Start with what you’ll use
A large telescope that takes twenty minutes to carry out and set up will show more than a small one, on the nights you use it. A pair of binoculars by the door will be used far more often. Experienced observers own both, and most of them started with binoculars. The aim of this guide is to help you choose the instrument that matches what you want to see and how you live.
What binoculars do well
Binoculars show a wide piece of sky, typically 5 to 7 degrees, upright and the right way round, with both eyes. That makes them the easiest way to find things and to learn the sky. They gather much more light than your eye, roughly 50 times more for 50 mm lenses, so they reveal hundreds of stars you can’t otherwise see.
Good binocular targets include:
- The Moon, especially craters along the line between day and night.
- Jupiter’s four large moons, if you brace the binoculars against something steady.
- Star clusters such as the Pleiades, the Beehive and the Double Cluster.
- Bright nebulae and galaxies: the Orion Nebula, the Lagoon Nebula and the core of the Andromeda Galaxy.
- Sweeping the Milky Way from a dark site, which no telescope does as well.
What they can’t do is magnify much. Saturn’s rings, the cloud belts of Jupiter and most double stars need a telescope.
Reading the numbers
Binoculars are labeled with two numbers, such as 10×50: the magnification, then the diameter of each front lens in millimeters. Dividing the second by the first gives the exit pupil, the width of the beam of light reaching your eye. In the dark your pupil opens to about 7 mm when you’re young and 5 mm or so later in life; light outside that is wasted.
| Size | Exit pupil | Notes |
|---|---|---|
| 7×50 | 7.1 mm | Bright, wide, steady; ideal for younger eyes |
| 8×42 | 5.3 mm | Light and versatile; good for travel and daytime use |
| 10×50 | 5.0 mm | The classic astronomy size; more detail, a little more shake |
| 15×70 | 4.7 mm | Impressive views, but needs a tripod |
What telescopes do well
A telescope’s most important number is its aperture, the diameter of the main lens or mirror. Aperture decides how faint you can see and how much fine detail is possible. Magnification comes from the eyepiece and can be changed, but it can’t add detail that the aperture doesn’t collect.
- Refractors use a lens. They are rugged, need no maintenance and give crisp views, but large apertures are expensive.
- Reflectors use a mirror. They give the most aperture for the money. The Dobsonian design, a reflector on a simple rocking base, is the classic recommendation for a first serious telescope.
- Maksutov and Schmidt-Cassegrain telescopes fold a long focal length into a short tube. They are compact and excellent on the Moon, planets and double stars, with a narrower field of view.
The mount matters as much as the tube. A wobbly mount makes high magnification useless. Simple altitude-azimuth mounts move up-down and left-right and are the easiest to learn; equatorial mounts track the sky’s rotation with one motion and are more useful for photography. Computerized “GoTo” mounts find objects for you but need power and alignment.
What you’ll actually see
Astronomy photos are long exposures, often hours, processed to bring out color. Through an eyepiece, the Moon and planets are sharp and genuinely thrilling, star clusters glitter, and nebulae and galaxies appear as gray glows, because our eyes don’t see color in faint light. The brightest galaxies show a core and perhaps a hint of structure from a dark site. Knowing that in advance turns disappointment into appreciation: that gray smudge is light that left another galaxy millions of years ago.
Traps to avoid
- Big magnification claims. A box promising “675×” on a small telescope is a warning sign. The useful maximum is about twice the aperture in millimeters, and on most nights the atmosphere limits you to 150–250× whatever the telescope. Try the magnification calculator.
- Spindly tripods. If the view shakes for seconds after you touch the focuser, you’ll spend the night waiting instead of observing.
- Too big to carry. Lift it, mentally carry it down your stairs, and imagine doing that on a cold night after work.
- Zoom binoculars and very small binoculars (under 30 mm) are poor for astronomy.
A sensible first choice
| Your situation | Start with | Why |
|---|---|---|
| Curious, not sure yet | 7×50 or 10×50 binoculars | Cheap to get right, useful forever, even after a telescope |
| Moon and planets from an apartment | 90–127 mm Maksutov, or an 80–102 mm refractor on a solid mount | Compact, quick to set up, sharp at high power |
| Deep sky, with a yard or a car | 150–200 mm Dobsonian | The most aperture for the money and the effort |
| Travel and dark-site trips | 8×42 binoculars | Small enough to always pack |
Whatever you choose, learn the sky with your eyes and binoculars first; stargazing for beginners is a good place to start. And remember that most of what you can see depends on your sky: why can’t I see it? explains how.