Reflector versus Refractor Telescopes

There are two primary types of telescopes, namely refractors and reflectors. The third type, the catadioptric or compound telescope, is simply a combination of the two. Both reflector telescopes and refractor telescopes have certain perks and drawbacks, but both of them serve essentially the same purpose; they simply go about it in different ways.

Refractor telescopes have been around since the early 1600's. They were notably used by Galileo and Kepler, and they are still a common find in the telescope market. Refractors operate under the same principle as binoculars. A convex class lens is placed inside the end of a tube. This lens collects and bends the light that enters it to a central focal point. Another, smaller lens at the opposite end of the tube acts as the eyepiece, and magnifies this image. Thus, when the telescope is focused on a light reflecting object, the image appears larger and clearer.

Reflector telescopes, on the other hand, were created nearly eighty years later by astronomer and physicist Sir Isaac Newton. Instead of passing light through glass at the front of the tube, reflectors reflect light off of a mirror placed in the back of the tube. The image is typically then reflected off of a smaller mirror placed inside the centre of the tube, and directed through the eyepiece somewhere on the side of the telescope. More recent models allow the secondary mirror to reflect the image through a small hole in the objective (primary, larger) mirror, so that the eyepiece can be located at the back of the telescope. Both the Hubble Space Telescope and the Keck Telescope are recent examples of reflectors.

Refractor telescopes, because they are closed at both ends by lenses, tend to be less susceptible to atmospheric changes and elements. This makes them a bit more reliable and consistent. Unfortunately, because refractors rely on bending light to create an image, chromatic aberration may occur. This means that images may appear to be discoloured, or may be encircled by rainbow coloured rings. Furthermore, very few large refractor telescopes are available. The objective lenses for refractor telescopes require large amounts of time and energy to create. The glass for the lens must be ground to a perfect shape, or the telescope will not work correctly. As lenses grow larger, the glass can become heavy and even more expensive to procure. For the amateur stargazer, however, there are usually no immediate problems with small refractor telescopes.

Reflector telescopes, because they are open at the top, are more likely to fall victim to elemental changes and dust. They require more upkeep than refractor telescopes. It is also necessary to align or "collimate" the optical pieces on a reflector telescope, which can be a confusing process for the beginning astronomer. Finally, while images viewed through reflector telescopes do not suffer from chromatic aberration, images in the mirror's periphery may undergo coma, a sort of stretching of the object in view.