Telescope Buying Guide Part 2: How to Select a Telescope

by Joe Rao, SPACE.com Skywatching Columnist

Date: 24 November 2009 Time: 12:49 PM ET

Eventually, there will come that time that you will finally purchase a telescope. First make sure you read PART 1. Next, proceed with caution.

Regardless of what type of scope you ultimately choose, never buy a telescope from a department store, a junk mail catalogue or one of those TV shopping channels! I'm sure you've seen ads like this in newspapers or on late-night cable TV:

"Bring the mysteries of the universe up close for your inspection. Study the sun, the moon, the planets, stars, comets and much, much more with this practical learning tool!

Such claims are a seductive lure to the astronomy neophyte or the well-meaning parent or grandparent wishing to get their prospective space-minded child an instrument that he or she can use to "discover the wonders of the heavens." But more often than not, such telescopes end up being a waste of money.

These are what we call "trash telescopes."

For the amount of money you might spend for such an instrument, you probably would be better off investing in good binoculars (again, see PART 1). Basically, a trash telescope is one that, along with having a poor, wobbly mount, has been advertised by the manufacturer as promising "spectacular views" of the moon or the rings of Saturn at magnifications of, say, 500-power or more.

These days, your best bet is a respected online telescope retailer. The best ones have a wide selection of brands and types and offer strong customer support after the sale. [See TopTenREVIEWS full Telescope Stores Review.]

Telescope types

Now, what type of telescope would you prefer? There are three primary types to consider:

A refracting telescope has a convex objective lens at one end and an eyepiece at the other. When most people think of a telescope a refractor immediately comes to mind; it's the type of instrument that Galileo used to discover the rings of Saturn and the moons of Jupiter in the early 1600s. Refractors tend to provide higher image contrast than a reflecting telescope, but the very best refractors are also quite expensive.

A reflecting telescope does not use an objective lens, but rather a concave mirror. Unlike a refractor, which is a sealed tube, reflectors are open at one end. The mirror (called the "primary") sends a cone of light up through the tube where a small flat mirror (called the "secondary") intercepts it and sends it to the eyepiece on the side of the tube. Reflectors have the advantage of being relatively cheap to produce, making them more affordable as opposed to a refractor.

A catadioptric telescope is a special type of reflecting telescope that possesses a correcting lens at the top to form the image. Most catadioptics are of the Schmidt-Cassegrain design. The light passes through the corrector, it reflects off the primary and then off a curved secondary, finally passing through a hole in the main mirror and reaching the eyepiece.

Choose quality over power

Unfortunately, too many uninitiated amateurs are "power happy." Don't be among them.

Any telescope will provide you with high magnification, but not only will you be increasing the size of the image, you'll also be increasing the effects of viewing an object through our turbulent atmosphere. The uninitiated (usually that "space-minded child") will likely want to "test drive" the telescope at its highest possible magnification. The result will be an enlarged and hopelessly fuzzy image that will be virtually impossible to keep in the telescope's field of view thanks to its defective and unbalanced design.

Before you know it, that "practical learning tool" has ended up in the back of a closest or up in the attic, never to be seen again!

Purchasing tips

Before investing your hard-earned money on a new telescope, consider these tips:

Go to your local bookstore or newsstand and browse through one of the various astronomy magazines where well-known and reputable manufacturers advertise quality astronomical telescopes this will fit your budget.
Many planetariums across the country offer courses on "How to Use a Telescope." Such classes typically assist both the prospective purchaser and those who wish to get the most out of an instrument that they already have.
Make an effort to contact local astronomical societies and attend star parties and other, similar events where he or she can look through various telescopes and talk to their experienced owners. Such gatherings are ideal "showrooms" for commercial as well as home-built telescopes.

If you're just starting out, or planning to purchase a telescope for a budding young astronomer, you might want to consider a 2.4-inch or 3-inch refractor, or a 4-inch or 6-inch reflector.

These relatively small telescopes have the advantage of being more portable than their big brothers. And they cost a lot less. There are some very good telescopes on the market that are available in the $150 to $400 range; refractors are generally at the higher end of this range. These small scopes are well suited for exploring the moon, spotting Saturn's rings, finding the pinpricks of otherwise invisible light that are Jupiter's moons. At the larger end of these small scopes, a viewer will get a modest glimpse of details in Jupiter's clouds and on the surface of Mars.

Among reflecting telescopes, you can choose between a tripod-mounted Newtonian telescope or the very popular Dobsonian style, which is mounted on a low profile, swiveling box-like base instead of a tripod.

Dobsonians are also popular because their mirrors can be huge. Many amateurs own "Dobs" in the 10 to 18-inch range. The large size of such mirrors allows them to collect light from very dim sky objects – which are why they are sometimes referred to as "light buckets."

I own a 10-inch Dob that can be readily transported in the back of my minivan. If you have access to dark, unpolluted skies, owning a Dobsonian can be a great asset, although the bigger the scope, the longer your setup time.

Don't aim too high

Buying the best telescope can prove frustrating for a beginner, which is one reason I suggest people think long and hard about their needs and desires, start with binoculars, and then pick their first telescope carefully.

Many first-time telescope buyers go right out and purchase a very expensive telescope, bring it home, try setting it up, and in the process get completely confused. One neighbor of mine did just this and ended up visiting me every other week with a "new problem," when it was simply a matter of properly adjusting the finderscope or aligning or collimating the mirror. She even queried me about the motor drive (used to take long-exposure photographs): "I thought it would automatically point the telescope to what I wanted to see." In the end, I think she realized that she was far in over her head.

Perhaps my neighbor was thinking of a "smart" or "go to" telescope. Such an instrument has a built-in computer that indeed will do the work of pointing the scope and finding objects for you. That's the good news.

The bad news is that many owners have been frustrated because they encounter difficulties just getting past the setup procedure, much of which has to be carefully done in the dark.

In fact my editor here at SPACE.com fell into this trap. Being a casual night-sky observer who delights in showing friends and family their first up-close views of the moon or blowing them away with a look at Saturn's rings, he enjoyed his simple, compact 3-inch telescope immensely. Then he moved up to a really heavy, complex 8-inch GPS-controlled smart telescope that required a small toolbox to carry all the extra eyepieces and other accessories. Being a very busy man with little time for setting up complex machinery, he became frustrated by all the moving parts and instructions. He admits to never having figured out how to run the thing, and so he sold it and is back to enjoying his simple 3-inch telescope immensely.

Bottom line: If you're impatient or aren't computer savvy, a smart scope is probably not for you. But once you've got some experience using a simple telescope, and you're ready to invest some serious time and money, a smart scope can help you find obscure and rewarding targets with relative ease.