Binoculars for Astronomy

Thinking about buying some binoculars to do some stargazing and not sure what sort are the most suitable?

When choosing binoculars for observational astronomy, you have to remember one crucial thing: there’s not a lot of light around. The most important thing your binoculars should do, therefore, is capture as much light as possible. Thus the aperture is the most important figure you should be looking for when shopping around.

A 7×35 binocular, for example, has a magnifying power of 7 (everything looks 7 times larger than normal) and an aperture (front objective lens diameter) of 35mm. Obviously, the size of the aperture limits how much light can be gathered—the bigger it is, the more light you will capture, and the more stars you will see.

ExitPupilAnother (less well know) factor you should take into consideration is the exit pupil. If you take light from a 35mm diameter objective lens, and magnify it by a factor of 7, it should come as no surprise that the resulting image has a diameter of (35 / 7 =) 5mm. This image is what you actually see in the eyepiece of your binocular.

Now, what’s interesting is that, on a bright day, your pupil may only have a diameter of around 2mm. But on a dark night, it may dilate to around 7mm. If your binocular has an exit pupil smaller than your own, then you aren’t seeing as much as you can possibly see.

A 7×50 binocular has an aperture of 50mm which is magnified by a factor of 7 so as to produce an exit pupil of (50 / 7 =) 7.1mm. As this is about the size of your own pupil, you’re not going to miss out on anything. (Note that if you live in the suburbs or otherwise use binoculars in a location which suffers from light pollution, your pupil may only dilate to 4, 5 or 6mm—not as much as it would under ideal conditions—so a 7.1mm exit pupil would be overkill.)

So, if you’re new to the game, I recommend that you get a binocular which has as large an aperture as possible and an exit pupil appropriate to the viewing conditions you are likely to encounter (if you’re unsure, or want to err on the side of caution, get one with an exit pupil of around 7mm). Many amateur astronomers start off with 7×50 binoculars—in case you were wondering.

Have fun!

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