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How Binoculars and Telescopes Work (1 Viewer)

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/articl...oculars-will-give-you-maximum-star-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
 
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.

Regards,
B.
 
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.
 
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
 
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 Köhler, 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
 
NO, no, no!

This thread is not about perception, apart from the apparent "brightening" of an extremely small light source. I offered an explanation for that, which so far has not been refuted.

Individuals may have different "perceptions" of the setting on a vernier calliper, but their perceptions have no influence on the setting, let alone the function of the instrument!

John
 
John,

To be pedantic, everything you see, or rather imagine you see, is perception. A product of the senses. The brain's interpretation of a heavily processed and manipulated stream of electrical impulses from the eye. This thread has everything to do with perception, as it is intrinsic to how binoculars and telescopes work. They are functionless without it. Read #7 again.

David
 
David,

I am trying to discuss the function of the optical/mechanical entities of binoculars or telescopes. Perception has no more to do with that than the understanding of any other instrument or machine.

I question too, whether you have given my posts any consideration. I do not dispute the validity of much of #7, just its relevance. However, the final paragraph seems to be geting rather close to some conspiracy theories. If you must have the last word, be my guest.

John
 
John,

Perhaps we should look at this as two different philosophical viewpoints?

The first, the instrumentalist approach, regards a binocular as a functional device in it's own right. The optical properties of objective diameter and magnification can essentially explain the primary aspects of its performance. The second, the visual approach, does not consider they are function on their own. They are simply devices that alter the effective focal length and pupil of the eye. The eye is a major parameter contributing to overall performance.

Professionals and amateurs have devoted considerable considerable amounts of time to chacterising their performance in different ways either through individual observations or group studies. The product is often expressed in some mathematical forumula. When it comes using this information to explain how these devices work, much of the time, in scientific terms there is little difference in the two approaches. However in some situations, including low light and astronomy, there is a major divergence. Particularly over the need to explain why targets appear more visible with magnification. The instrumentalists need to add some amplification component to the basic laws of optics in order to explain it. The visualists say there is nothing special required, it's just what the eye does naturally. It's just how visual perception works.

David
 
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Limiting visual magnitude of white stars.

100mm aperture f/12 high quality doublet refractor.

Good conditions, experienced observer, good eyesight, dark adapted, averted vision, dark site.

20x 11.6 limiting visual magnitude.
35x 12.0
70x 12.5
85x 12.7
135x 13.1
200x 13.4

The 1.8 magnitude difference is about 5 times.

There will be variations, but not by much.
The eye pupil will be greater than the exit pupil at all magnifications.

20x will need a 60mm fl eyepiece. These are available in 2 inch barrel commercially.
200x will need very good conditions.
In northern Europe these are fairly common at 3 a.m.

The above table is a result based on many different telescopes of different sizes but reduced to a 100mm size.
It is also based on the reported results of many experienced observers, as well as personal experience.

Regards,
B.
 
I 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?

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.

For binoculars I use NELM + 2.5 Log(D) + 2.5 Log(M)
with NELM the naked eye limiting magnitude, D the objective size in cm, and M the magnification.
Picked up this formula several decades ago and it's easy enough to remember...
 
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