Binoculars for Astronomy

Did you know that a beginning study of the stars can be made with just a pair of astronomy binoculars? Most beginners believe that to study astronomy requires the use of a telescope, and the general rule seems to be the bigger the better. In fact many amateur, and even some professional, astronomers wouldn't dream of dispensing with their binoculars. Certainly they also have their telescopes for major viewing, but many find themselves using their binoculars far more often simply because of portability and ease of use.

What can you see with just binoculars?Earth's MoonCredit: Nasa Public Domain

Some pretty amazing stuff, actually. You can find star clusters, galaxies, and make a pretty decent study of the moon's craters. And if you find your hands or arms growing tired of holding the binoculars, simply invest in a cheap tripod to free your hands.

What are the advantages of using binoculars for astronomy rather than a telescope?

  • They are lighter and much more portable. Many an amateur astronomer has a telescope in their closet that is simply gathering dust because they simply don't want to take the time to drag it out and set it up. In the time it takes to get your telescope set up for night sky viewing, you could have already been studying the heavens with your binoculars.
  • They are less expensive. You can find a decent pair of astronomy binoculars for under $100.00. (And in a pinch, you can use your everyday outdoor type binoculars, too.)
  • Binoculars give a much wider view than a telescope. This makes celestial objects much easier to find. For a beginner, finding an object in the night sky can be trying with a telescope.
  • With telescopes you are limited to a one-eye view, which naturally distorts your ability to see clearly. Using binoculars for astronomy allows you to see what you are looking at much clearer.

What do you look for in a good pair of astronomy binoculars?

On the back of almost all binoculars, you will find two numbers. Example: 10x50. The first number represents the magnification of the binoculars, the second number tells the aperture, or size of the front lenses measured in millimeters.

At first glance you would think that the higher the magnification, the better. However the truth is that with binoculars, once you get past 10x, they become pretty hard to hold steady without a tripod. There is a lot of debate about what magnification is best for stargazing, but if you stick with 7x, 8x, or 10x, you should do okay.

Binocular Description PlateCredit: Wikimedia CommonsNow we add in the second number, the aperture. Here we do a very simple calculation to come up with something called the exit pupil. To determine the size of the exit pupil you simply divide the aperture by the magnification. So an 8x56 pair of binoculars would have an exit pupil of 7-mm.

This is an important number to consider when buying your binoculars. The exit pupil should fit inside the pupil of the users' eye, and in the dark not everyone's pupils dilate to the same size. The general rule is that people under 30 or so need an exit pupil of around 7-mm. As you get older, you need a smaller one, such as a 5-mm exit pupil. So the younger crowd might want to consider an 8x56 or 7x50 pair of astronomy binoculars, and the older astronomers may be better off with a 10x50.

Lastly, when looking for a good pair of binoculars for astronomy, look at the specification sheet to determine what prism glass was used. A quality prism glass will ensure that you get all the light you should. Look for BAK-4 glass, rather than BK-7 glass. It does make a difference.

Why wait until you can afford a telescope to study all the heavens have to offer? Start viewing the night sky now with a pair of astronomy binoculars!