Tips for Choosing a Telescope for a Beginning Astronomer

Buying your family’s first astronomical telescope is one of the best small investments you can make. And, buying an inexpensive astronomical telescope at your local superstore may improve your golf game or bowling score.

Why? Because that scope may soon oust clubs or ball from their closet space giving you more excuse to get out of the house with them instead of enjoying astronomy with your family. Sadly, many big-box telescopes boast observatory performance for a few dollars. If NASA could have gotten by so simply, why would we have put Hubble Space Telescope in orbit?

Bigger is better. Big and in-your-face get the sale. The important thing to remember about telescopes, however, is not their magnification but the amount of light they can swallow. That “big” you want to look for is in the aperture. Aperture is related to the size of the light-gathering lens or mirror in a telescope. No matter how expertly crafted the instrument, its useful magnification is limited by how well it can gather and transmit light. After all, light makes up information. More light, more savvy. Technically we say higher resolution.

In order to see details in an image, especially deep-sky objects like star clusters, nebulae and galaxies, a larger amount of light conveys more of those tiny details to your retina. A large aperture grabs more of the sky and extracts all that light from it. Any telescope can magnify far beyond its effective aperture, but no matter how much so, you will never actually see more. Too little light (aperture) spread out over too big a field (magnification) makes the image dim and useless.

Aperture is often stated as the size of the big lens at the front of the telescope or, if it is a reflector type, the main mirror at the back of the tube. (Either way, this light gatherer is called the objective.) More technically, aperture is indicated by a simple equation, familiar to photographers and astronomers as f-ratio, which is the ratio of the diameter of the objective to its focal length. Without going into Optics 101, which is beyond the scope of this overview (…ooooops!), it can be said that scopes with small f-ratios, commonly called “fast” scopes, feature low power and rich, wide, window-in-the-sky views. This is similar to binoculars. Instruments with higher f-ratios are for concentrated viewing at the edge of resolution in their magnification range, or detailed views of bright objects like the moon, and they are generally more expensive and not as easy for young amateurs to use.

You’re not likely to see f-ratios or other specs stated on consumer outlet big-box scopes. A bit of external tire kicking may be necessary since the scope can’t be “test driven” inside a blister pak or sealed box in the store. Try to find or look at the size of the front lens, and remember this rule: Amateur telescopes, regardless of type or quality, can give you a good image up to 50x or 60x magnification per inch of objective diameter. A commonly available 4-inch reflector, for instance, will perform reliably up to 225x or a little more if the quality is good. Beyond that, you are magnifying only darkness. If that seems nonsensical, claims of several hundred power magnification for a scope with a one or two inch lens are the proof.

Scopes having inflated claims are often cheaply produced plastic toys. Their tiny molded eyepieces that pretend to squeeze such magnification out of the sky are virtually useless. That said, serious astronomers have been known to use them…for dustcaps!

With a little experience, you can spot these bogus attention getters and avoid them. If you’re truly devoted to your child’s budding fascination with the heavens, don’t squander twenty dollars on pretentious plastic that will bump the hibernating action figures out of the toy chest. Excellent quality starter scopes are available from a number of reputable astronomical and recreational optics dealers, many priced below $100. Not that much more than big-box, it’s a modest investment in a priceless gift for a lifetime.

Got binoculars? You’ll be amazed at the wonders they can reveal for you. Galileo’s first scope couldn’t match up to a modern 8-power pocket sports binocular! When the birds have gone south and the final scores are in, treat yourself to a new acquaintance with your binoculars. Many amateur astronomers use them more than telescopes to study the wonders of our galaxy. Again, the larger apertures rule. At least 40- or 50-millimeter glasses are needed to open the curtain. Serious amateurs prefer 7×50 to 20×80 instruments. Many comets have been discovered by using binoculars in this power range.

Binocu-lingo: The first number, such as 7, 10, 20, etc., is the power (“x”) or amount of magnification. The second number is the diameter of the objective lenses (the big ones at the front) in millimeters. An inch is about 25 millimeters. For astronomical use, standard binoculars are better than compact models, and the objective number should be at least four times the power number.

Standard binoculars give awesome views of the star field we live in. With a bit of inexpensive foresight, you can get acquainted with some stars on non-bowling nights…by turning a focusing knob, not a tuning knob.