (luminance) of a target, only reduce it, but magnification can increase it's apparent brightness. To explain how you need to consider the eye.

David
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