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So You Want to Buy a Telescope?

Through the Looking Glass

Alignment of Jupiter, Venus and the Moon to form a frowning face in the night sky. Jupiter is in the upper left and Venus is in the upper right. Taken on December 1, 2008.

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Since Galileo’s time, millions of people have delighted in owning a telescope. If astronomy is a new hobby for you and a telescope is in your future, look-y here.

Telescope Buying Tips


Rule 1: Ignore claims about power or magnification.
Most people equate “high power”—say, 300x or 600x (expressed as “300 power” and “600 power,” respectively)—with “better telescope” but the importance of magnification is often exaggerated. Most celestial objects look best (i.e., clearer, sharper, and with no portion of the target outside the field of view) through a modest 50x to 120x eyepiece. Higher power only makes an image blurrier, thanks to our atmosphere’s fuzziness.

Any eyepiece with any power can be inserted into any telescope. (Yes, the power lies in the eyepiece.) Some companies include a 450x eyepiece to satisfy customers who suffer from “high-poweritis,” but that degree of magnification produces an image that is dark and blurry.

Rule 2: Focus on the tube: the fatter, the better.
Rather than ask about a telescope’s power, inquire instead about its size. The answer, expressed in inches, will be more meaningful than any information about magnification. The diameter, or aperture, reveals a telescope’s true value; specifically, how much light its lens, or mirror, can gather. For example, a telescope, or tube, that is 10 inches in diameter gathers, or lets in, more light than one that is six inches in diameter, and a telescope that is six inches in diameter gathers more light than one that is three inches in diameter. (The length of the tube is unimportant.) To better understand this principle, look through the wrong end of several telescopes of different diameters.

Rule 3: Curb your enthusiasm—at least at first.
Most people who get a telescope expect to see billions of heavenly bodies right from their own backyard. In fact, there are only about a dozen objects in the sky that appear spectacular; everything else looks colorless, smudgy, and blurry, including all of the galaxies and nearly every nebula. (High power won’t alter a smudgy view; under excessive high power even the Moon would appear blurry.)

Some amateur astronomers love the faint smudges, and they train their eyes to recognize subtle features, such as tiny dark streaks on galaxies (aka dust lanes, or spaces between spiral stellar arms) and dark nebulae thousands of light years across. The majority of raw beginners will not grasp or be impressed by the thousands of galaxies and nebulae that could be viewed through a small telescope because they do not know what they are—yet.

Most novice astronomers use a telescope to view the Moon (except in its full phase, when its craters and mountains seem to vanish), Jupiter, Saturn, a few nebulas, and a few double stars. (Backyard amateurs rarely use their telescopes for seeing single stars.) Half of the stars in the sky are double stars; they look single to the naked eye, but their beautiful contrasting colors are visible through a telescope. To find any of these, you must know where in the sky to look.

Rule 4: Start small
It’s probably a safe bet that, over the centuries, a few hundred thousand telescopes have ended up in attics, gathering dust. If astronomy is a new interest, make your first telescope a relatively inexpensive one—say, under $500. Something at that price will display exquisite images when pointed to the appropriate targets; just don’t expect colorful swirling galaxies and nebulae that resemble the photos in magazines, and remember: You can always upgrade.

Many beginners look for portability and ease of set up; but those conveniences too often define telescopes that are too flimsy to provide a steady image. If portability is important, get an 8-inch Schmidt-Cassegrain and expect to pay $1,500.

If you want a hassle-free, durable instrument that never needs adjustment, get a refractor. Galileo’s telescopes were refractors, the simplest type still used today, with a lens at one end. The user peers through an eyepiece at the opposite end and literally “looks through” the instrument. A 2.4-inch (60mm) beginner’s refractor can be had for as little as $100. If you can afford more, get one that is at least a three inches in diameter (about 75mm); four inches (100mm) would be better, and five (125mm), although expensive, would be ideal.

A 6- to 10-inch reflector telescope with a motor drive to track sky objects automatically as they move during the night is a wonderful tool. Isaac Newton invented the reflector, which uses a mirror to gather light. The user looks through the side of the tube and sees the image through a series of reflections. These are good instruments in the 6- to 12-inch size but they are not easily portable. This type is large and awkward, and must be carried in and out of the house for each use.

Today, many amateur astronomers prefer catadioptric, or mixed-type, instruments. These have lenses and mirrors, and “fold,” or bounce, the light back and forth to produce a portable (albeit pricey) instrument with a short tube, that also serves well for terrestrial uses such as bird watching.

Rule 5: Open both eyes.
One of the most undervalued tools on the amateur astronomer’s shelf is a pair of binoculars. These are unbeatable for delivering a bright image (brighter than that of most telescopes!), a wide field, and a stereoscopic view. In fact, binoculars are a better choice for some objects than, say, the Keck telescope. Binocular magnifications are too low for viewing planets, but they are fine for sighting loose star clusters and for sweeping the Milky Way. Keep this in mind:

  • Binoculars over 10x cause images to shake too much, so avoid them.
  • Image-stabilized models are preferable, but they are expensive.
  • In order to have an image that is bright enough for astronomical use, make sure that the second number in the specs (as in 7x35, expressed as “seven power”) is at least four times greater than the first. However, any binocular whose second number is 30 or higher and whose magnification is 7 to 10, can provide satisfying views of many celestial phenomena.

Rule 6: Join a Club.
Local astronomy clubs exist in nearly every part of the country and welcome enthusiasts of all experience levels. Most clubs hold monthly sky-observing sessions with impressive telescopes. A club is one of the best way to be explore to the wonders of the night sky, and learn more about the equipment. Many have electronic newsletters, with articles and all sorts of astronomical advice; if a newsletter is offered, be sure to sign up.

Only a handful of major observatories make their telescopes available to the public, although many schools and science museums have telescopes and have programs and open nights throughout the year. Contact the science or astronomy departments.

Telescope Viewing Tips

Tip: Scoping Out a Viewing Site

  • Telescopes belong on a lawn—not on a wooden deck (a deck vibrates, even if you can’t feel it) or pavement, although pavement is better than a deck. Pavement heats up during the day and, as the heat from it dissipates after sundown, that warm air acts like the wiggly air you see rising from a radiator or wood stove to distort an image as seen through a telescope.
  • Telescopes also should not be used at an open window. The same warm air principle distorts the view: heat from the house escapes through the open window, making for turbulent air. Avoid closed windows, too, as the quality of the glass will distort an image.

Tip: Red Light, Green Light

  • It takes at least ten minutes to acquire night vision (that is, to see well in the dark). A red light enables you to read a star chart without losing that dark sensitivity. Pick up a red LED flashlight in a hardware store or home supply store, or use red cellophane and rubber bands to cover a flashlight.
  • Red lasers do not cast a beam into the sky, but green ones do, especially at a dark site away form artificial lights. They look "cool" and allow a knowledgable stargazer to point out celestial objects to friends. (However, never point any laser at a person's eyes or aim a green laser at an airplane. You can harm the eyes of the pilots or passengers, and it is a federal offense.)

See for Yourself
As a calendar of the heavens, The Old Farmer’s Almanac provides accurate sighting times for a variety of celestial events and highlights throughout the year. Get a copy of the Almanac hereAlso, check out our Monthly Sky Watch on this Web site.

To get more detail on buying a telescope, read our two-part blog "How to Buy a Telescope."

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