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How Binoculars and Telescopes Work

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Old Tuesday 17th September 2019, 11:10   #26
Tringa45
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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
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Old Tuesday 17th September 2019, 13:04   #27
typo
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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
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Old Tuesday 17th September 2019, 19:17   #28
Tringa45
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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
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Old Wednesday 18th September 2019, 06:28   #29
typo
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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

Last edited by typo : Wednesday 18th September 2019 at 07:06.
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Old Wednesday 18th September 2019, 15:30   #30
Binastro
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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.
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Old Saturday 21st September 2019, 09:53   #31
Mark9473
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Quote:
Originally Posted by wllmspd View Post
I have always used 2.7+5log10(D). From Nortons. Interesting the first number isnt so carved in Stone! What was your suggested formula?
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Originally Posted by Binastro View Post
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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Old Saturday 21st September 2019, 15:01   #32
Binastro
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Hi Mark,
That seems to be a reasonable formula for binoculars.

Regards,
B.
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