Comparing Telescope Types – AstroGear Today

Confused by all the different telescope types? Don’t be. All telescopes have one purpose; to gather light. The human eye’s light gathering ability is limited by its small opening. A telescope is, in effect, a “light gathering amplifier.”

But there are many ways to do this. Telescopes fall into three broad categories. A refractor is what people typically think of when they hear the word “telescope” – a long tube with a lens at the front and an eyepiece at the back.

Advantages of refractors include very sharp images for their size. Also, their sealed tubes keep dust and dirt at bay, and they usually require little to no optical adjustment. On the downside, they are the most expensive of the three designs. Large refractors can become unwieldy beyond 4” aperture or so. If you want a big “light bucket” to see tiny, dim, distant galaxies, a refractor may not be for you. But if you spend your time viewing the moon or planets, a refractor may be your ideal choice.

A reflector uses a mirror at the bottom of a tube to collect light (contrast this to the refractor, above, which uses a lens at the front to gather light). In the simplest form of reflector a small secondary mirror near the top of the tube diverts the light out the side of the tube to the eyepiece. This “Newtonian” reflector, which includes the popular Dobsonian telescope, is the least expensive of all designs. If you’re looking to get the biggest bang for your buck, the reflector is for you. But the reflector has a couple of disadvantages. There is an occasional need to re-align its optics, and its open tube can be prone to collecting dust.

Newcomers are often confused when they first see a reflector. Unlike a refractor, where the user looks through the back of the tube, a Newtonian reflector requires the viewer to look through the side of the front of the telescope. Don’t be embarrassed if you make this mistake the first few times. Hang around enough telescopes and soon you’ll be a pro at this in no time.

A catadioptric (or compound) telescope uses a combination of lenses and mirrors to gather and focus light. It can be thought of as a blend of the refractor and reflector. Like the reflector, it gathers light with a mirror. Like a refractor, the observer looks through the back of the telescope, by bouncing the light back and forth inside the tube.

Why do this? Compound telescopes effectively “fold” the light path of a reflector into a shorter, more compact tube. Their short, stubby appearance makes them immediately recognizable in the observing field. Compound telescopes are often the first ones fitted with the latest electronic wizardry, so they can be popular with the computer-literate crowd.

The compound telescope costs more than the simpler Newtonian reflector, but usually less than a refractor. On the downside, the compound telescope contains more optical elements than the other designs, giving them the (often undeserved) reputation as being slightly less sharp than refractors or reflectors. They are sometimes (again, often unfairly) referred to as a “jack of all trades, master of none” telescope.

As you can see, there is no such thing as a perfect telescope for everyone. Luckily, excellent views can be obtained with telescopes of any design. Whatever you buy, be sure and use your telescope. Enjoy the views, and clear skies to you all!

About Ed Ting

Ed Ting is a well-known amateur astronomer. His work has appeared in Sky & Telescope, Night Sky, Skywatch, Amateur Astronomy, Discover, and Popular Mechanics magazines. His web site,, is a widely-read telescope review web site. He is a National Science Foundation Ambassador to Chile and a NASA Solar System Ambassador.

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