Starting out in astronomy and wondering what to buy as your first telescope? There’s a simple answer to that question: don’t buy one, buy two. We are of course talking about binoculars – a valuable tool in the armouries of most active observers.
There are hundreds of astronomical bodies that a pair of binoculars will bring into view for you.
Not only will they let you see many more objects than you can with your naked eye, but the detail and colour in those objects become a lot richer.
With binoculars, Brocchi’s Cluster actually looks like a coathanger and the Orion Nebula becomes a fantastically detailed painting of light.
The Milky Way is no longer a tenuous glowing band, but a knotted tangle of stars, interspersed with mysterious dark patches.
Albireo goes from being an ordinary-looking star that marks the head of Cygnus to an exquisite juxtaposition of gold and sapphire.
And you can easily see galaxies by the light that left them millions of years ago, when our ancestors were barely Australopithecines.
Binoculars are still suitable even if you want to do ‘serious’ astronomy.
There are variable star observing programmes specifically for binoculars, and their portability makes them ideal for taking to the narrow track where a lunar graze or asteroid occultation is visible.
Alternatively, you could wrap up warm, lie back on your garden recliner and just enjoy the objects that the binoculars let you find as you cast your gaze among the stars.
Before you realise it, you have begun to learn the sky and you’ll soon be able to navigate around it better than the entry-level Go-To telescope you nearly bought instead.
Best of all, you can have this complete observing system for two eyes for less than the price of one reasonably good telescope eyepiece.
What to look for
Binoculars are classified by two numbers that refer to their magnification and aperture.
So a 10×50 pair of binoculars has a magnification of 10x, and each of the objective lenses has an aperture of 50mm.
These numbers also enable you to calculate the size of the circle of light – or ‘exit pupil’ – that emerges from the eyepieces: all you have to do is divide the aperture by the magnification.
So a 10×50 pair of binoculars has an exit pupil of 5mm.
The exit pupil should be no larger than the dark-dilated pupils of your eyes: anywhere between 4-6mm is fine for your first pair of binoculars.
Larger apertures potentially show you more, but may need mounting if you want steady views over prolonged periods.
Common sizes are:
- 8×40, which almost anyone over the age of 10 can hold steadily
- 10×50, which most adults can hold steadily (this size is a popular compromise between size and weight)
- 15×70, which really needs to be mounted, although it can be briefly handheld
You should also check that the distance between the eyepieces, or ‘interpupillary distance’ will adjust to your eyes.
If you observe while wearing glasses, ensure that the binoculars have enough distance (‘eye relief’) from the eyepiece to your ideal eye position; 18mm or more should be fine.
There are two basic types of binoculars: Porro-prism and roof-prism.
In any price range, roof-prisms are lighter, but Porro-prisms tend to have better optical quality.
Once you’ve decided on size and type, get the best quality you can for your budget and start exploring the night sky.
Can I use any old binoculars?
In principle, yes: even plastic-lensed 4×20 toy binoculars can show you astronomical objects that you otherwise couldn’t see, such as the moons of Jupiter.
So, if you already have a pair of small binoculars, for example a 6×30 or 8×32 pair, try them out under the stars: you’ll be amazed at how much more you can see.
Even toy binoculars can give you a decent view of the night sky
The optical quality will also make a difference and you may find that there are things you can see with good-quality small binoculars like 8x42s that are beyond the capability of an entry-level 15×70.
But avoid zoom binoculars: good ones don’t exist.
Better than a telescope?
If your passion is planetary detail, close double stars, globular clusters or planetary nebulae, then you should consider buying a telescope.
But for the rest of the visible Universe, binoculars are the better option.
Setting up handheld binoculars takes a few seconds, and even mounted ones can be set up in a few minutes, so you’ll be observing long before your Go-To telescope-using buddies are ready to start.
You can also use them for impromptu sessions where it would be too much trouble to set up a telescope.
Many objects are ideally framed in the wider field of handheld binoculars: asterisms like Kemble’s Cascade or the Leaping Minnow overflow most telescope fields, as do large open clusters such as the Pleiades and the Beehive Cluster.
Even large faint objects like the Triangulum Galaxy and the North America Nebula can be easier to see in budget 10×50 binoculars than in amateur telescopes of several times the price.
Stephen Tonkin is the author of Binocular Astronomy