Telescopes 101

Fifteen things you need to know before buying a telescope.
By | Published: April 1, 2014

Buying a telescope is a big step, especially if you’re not sure what all those terms — f/ratio, magnification, go-to — mean. So, to eliminate confusion and make sure you understand what you’re buying, here’s what to check out before you write the check out.

1. I know telescopes make things appear bigger, but what exactly do they do?

A telescope’s main purpose is to collect light. This property of telescopes allows you to observe objects much fainter than you can see with your eyes alone. Galileo said it best when he declared that telescopes “revealed the invisible.”

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Astronomy: Roen Kelly
Light-gathering power: Why “bigger is better”: A 4-inch-diameter mirror has four times the light-gathering area of a 2-inch-diameter mirror. A larger scope gathers more light, so you can observe fainter objects. The faintest star you can see through your telescope — “limiting magnitude” — depends on the size of the objective. A star field is shown above through a 2-inch (bottom left) and a 4-inch (bottom right).

2. When I buy my first telescope, will it be complete, or will I have to buy additional items to make it work?

Most telescopes marketed to beginners are complete systems, ready for the sky as soon as you unpack them and perform whatever setup procedures the manufacturer recommends. Some high-end models are sold as “optical-tube assembly only” versions. This means what it says: All you’re purchasing is the optics in the tube — no mount, tripod, or accessories.

Refractors usually need a star diagonal because of their design. A star diagonal bends the light from your target object 90°, so without one, you find yourself in some awkward positions when you’re observing objects high in the sky. The star diagonal fits into the telescope’s focuser, and the eyepiece fits into the star diagonal. Most manufacturers that provide complete refractor telescopes include a star diagonal.

3. I’m interested in observing, but I don’t know which scope to buy. What should I do first?

Your first step should be to learn all you can about telescopes: what types are available, what manufacturers sell them, and what words describe them. Page through the ads in each issue of Astronomy magazine, and you’ll see a range of what’s available.

For each telescope that interests you, visit that manufacturer’s website. When you’re finished reading this article, refer back to recent issues of Astronomy and carefully read telescope reviews (subscribers can read them online in the Equipment Review Archive). You may not understand everything discussed in each review, but you’ll get a taste of telescope terminology. By doing this, you’ll learn what’s important to observers when they use a telescope. You’ll get a feel for optical and mechanical quality, ease of use (including portability), extra features, and — perhaps most importantly — which objects the telescope excels on.

Telescope types
The three main types of telescopes use lenses, mirrors, or a combination of both.

Refracting telescopes use lenses — combining at least two, and as many as four, pieces of glass — as their objective (the primary light-gathering device).

Reflecting telescopes use mirrors to gather and focus light. In a Newtonian reflector — the most common type — light reflects from the primary mirror (whose surface is ground into a parabola so light comes to a common focus). The light strikes a smaller, flat secondary mirror near the top of the tube. The light then is bent 90° and enters the eyepiece through a small hole in the tube.

Catadioptric, or compound, telescopes incorporate a primary mirror coupled with a corrector lens placed at the front of the tube. The primary mirror’s curve is ground to a simple shape, usually a sphere. Ordinarily, a spherical mirror would introduce aberrations in the viewed image, but the corrector of a catadioptric scope “pre-bends” the light before it strikes the mirror. Two popular catadioptric telescopes are the Schmidt-Cassegrain and Maksutov-Cassegrain designs.

Astronomy: Roen Kelly

4. Is it true that I should purchase binoculars before a telescope?

No. I used to give this advice to beginning amateur astronomers, but not any more. The view through binoculars — especially from a light-polluted site — often proves disappointing to beginners. However, high-quality binoculars are a valuable observing accessory. Large star clusters look great through binoculars, as do the Milky Way’s band and the Moon.

5. What are the sizes of “small,” “medium,” and “large” telescopes?

I can’t speak for every reference to small, medium, or large telescopes — these terms are not standardized — but here at Astronomy, when we refer to a “small” scope, we specifically mean a telescope with an aperture (size of the lens or mirror) less than 4 inches. The type of scope doesn’t matter, and neither does the quality. Many small telescopes manufactured today are high-quality instruments I am proud to use.

“Medium” telescopes have apertures from 4 to 10 inches. This category is the size most amateur astronomers use. One of the most popular scopes is an 8-inch Schmidt-Cassegrain.

Finally, any telescope with a lens or mirror larger than 10 inches is “large.” More large telescopes are in use today than ever before. The introduction of Dobsonian mounts is largely responsible for this. Newtonian reflectors coupled to alt-az Dobsonian mounts (see “Telescope mounts” later in this article) dramatically reduce the prices (and weights) of large telescopes from what they would be with a motor-driven, equatorial mount.