Choosing a Telescope Help

By — McGraw-Hill Professional
Updated on Sep 19, 2011

Choosing a Telescope

If you want to see planetary detail or intricate features on the Moon, you will need a telescope.  Here we’ll take a pragmatic view. If reading this chapter tempts you to go out and buy a telescope, that’s fine, but don’t spend more money than you can afford. Sleep on the idea before you act on it.

There are dozens of telescopes available for hobbyists. Some are inexpensive; others cost as much as a car. This chapter isn’t meant to be a shopping guide, but you should be aware of certain assets and limitations of the most popular hobby telescope designs. These include the Keplerian refractor, the Newtonian reflector, and a specialized form of Cassegrain reflector known as a Schmidt-Cassegrain telescope (SCT).


Before we compare the virtues and vices of the various telescope designs, there’s a specification you should know about. It is called the focal ratio , or f-ratio . This specification will appear as the letter f , followed by a slash, followed by a number.

If you’ve done much photography, you know about the f -ratio of a camera. In telescopes, the meaning is the same, but the dimensions are larger. The f -ratio is equal to the focal length of the objective divided by its diameter or aperture (Fig. 20-3). Thus, for example, an objective whose diameter is 20 cm with a focal length of 200 cm is an f /10 objective (as shown at A ). If the focal length is cut to 100 cm, it is an f /5 objective (as shown at B ).

Your Home Observatory Location, Location,
Location! F-ratio

Figure 20-3. The f -ratio of an objective is equal to the focal length divided by the diameter of the lens or mirror. At A , an f /10 objective; at B , an f /5 objective.

In general, objectives with smaller f -ratios make it possible to get larger absolute fields of view than is the case with large- f -ratio objectives. However, there is a tradeoff: The smaller the f -ratio, the more difficult it becomes to engineer the optics to provide quality images. This is especially true of refracting telescopes.

Keplerian Refractor

The Keplerian refractor, as you will remember from Chapter 17, uses a convex lens as the objective and a convex lens (actually, a set of two or more convex lenses) as the eyepiece. A well-made Keplerian refractor, mounted on a sturdy base, is a joy to use. The images are sharp, the viewing is stable, and the positioning of the telescope is intuitive. You can get great results without a lot of hassle. A poorly made refractor, however, can be, like anything shoddy, a source of frustration.

Refracting telescopes for amateur use range in diameter from about 50 mm (2 in) to 150 mm (6 in). The cost increases dramatically as the objective diameter increases. Another factor to consider is the manner in which the objective is made. Achromatic objectives are the most common; they consist of two different lenses, having different refractive indexes, glued together. This helps to reduce chromatic aberration, the tendency for focal length to vary with color, producing “rainbows” around stars and blurring planetary and lunar images. The greater the f -ratio, the less likely you are to have trouble with chromatic aberration in a refracting telescope if all other factors are constant. A typical value is f /10.

Apochromatic objectives provide the highest quality in refracting telescopes. As you should expect, telescopes that use this type of objective are expensive. Serious refracting-telescope lovers, to whom price is no object, seek apochromatic refractors. The f -ratios are generally smaller than those of the achromatic refractors, sometimes as low as f /5, and the image contrast is superior.

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