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Astronomy Binoculars

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Astronomy Binoculars

Postby Guest on Sun Dec 18, 2005 2:38 pm

I'm looking for a pair of binoculars to use mainly, but not exclusively, for astronomy (nebulae, star clusters, etc.). I don't want to lug along a tripod, so I expect something like the Nikon 7 X 50 would be a good choice. However, I see that I can get a pair of Canon 15 X 50 image stabilized binoculars for almost the same price. Since they have the same nominal light gathering power and the Canons are image stabilized, so that I wouldn't need a tripod, it appears that the only downside to the Canons is the smaller FOV. Have I missed something here?
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Postby opticsplanet.com on Sun Dec 18, 2005 3:14 pm

Hi

They both have the same size objective and are both quality optics, but the higher magnification of the Canon at 15x will result in considerably less image brightness, since the Canon will have a much smaller exit pupil (first number divided into second number) of only 3.3 compared to the 7 of a Nikon 7x50.

No big deal if you are targeting brighter objects and want to resolve more detail in globulars and open star clusters, but definitely a negative if you are trying to trace the outline of very faint nebulae or objects with low surface brightness such as M33. The smaller exit pupil of the 15x50 (3.3) will also result in less viewing comfort than the 7mm exit pupil of a 7x50 over long observing sessions.

In general, I recommend a binocular with solid optics, full multi-coating and a 5mm exit pupil or more for an astronomy binocular. It need not be designated as an astronomy model; in fact, many conventional 8x40s or 10x50s are good choices here. I personally prefer to handhold and go without a tripod and typically use 7-10x binos for nearly all my observing. Getting away from all the equipment and set-up is much of the fun for me.
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Postby Guest on Mon Dec 19, 2005 4:11 pm

Thank you for the reply. That is (of course) the same answer I've gotten from several textbooks I've referred to. There is, however, an element of that answer that I do not understand. Perhaps you can clear up my confusion. It seems to me that, since the magnification is the ratio of the entrance pupil to the exit pupil, we are taking the two systems considered to have the same entrance pupil (50 mm), but two different exit pupils. Since the two instruments have the same entrance pupil, don't they thus intercept the same angle cone of light from the object and just distribute it across different exit pupils? If the object is a star, and the eye images the entire exit pupil of the instrument onto the retina, isn't the irradiance on the retina the same in both cases?
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Postby opticsplanet.com on Mon Dec 19, 2005 4:38 pm

Hi

Image brightness, not magnification, is the ratio of exit pupil to entrance pupil, with entrance pupil referring to the opening (pupil) of your eye, not the objective lens of the binocular. There are upper and lower limits to image brightness, depending on the width of your entrance pupil at any given moment. but assuming your eyes are dark adapted and your entrance pupils are 5mm or more, an exit pupil of 7 will always appear brighter than an entrance pupil of 3.3.

One of the most basic laws of optics is that as magnification goes up, image brightness goes down. This applies to all optical instruments, whether binoculars, telescopes, riflescopes, microscopes, cameras, and so on. An easy way to demonstrate this is to zoom up magnification on any instrument that has a zoom eyepiece. The only way to compensate for this loss of brightness is to increase the size of the front lens correspondingly as you increase magnification.

All of this assumes, of course, that everything else - lens quality, optical coatings, glass type - is equal. When comparing two good optics such as the Canon and Nikon, however, we are safe ground as to comparisons.

Hope this helps. Thanks for posting such good questions.
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Postby Guest on Mon Dec 19, 2005 6:46 pm

Joanie,

I think I have it now. I was referring to the entrance and exit pupils of an afocal instrument (the binoculars), not the eye. The key is that when an extended source is considered, the irradiance in the image plane depends only on the solid angle subtended by the exit pupil. Thus, when the exit pupil of the instrument is less than the entrance pupil of the eye, the total energy incident on the retina is reduced as the ratio of the exit pupil of the instrument to the entrance pupil of the eye (i.e. goes down with increasing magnification). All of this refers to an extended, lambertian source and originates in the subtleties of the integration of the radiation from an extended source. For the case I had in mind, an unresolved point source (like a star), the illumination of the retina depends only on the aperture stop in the system, and not on its magnification (W.J.Smith, Modern Optical Engineering, 2nd Ed., pg.231). Thanks for you patience.
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Postby opticsplanet.com on Tue Dec 20, 2005 12:46 pm

Wish I had said all that, but if I had, I would probably be writing books instead of these humble efforts on our forum. LOL! Please believe me when I say it's been a pleasure. Definitely beats the usual "How far can I see?" questions I get this time of year.
----------------------
Your personal optics expert
Joanie (Jne) K
http://www.OpticsPlanet.com
Phone: (888) 263-0356
Fax: (847) 574-6820
opticsplanet.com
Site Admin
 
Posts: 4044
Joined: Tue Sep 23, 2003 11:18 am
Location: Prospect Heighs, IL


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