How To Choose Binoculars for Astronomy

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Binoculars are ideal instruments for beginner astronomers. They are portable, easy to hold steady, and you can comfortably see most celestial bodies with them.

Your binoculars will have a pair of numbers associated with them, like 10 x 50, 7 x 50 or 7 x 35. 

The first number is the magnification. This tells you how zoomed in you are – that is, how much closer the object appears than it is. So for a 7 x 35 pair, objects look 7 times closer than they are. As magnification increases, shakiness increases and brightness decreases. You can try this with your phone camera – hold it up in the air with your hand, and zoom in as far as it lets you. You’ll notice that the image gets really unsteady, and may get dimmer. This is a problem with high magnifications – anything above 10x will need a tripod to keep steady.

The second number is the aperture (in mm). Aperture tells you how big the lens is, and therefore how much light it can collect. A 7 x 35 pair has lenses that are each 35 mm across. As the aperture increases, each lens gets bigger and disproportionately heavier. An aperture of above 50mm makes binoculars significantly heavy to hold up for long periods.

The ratio of aperture and magnification are constrained by a third quantity – the binoculars’ exit pupil. The exit pupil of the binoculars is aperture/magnification.For a 7 x 35 pair, the exit pupil is 35/7 = 5mm. Your eyes have an exit pupil too – this is the maximum size your pupils dilate to in darkness. For proper viewing, the binocular exit pupil needs to roughly match the exit pupil of your eyes. Depending on your age, your exit pupil will fall between 4mm and 7mm. A 5mm pair is generally a safe bet for most people.

A good starting pair of binoculars is a 7 x 35 or 10 x 50 pair. You can find these at almost any camera/video equipment store. If you want a smaller lighter pair that you can hold up easily, pick 7 x 35s. If you want more magnifying power and don’t mind the weight, go with a 10 x 50 pair. Initially you don’t need to invest too much money into them – any pair will do as long as they are sturdy and you can see through them comfortably.

If you do consider it a worthwhile investment and you know you’ll use them often, Bushnell Falcon 133410 Binoculars with Case (Black, 7×35 mm), Nikon 8248 ACULON A211 10×50 Binocular (Black) or the more expensive Pentax SP 10×50 WP Binoculars (Black) come highly recommended.


Other Useful Articles:

How To Focus and Aim Your Binoculars

Why You Shouldn’t Buy a Telescope First

 

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