telescope binoculars - advice

[pmp613]

Hi folks-
Pardon a newbie, but I wanted some advice, and I hope this educated community can help out.

In considering binoculars for night sky viewing - what are the tradeoffs to take into consideration? (Let's limit the discussion to binoculars that can be relatively easily transported - say up to 100 MM objective.) It's not clear to me what combination of factors is going to give me the best experience - is there a formula for magnification/exit pupil/etc. to help figure this out?

Many thanks,
Ted

[Chris Peterson]

In considering binoculars for night sky viewing - what are the tradeoffs to take into consideration? (Let's limit the discussion to binoculars that can be relatively easily transported - say up to 100 MM objective.) It's not clear to me what combination of factors is going to give me the best experience - is there a formula for magnification/exit pupil/etc. to help figure this out?

As with any telescope, all things being equal, there is a significant increase in performance as you increase the aperture. When you buy binoculars, the primary specs you get are aperture and magnification. The vast majority of people using binoculars for astronomy do so for their convenience, meaning they want small enough instruments that they can be comfortably hand held, and low enough magnification that the view is relatively steady. Binoculars around 8X50, sometimes 10X50 are popular for astronomy. So are image stabilized binoculars with somewhat higher magnification.

Anything much more, either in aperture or magnification, will require some sort of mounting system. Certainly, people do that, but this is now a specialty instrument and you need to consider whether a more conventional telescope would be a better choice.

So the primary tradeoffs are weight against aperture (brighter image), and magnification against hand held steadiness. Fortunately, there are a large number of targets that require neither a large aperture nor much magnification.

[pmp613]

Chris-
Thanks for your reply. And - full disclosure - I have opened up an online store selling telescope binoculars (i am not going to post the url here) and am trying to learn more to serve my customers better. And, I also freely admit to not being very knowledgeable.

I personally prefer using binoculars - I find them more comfortable for extended viewing, even if I employ a tripod, or steady them in some other way.

When you say "Aperture" - there are two apertures - the objective lens and the exit pupil measurement. Is there any kind of ratio or other calculation that can be derived from this, including the magnification as well, to help guide someone who wants to make a purchase decision?

I would love to be able to present a formula of some sort:
exit pupil X * objective lens Y * magnification Z - you can count the craters on the moon.
exit pupil X * objective lens Y * magnification Z - you can count the rocks in the craters...

Thanks for your patience, and I hope I haven't violated any of the forum rules.

Ted

[Chris Peterson]

When you say "Aperture" - there are two apertures - the objective lens and the exit pupil measurement. Is there any kind of ratio or other calculation that can be derived from this, including the magnification as well, to help guide someone who wants to make a purchase decision?

In normal usage, "aperture" always refers to the entrance or objective size.

The exit pupil is simply the entrance aperture divided by the magnification. There is limited value in increasing the aperture of a telescope beyond the point where the exit pupil is larger than the pupil of the observer's eye. For astronomical viewing, that value is usually taken as 7mm (but does vary somewhat between different people).

So your basic 7X50 binoculars, probably the most popular for astronomy, have an exit pupil of 7.1mm- just right. There would be no point to making 7X binoculars with a larger aperture, because the additional light would be wasted. If you wanted some big, 100mm aperture binoculars, you would want at least 100mm/7mm = 14X or higher. Of course, such binoculars couldn't practically be used handheld- they would be too heavy, and too hard to hold steady.

How much magnification people require depends on their observing goals. You could specify an "ideal" binocular by first determining the desired magnification, and then figuring out what aperture, used at that magnification, gives a 7mm exit pupil. You could calculate magnification by considering that the unaided eye has a resolution of about one arcminute (the Moon is 30 arcminutes across). So at 7X, you could see features, like craters, that are 1/7 of an arcminute. But most people don't look at low power optics this way. When it comes to binoculars, it is common to simply consider what magnification you can use and still keep the image steady. That's around 7X for most people, although certainly a few can go higher and still keep things steady.

[harry]

G'day from the land of ozzzz

I'm thinking of building a quad system telescope.
With a computer control system.
Its in the pipe line for now.

Now I have to learn how to do it.

[BMAONE23]

here are some possibilities
http://www.foothill.net/~sayre/Binocular.htm
http://en.wikipedia.org/wiki/Large_Binocular_Telescope
http://www.popularmechanics.com/home_jo ... 20175.html

[harry]

G'day BMAone23

Thanks mate for the links.

[pmp613]

Chris-
Thanks again for your thoughtful replies.

So, then what is the difference between a 100mm aperture with a 14X magnification, and a 25X? What is the meaning of the smaller exit pupil? If I understand you correctly, the larger magnification is less light efficient, and if one was looking at a dim area, there would be less light getting to the viewer's eye. On the other hand - would higher magnification compensate in some way?

Ted

[Chris Peterson]

So, then what is the difference between a 100mm aperture with a 14X magnification, and a 25X?

The difference is simply the amount of magnification. Either will effectively use the full aperture, since in both cases the exit pupil is 7mm or smaller.

What is the meaning of the smaller exit pupil? If I understand you correctly, the larger magnification is less light efficient, and if one was looking at a dim area, there would be less light getting to the viewer's eye. On the other hand - would higher magnification compensate in some way?

A smaller exit pupil isn't generally significant. What matters is that when the magnification becomes too low, the exit pupil becomes larger than the pupil of the eye. At that point, the image no longer gets brighter as the aperture size increases. For a given objective, as you increase the magnification the exit pupil gets smaller, but the amount of light entering the eye isn't reduced (higher magnification isn't "less light efficient"). The apparent brightness may decrease, because the available light is spread across more of the retina, so the number of photons per unit area goes down. That's a perceptual issue which affects different viewers in different ways. With many objects, the effect is partly offset by using higher magnification, which decreases the impact of sky glow, and allows more of the retina to be involved in detecting detail.

[pmp613]

Chris-
Thanks again for your patience. I certainly understand more now. I freely admit to rereading your posts several times before it sinks in.

BTW, I went out to your site. That observatory looks like a real labor of love.

Ted