Types of telescopes

What a telescope actually does

A telescope has two jobs. Its main lens or mirror, called the objective, collects far more light than your pupil can, and then brings that light to a focus. An eyepiece, a small magnifier, lets you look at that focused image.

Collecting light is the more important job. Most things in the night sky are faint rather than small, so a telescope that gathers more light shows more. Magnification comes second and is set mostly by the eyepiece, not the telescope.

Refractors: lenses at the front

A refractor uses a lens at the front of a closed tube, the classic shape most people picture. Light passes through the glass and bends to a focus at the back. The sealed tube keeps out dust and air currents, and the optics almost never need adjustment.

Refractors give crisp, high-contrast views of the Moon, planets and double stars, and they are ready to use quickly. Their drawback is cost per inch: large lenses are expensive and heavy. Simple lenses also bend colors slightly differently, which can add a faint purple fringe around bright objects; better designs with special glass reduce this.

Reflectors: Newtonians and Dobsonians

A reflector uses a curved mirror at the bottom of the tube instead of a lens. Isaac Newton's design, still common today, bounces the light back up to a small flat mirror that sends it out the side of the tube to the eyepiece. Mirrors show no color fringing and are much cheaper to make in large sizes.

A Dobsonian is a Newtonian reflector on a simple wooden or metal box mount that swivels on the ground. It is the easiest way to get a large aperture, such as 6 to 10 inches, for a modest budget. The trade-offs are bulk, an open tube that needs time to cool to outdoor temperature, and occasional collimation, which means lining up the mirrors with a simple tool.

Catadioptrics: Schmidt-Cassegrain and Maksutov

Catadioptric telescopes combine a thin corrector lens at the front with mirrors inside. The light is folded back and forth, so a long focal length fits into a short, portable tube. The Schmidt-Cassegrain uses a thin, almost flat corrector plate; the Maksutov uses a thicker, deeply curved lens.

Both are popular on computerized mounts and travel well. Their long focal lengths suit the Moon, planets and small targets, but give a narrower field of view for large star clusters and nebulae. They also cost more per inch of aperture than a Dobsonian and take a while to settle to outdoor temperature.

Aperture: the number that matters

Aperture is the diameter of the main lens or mirror, usually given in inches or millimeters. Light-gathering power rises with the area of the opening, so doubling the aperture collects four times as much light. A 6-inch telescope gathers roughly 470 times more light than a fully dark-adapted eye.

More aperture also means finer detail, because a wider opening can separate closer points of light. That is why experienced observers compare telescopes by aperture first, and treat magnification figures on the box as close to meaningless. The best aperture, though, is the largest one you will actually carry outside.

Mounts: alt-az, equatorial and GoTo

An altitude-azimuth (alt-az) mount moves in two simple directions: up and down, and left and right, like a camera tripod head. It is intuitive and quick to set up; the Dobsonian base is a type of alt-az mount.

An equatorial mount has one axis tilted to point at the celestial pole, near Polaris for northern observers. Once aligned, turning a single axis follows stars as Earth rotates, which helps at high magnification and for photography. It is heavier and takes some learning.

GoTo mounts, of either type, have motors and a small computer. After a short alignment on a few bright stars, they point at thousands of objects and track them. They are convenient but need batteries or a power supply, and the computer cannot make up for a wobbly tripod.

How to choose between them

If you want the brightest, most detailed views for the least money and have room to store it, a Dobsonian reflector is hard to beat. If you want something light, low-maintenance and quick for the Moon and planets, a small refractor fits. If portability and a computerized mount matter most, a Maksutov or Schmidt-Cassegrain is a good match.

Whatever the design, try to look through a few at a local astronomy club or public star party before deciding. Never point any telescope at the Sun unless a certified solar filter meeting ISO 12312-2 covers the front of the tube; the focused light can blind instantly.