Ritter Planetarium and Brooks Observatory

Telescope FAQ

Buying a Telescope FAQ

1. When should I buy a telescope?

Before buying a telescope you should be familiar with most of the major constellations, be proficient in the use of star charts, and have a fundamental understanding of some basic astronomy. One of the best ways to accomplish these is to spend as much time as you can exploring the nighttime sky with a good pair of binoculars.

2. How much should I expect to spend?

How much to spend on a telescope is a very difficult question. Depending on your needs and interests you can spend as little as a hundred dollars, and as much as ten thousand dollars, or more. The important thing to consider is not how much you spend, but how the telescope you buy meets your needs. The most important thing you can do before investing hundreds or thousands of dollars in a telescope is to do some reading and research, and ask yourself what you want to do with the telescope.

3. Which type of telescope is best?

There are three main types of telescopes commonly available to amateur astronomers: refractors, reflectors, and Schmidt-Cassegrains (here  is a nice description of the difference between a refractor and a reflector). Of the three there is no single ``best choice''. It all depends on your interests and needs. If portability is an important factor for you, (and it should be if you live in a bright urban area) then a small refractor or a Schmidt-Cassegrain has a lot of advantages. If you are interested in faint diffuse objects like galaxies and nebulae then a reflector might be your best choice. If you are interested in planets then a long focal length refractor, or Schmidt-Cassegrain might be your best bet.

Any telescope will work reasonably well on any type of object. It just so happens that some telescopes are better suited for certain types of objects than others. There is no telescope on the market that is the best choice for every type of observing. Picking a telescope is a series of trade-offs.

4. How big should a telescope be?

The most significant way to measure a telescope is its aperture. With refractors aperture measures the diameter of the lens at the top of tube. This is the lens that collects light. A refractor should measure at least 2.4 inch, or 60 millimeters across. For reflectors the measurement is the diameter of the mirror at the bottom tube, which collects the light. For reflectors the minimum should be about 4 inches. Schmidt-Cassegrains, like reflectors, the aperture is a measurement of the diameter of the light collecting mirror at the bottom of the tube, and should measure about four inches. Although there are exceptions, these are minimum aperture worth considering. These telescopes range in price from a low of about one hundred dollars to a high of about one thousand dollars.

Larger is better, but a smaller, high quality, telescope is to be preferred over a large, poor quality telescopes without any of the accessories. A three to four inch refractor, a six to eight inch reflector, or an eight inch Schmidt-Cassegrain are all excellent choices, and are probably the most commonly used telescopes by serious amateur astronomers. These telescopes, well equipped, range in price from about five hundred dollars to three thousand dollars.

5. What about all those little lenses, what are they for?

Those little lenses are called eyepieces and they act like magnifying glasses to magnify the light collect by the primary lens or mirror. The smaller the number printed on it, the higher the magnification. You should have at least three eyepieces, though more is better. One should yield a low magnification, one a high magnification, and one in between.

Tocalculate your magnification you first need to know your focal length. There are two ways to do this. The easy way is to look on the telescope, or in the instruction book. It might read something like ``fl=700mm''. In this example the focal length of the telescope is 700 millimeters. Some telescopes won't tell you the focal length, but will tell you the focal ratio. In these cases you need to multiply the focal ratio by the aperture and convert to millimeters. For example, a six-inch telescope with a focal ratio of 8 (or f number 8) would have a focal length of 6 inch X 8, or 48 inches. 48 inches is the equivalent of 1200 millimeters. We just multiplied by 25 millimeters per inch.

Once you know your focal length, all you have to do is divide it by the number on the eyepiece, which is the focal length of the eyepiece. So a telescope with a focal length of 700 millimeters, using an eyepiece, which has a focal length of 18 millimeters, yields a magnification of 38X. The same telescope with an eyepiece that has a focal length of 5 millimeters will yield a magnification of 140X.

6. The higher the magnification the better, right?

Wrong! A lot of telescope manufactures would like you to believe this, but it is far from the truth. A good general rule of thumb is that any magnification above 50 per inch of aperture is useless. Even 50 per inch is seldom useful. The optimum magnification for a telescope varies from object to object.
For planets higher magnification is to be preferred, but for nebulae and galaxies lower magnifications often yield the nicest views. The highest useful magnification is also a function of such things as the quality of the optics, the stability of the mounting, and the weather conditions at the time of observing.

7. What other accessories do I need?

Anassortment of eyepieces is essential. Another essential accessory is a viewfinder. A viewfinder should have an aperture of at least 30mm, with 40mm being a little better, and 50mm being nearly ideal. The viewfinder should also be adjustable to allow for precise alignment with the main telescope. A diagonal prism is often very convenient to have. A set of colored filters is very nice to have. Barlow lens have their uses, but really aren't essential. Solar filters are very dangerous, and should never be used. Other accessories are often useful or essential for certain types of observations, but the above is all you really need to begin observing.

8. Is the mounting really that important?

Absolutely. A smaller telescope on a very sturdy equatorial mount is to be preferred over a larger telescope on a flimsy mounting. You should pay as much attention (if not more!) to the mechanical mounting of your telescope as you do the optics. An optical superior telescope on a poor mounting is almost useless.

9. Which type of mounting is best?

There are basically two type of mounting commonly available, each with several variations. The alt-azimuth mount often found on small refractors is acceptable for casual stargazing, but is not to be preferred unless size and weight are crucial factors.

The better choice is the equatorial mount, which allows you to track an object by moving only one axis. Setting circles and clock drives greatly enhance the utility of equatorial mounts and should be considered essential for any serious observer, or if you are contemplating astrophotography.

Fancy electronics that move the telescope automatically, or give digital readouts of where the telescope is pointing has both advantages and disadvantages for the novice. Fortunately they don't have to be purchased with the telescope.

10. My eight year-old daughter wants a telescope for her birthday. What do you think?

Great! If the child is ready for a telescope (See question 1) it should be a fairly easy to use, lightweight instrument that the child can operate by herself. Small alt-azimuth refractors are very well suited as first telescopes for smaller children. They have an additional advantage of being very durable!

11. Where does one go to buy a telescope?

That depends on what type of telescope you are interested it. Many department and catalog stores carry small inexpensive refractors that are of acceptable quality as a small first telescope. Optically these telescopes are OK, but don't expect to use the very magnifications with any success. Stick to the lower magnification, say under 75X. As soon as you get the telescope home, throw the solar filter out! These cheap little filters have been known to crack or melt thus exposing the observer to the very real possibility of being blinded by the sun.

For a larger, higher quality instrument you have two choices. There are some telescope dealers locally and in the Ann Arbor-Detroit area. Most are actually photography stores that also carry telescopes. You can find them in the Yellow Pages under "Telescopes." They usually carry only a few telescopes, and because of the low volume of sales their prices are often (though not always) higher than mail order retailers. Never the less if you value immediate deliver, a knowledge sales persons, and the ability to see the actual telescope you are buying it might be worth the additional cost.

The best source for information on mail order is the advertisements in Sky and Telescope and Astronomy magazines. Many of the dealers advertising in these magazines have been doing so for decades and that does imply a certain degree of integrity. If you have any doubt, check the back issues of these magazines at the library and see how long they have been advertising. You might also consider calling the Better Business Bureau in the city that the dealer is located in and get any information that they might have.

Last Updated: 6/27/22