Before a telescope,
try binoculars.

Many people buy a telescope as their first astronomy purchase and are disappointed. Telescopes take time to set up, show a tiny patch of sky, and can be hard to point at anything. A pair of ordinary binoculars avoids all of those problems. They are quick to use, show a wide field of view, and present the sky the right way up. You may already own a pair that is good enough to start.

This field note explains what the numbers on a pair of binoculars mean, how to choose a pair for night use, and what to look at first.

Reading the two numbers

Every pair of binoculars is labeled with two numbers, such as 7x50 or 10x50. The first number is the magnification: 7x makes things appear seven times closer. The second number is the diameter of each front lens, called the objective, in millimeters.

For daytime use, magnification gets most of the attention. At night, the front lens size matters more, because the job is to collect as much faint light as possible. A larger objective gathers more light. Astronomy.com notes that 50 mm lenses gather about twice as much light as 35 mm lenses.

Exit pupil: the number that is not printed

Divide the lens size by the magnification and you get the exit pupil, the width of the beam of light that leaves each eyepiece. For 7x50 binoculars that is about 7.1 mm. For 10x50 binoculars it is 5 mm.

The exit pupil matters because your own pupil can only open so wide. In the dark, a young person's pupils can open to about 7 mm. That ability decreases with age, and Sky & Telescope suggests that a 5 mm exit pupil is a better guideline for middle-aged and older observers. If the beam from the binoculars is wider than your pupil, some of the light simply does not enter your eye, and you have paid for aperture you cannot use.

There is a second reason to favor a smaller exit pupil. Under a suburban sky, your pupils will not fully open anyway, and a higher magnification darkens the background sky while keeping stars bright. That makes faint objects stand out better.

7x50 or 10x50?

These are the two traditional choices for astronomy, and both are good.

• 7x50: wider field of view, easier to hold steady, and very bright images. A good match for young observers under dark rural skies.
• 10x50: a little more magnification and a smaller exit pupil. Often the better all-round choice for adults, and for observing from towns and suburbs.

Astronomy.com suggests about 10x as the practical limit for hand-held use. Above that, most people cannot hold the binoculars steady enough for a sharp view. Larger "giant" binoculars, such as 15x70 or 20x80, can show much more but really need a tripod.

Avoid very small binoculars such as 8x21 or 10x25 for astronomy. They are excellent for hiking because they are light, but their exit pupils are small, and the sky will look dim through them.

Holding them steady

Even small tremors in your hands are magnified along with the view. A few simple habits make a big difference.

• Brace yourself. Lean against a wall, a car or a fence post, or sit in a chair with arm rests and rest your elbows.
• Lie back. Looking high up is much easier if you are reclining. A garden lounger is one of the best pieces of astronomy equipment you can own.
• Hold near the front. Gripping the binoculars further forward along the barrels can reduce shake.
• Use a tripod adapter. Most binoculars have a threaded socket hidden under a cap at the front of the central hinge. An inexpensive L-shaped adapter lets you mount them on a camera tripod.

Before you start, focus carefully on a bright star. Many binoculars have a separate dioptre adjustment on one eyepiece. Focus the main wheel with that eye closed, then adjust the dioptre ring for the other eye until the star is a sharp point in both.

What to look at first

The best first targets are bright and large, because binoculars excel at wide views.

• The Moon. A crescent or half Moon shows craters and mountains along the line between light and shadow. A full Moon is flat and dazzling, so try a few days before or after. Our Moon field note explains the phases.
• Jupiter's moons. Up to four tiny points of light in a nearly straight line beside the planet, changing position from night to night. Read how to tell a planet from a star to find it.
• The Pleiades. A compact group of blue-white stars in Taurus, easy to see without optics but stunning through binoculars.
• The Orion Nebula. Below Orion's belt, a faint misty patch around a group of stars. It is a cloud where new stars are forming.
• The Andromeda Galaxy. From the northern hemisphere and a dark site in autumn, it appears as an elongated smudge. The light you see left it about 2.5 million years ago.
• The southern treasures. From the southern hemisphere, sweep along the Milky Way through Crux, Carina and Centaurus for rich star fields, the Carina Nebula and the globular cluster Omega Centauri. The Large and Small Magellanic Clouds are companion galaxies visible without any optics from a dark site.

Do not expect color or detail like the photographs in books and online. Most nebulae and galaxies look gray and faint to the eye. The pleasure comes from finding them yourself and knowing what you are looking at.

A strict rule about the Sun

Never point binoculars at the Sun or near it, even briefly, and even at sunset. The optics concentrate sunlight and can cause instant and permanent eye damage. Viewing the Sun safely requires a certified solar filter fitted securely over the front of each lens. Our eclipse safety field note explains the rules in detail.

Before you buy anything new

Try the binoculars you already have first. Even a pair of 8x42 birding binoculars will show Jupiter's moons, the Pleiades and the Moon's craters. If you enjoy the experience, upgrade to a 7x50 or 10x50 pair and a tripod adapter. Use the darkness planner to find a night when the sky is dark and the Moon is where you want it.