The sky — astronomy

Binoculars or Telescope: Where to Start

For most beginners, a pair of ordinary binoculars (for example 7×50 or 10×50) is the best first instrument: easy to use, and enough to show lunar craters, Jupiter’s four brightest moons and star clusters. A telescope pays off when you want finer detail, such as Saturn’s rings. What matters most is aperture, the diameter of the main lens or mirror, not the advertised magnification.

Almost everyone who starts looking up eventually asks what to buy. The answer depends on your goals, but there is a solid starting point: binoculars.

Why binoculars make a good start

Binoculars are simple. You hold them with both hands, look with both eyes and see an upright image. There is no assembly, no mount to balance, and they are ready in minutes. Their wide field of view also makes it much easier to find your way around the sky.

You will often see numbers such as 7×50 or 10×50 printed on them. The first number is the magnification (how many times larger an object appears); the second is the diameter of the front lenses in millimetres. With such a pair you can see:

• the craters and dark plains of the Moon;
• the four brightest moons of Jupiter, which Galileo recorded in 1610 with an even simpler instrument;
• the star cluster known as the Pleiades, spread out as a rich field of stars;
• star clusters, some nebulae and the Andromeda Galaxy as a faint smudge;
• the dense star fields of the Milky Way.

Binoculars stay useful for life: even owners of large telescopes use them for a quick sweep of the sky. If you are not sure the hobby will suit you, they are the lowest-risk first step. For tips on getting started, see How to Look at the Sky.

The main types of telescope

Telescopes gather light with either a lens or a mirror. There are three broad families:

Refractor. It uses a lens as its objective. The tube is sealed, images are crisp and maintenance is minimal. For the same diameter, though, it costs more than a mirror-based design. It suits the Moon, planets and double stars.

Reflector. It uses a concave primary mirror. The best-known layout is the Newtonian. It offers the largest aperture for the money, which is why amateurs like it. Occasionally the mirrors need collimation (alignment).

Catadioptric. It combines lenses and mirrors (for example the Schmidt-Cassegrain and Maksutov designs). The tube is short and portable, but prices are generally higher.

Besides the tube, the mount matters too. A Dobsonian, a simple rotating box-and-base design, is popular with beginners because it is steady and easy to point. A wobbly mount can ruin excellent optics, because at high magnification every vibration is amplified.

Aperture versus magnification

The most common mistake is buying by advertised magnification. Magnification depends on the eyepiece (the interchangeable lens you look through) and can be changed. The real limit is aperture: the diameter of the objective lens or mirror.

Aperture controls two things:

1. Light grasp. It grows with the area of the objective, so with the square of the diameter. A 100 mm aperture collects about four times as much light as a 50 mm one.
2. Resolution. A larger aperture can separate finer detail, such as close double stars.

As a rule of thumb, amateurs consider the maximum useful magnification to be about twice the aperture in millimetres. An 80 mm telescope therefore works sensibly up to about 160×. Beyond that the image is bigger but dimmer and blurrier. This is why claims such as 500× on a small telescope say nothing about quality.

Binoculars have one more number to know: the exit pupil, the width of the beam of light leaving the eyepiece. You get it by dividing the aperture by the magnification, so 10×50 gives 5 mm. A dark-adapted human pupil opens to roughly 5–7 mm, and it tends to narrow with age.

Realistic expectations

Space-telescope pictures and long-exposure astrophotos do not look like what you see at the eyepiece. The eye cannot accumulate light over time, so galaxies and nebulae appear as faint, greyish patches. That is normal, and many observers find a special thrill in knowing that the light has travelled for millions of years before reaching their eye.

Planets are more rewarding. In a small telescope Saturn shows its rings, and Jupiter shows cloud bands and moons that change position from night to night. Venus shows phases like the Moon. Colours, however, are subtler than in photographs.

A few things improve any observing session:

• a dark site away from city lights;
• several minutes to let your eyes adapt to the dark;
• steady air, since atmospheric turbulence often limits the view more than the instrument does;
• patience, because observing skills grow with practice.

As a simple guide: if you want to begin right away with little effort, choose binoculars. If you already know you want planets and finer detail and you are happy to learn your instrument, choose a telescope with the largest aperture you are still willing to carry and set up. The best instrument is the one you will actually take outside.

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