how to… choose binoculars for astronomy

Ensure you make the right choice when buying kit

Binoculars make great 'grab-and-go' instruments for the astronomer. Depending on the size and type, they can show you many craters on the Moon, the moons of Jupiter and the grandeur of the Milky Way. In fact, there are many deep-sky objects that look better through binoculars than through some telescopes!

When choosing binoculars for astronomy, the important parameter is the aperture – or the diameter of the front lens. For comfortable hand-holding you don't want them too big, so a good size is 50mm or thereabouts. Magnification is less important, but should be somewhere between 7x and 10x. These, combined, give the number you frequently see inscribed on binoculars, for example 10x50 is 10x magnification and 50mm aperture. The ratio between magnification and aperture provides the field of view, or how much area of sky you can actually see through them. With 10x50 instruments, it is usually around 5 degrees.

The magnification also affects the amount of light entering the pupil of your eye through something known as the 'exit pupil'. This can be seen as a circle of light at the eye lens if you hold the binoculars at arm's length in the daylight. Ideally, you want this exit pupil to match the diameter of the pupil of your own eye once it has become dark adapted. In older adults, this doesn't get much larger than about 5mm, although it can be as much as 7mm in youngsters. As you can see, 10x50s giving an exit pupil of 5mm are ideal. That is not to say that you shouldn't use 7x50 or any other similar specification though.

Optical coatings are important for good light transmission as well, so it's best go for binoculars which state that they are fully multi-coated. When you are looking at faint objects, as most things in the night sky are, you want as much of the light to reach your eyes as possible. If you wear spectacles, you will also need something called 'long-eye relief', which means that you won't have to remove your glasses in order to use them for your night-sky tours.

"There are many deep-sky objects that look better through binoculars"