Right next to light, the telescope is an astronomer’s most valuable tool. There are so many different varieties of telescopes, it can be hard to keep them all straight. But they can all be sorted into a few basic types, and that makes choosing one a lot simpler.

Two very common types are reflectors and refractors, and each one in the image above is one of these. You can tell a reflector by its cylindrical design. They all look like cylinders, you say? Well…refractors are a little bit different.

Take the two telescopes on either end of this lineup, for instance. These two—the far left and the far right—are refractors. And you may notice that, unlike most of the rest, they’re not perfect cylinders.

Look closely. You’ll see that, not only is the end pointing up a bit wider than the rest of the telescope, but there’s a little tiny piece tacked onto the end. That same little tiny piece is tacked onto the side for the reflectors.

Every reflector and every refractor can be recognized by these basic qualities. But what they do with light is more important.

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