Society for Popular Astronomy

Using and choosing binoculars for astronomy

There are lots of uses for binoculars in astronomy, and even advanced amateurs keep their trusty binoculars handy. They can show many more stars than you can see with the naked eye, and yet have a wide enough field of view that you can recognise the star patterns you’re looking at. Unlike astronomical telescopes, they show objects the right way up and not upside down. And there are some objects that are actually best seen in binoculars – star fields in the Milky Way, and bright comets (should you be lucky enough to see one), for example.

The downside of binoculars is that they have limited magnification, so you can’t see any detail on the planets, and nor can you see the smaller and fainter deep-sky objects such as most galaxies. For those observations you definitely need a telescope. But in terms of usefulness they rank very highly – and the good thing is, you don’t need to spend very much on them. Around £50 will get you a perfectly serviceable pair, which should last you a lifetime. And of course you can use them for many other things than astronomy.

Which binoculars should you choose?

 

To start with, you need to get to grips with the terminology. Binoculars are described by two figures, such as 10×50 (pronounced ‘ten by fifty’). The first figure is the magnification, which is 10× or ten times, and the second figure is the size of the objective lenses in millimetres.

As with telescopes, the magnification depends on the eyepiece you use, though in the case of binoculars the eyepieces are not usually interchangeable. Magnifications are typically between 6 and 12 in everyday binoculars, though specialist binoculars may have magnifications of between 15 and 25 or even more. Invariably, as the magnification increases, the field of view goes down, though in the more costly models you can get a combination of a fairly wide field and a high magnification, within limits.

Although the physical size of the binoculars isn’t affected by the magnification, in order to provide a bright image it is usual for the higher magnification binoculars to have larger lenses.

Occasionally you will see zoom binoculars, with a range of magnifications. These are best avoided for astronomy, unless they are extremely expensive, because the optical configuration needed to provide the zoom mechanism is usually a compromise, and results in poor optical quality or a small  field of view, or both. The mechanism is also likely to suffer from wear or get out of line. Some manufacturers are notorious for offering binoculars with small apertures and extreme magnification ranges, often by mail order. No serious astronomer would buy them, let’s just say that.

So what’s best for astronomy? Here are some typical binocular sizes and their merits or demerits.

8×30 Lightweight and compact, easy to hold. Wide field of view but limited value for astronomy because of the comparatively low magnification and image brightness.

8×42 Somewhat larger but still fairly lightweight. Brighter image than 8×30s.

10×42 Higher magnification allows better views of small objects than 8×42s, while still comparatively light. Good for finding constellations in light-polluted skies.

7×50 Very bright image and wide field of view and often regarded as a night binocular for military use. But older users (over 40) may not be able to see the full brightness of the image because their pupils don’t open wide enough (7 mm needed) to admit all the light. Excellent for viewing star fields and the larger nebulae, but rather heavy for many to hold gazing at the skies for long periods of time.

10×50 A good all-round binocular. The higher magnification than 7×50s means that the field of view is smaller but the higher magnification allows the smaller objects, such as some galaxies, to be seen more easily. Again, heavy to hold for long periods but regarded as the standard binocular for astronomy. Some galaxies, such as M51 and M81/M82 become easily visible in reasonable skies.

12×50 Same weight as other 50 mm binoculars but the higher power allows fainter and small objects to be seen at a cost of smaller field of view. With the higher powers, it can be harder to know which objects you are looking at, so some practice is needed.

15×70 These come into the category of specialist astronomy binoculars. The higher magnification and larger size give excellent views of a wide range of objects, and the fainter objects such as the Crab Nebula and galaxies in the Virgo Cluster now become easy to see. But they are pretty weighty and you wouldn’t want to hold them up for any length of time unless you have particularly strong arms. Some models are excellent value, but binoculars of this size and larger are less useful as general-purpose daytime binoculars unless they are tripod-mounted.

20×60 Equivalent to 15×70s in many ways – the higher power makes small objects such as galaxies visible, despite the smaller aperture.

25×105 Monster binoculars, requiring a tripod. Don’t get these as your only binoculars, but as luxury items they can give superb views of a range of deep-sky objects and can even show a little detail on Jupiter and the rings of Saturn.

The field of view of typical binocular sizes, compared with the Pleiades cluster and the Moon


Holding binoculars steady

The steadier you can hold binoculars, the more you will see. Resting them on something seems like a good idea but is difficult when the object you are viewing is high up. In the same way, although most binoculars can be mounted on tripods, the standard photographic tripod gets in your way when you’re trying to observe objects nearly overhead.

Image-stabilised binoculars are costly but they can outperform other binoculars in their size class, when hand-held, as the stabilisation makes it possible to see much finer detail and fainter stars that you might otherwise miss as you try to view a jiggling image.

Using 25×105 binoculars on a purpose-built tripod. The binoculars have a 45º prism to avoid a cricked neck when viewing overhead, and the mounting has a counterweighted pivot to keep them clear of the tripod. But ordinary photographic tripods are not so convenient

Testing binoculars

If you get the chance to test a range of binoculars before you buy, a good test is to view a small object against a bright sky, such as a chimney pot or TV aerial. Look carefully for signs of false colour around the edges of the object. The more expensive models tend to have barely noticeable false colour, wider fields of view and very little image distortion. Check for flatness of field by looking at a wall where everything is the same distance from you. The view should be sharp from edge to edge.

The cheaper binoculars are more likely to have a less rugged construction. The prisms that reduce the length of the light path and bring the image the right way up can become misaligned through rough treatment, giving overlapping images that can be uncomfortable to view, and it usually costs more than the binoculars are worth to get them repaired, even if it is possible. However, for astronomy you needn’t buy the rugged binoculars that are recommended for field use, such as for bird spotting, which are often waterproof. Few astronomers observe in the rain!

Objects for binocular viewing

What can you see through binoculars?

With standard 10×50 binoculars and reasonable UK country skies (that is, away from city lights but not necessarily free from all light pollution) you should see the following types of object.

Stars down to about magnitude 9 or 10, which also includes the planet Neptune and the brighter minor planets such as Ceres and Vesta, which all appear starlike. Binoculars are also good for observing a range of variable stars.

From towns and cities, binoculars enable you to see the stars in the fainter constellations, such as Cancer and Aquarius, which might not otherwise be visible.

The bright planets are of course visible, though with no details visible other than the disc of Jupiter and its four Galilean satellites. Saturn appears elongated, but in honesty you can’t really see that it has rings as the gap between the rings and the planet is not visible. The phase of Venus is clearly visible when it is at half phase or a crescent. Binoculars are also great for locating Mercury and Venus, and indeed the thin crescent Moon, in a twilight sky after the Sun has set, and once you have found them with binoculars you will find it much easier to spot them with the naked eye.

The larger and brighter comets are visible. Each year there may be one or two which can be seen with binoculars, usually newly discovered ones, but don’t expect too much. A faint elongation of the fuzzy blob is often all you see of a tail, though comets are strongly affected by any light pollution and in a better sky you may see a lot more.

A good number of the larger star clusters, including most if not all of the 27 in the Messier Catalogue. The smaller ones, such as M103, are not very obvious, but there are a number of other clusters in the NGC which are visible.

The Andromeda Galaxy, M31, as seen through 15 x 70 binoculars by Michael Hezzlewood of Burnley

Globular clusters show as circular fuzzy blobs. Many of those in the Messier catalogue are visible, but you won’t see individual stars in them.

The Milky Way is transformed into countless stars, as long as your skies are good enough to be able to see it with the naked eye.

The brighter nebulae, such as the Orion Nebula, the Lagoon Nebula and the Omega Nebula, though these are more susceptible to light pollution than star clusters. The distinctive shape of each one is obvious in 10 x 50s. The North America Nebula is visible in good skies, though it is not very obvious.

One or two dark nebulae are visible, such as ‘Barnard’s E’, but not the Horsehead Nebula.

A number of galaxies, including the Andromeda Galaxy (M31), the Whirlpool Galaxy M51, M81 and M82. Some galaxies in the Virgo Cluster are visible, including M87 and M49, as small fuzzy blobs.

But binoculars are virtually useless for observing the aurora (Northern Lights), which covers a much larger area of sky than the field of view, and for meteors, which are gone as soon as you see them. Satellites are just a moving point of light which is difficult to track on account of the speed of their movement through the sky, and the International Space Station similarly appears as a point of light through 10×50s. You might see faint meteors and satellites passing through your field of view as you look at other objects, though.