The Difference Between Reflectors and Refractors

by Fraser Cain on May 21, 2008

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When you choose a telescope, there are two main kinds you can pick from, reflectors and refractors. Both can be wonderful for viewing the night sky. They use basically different methods to boost light from dim objects in the sky. Here’s how they work, and how they’re different.


Refractor Telescopes
Here’s what’s inside a basic refractor telescope. The job of the objective lens, opposite the eyepiece end, is to gather the light coming from a distant object, such as a star, and bend it into a single point of focus. A second lens’ (the eyepiece) job is to enlarge that focused image for our retina; it acts as a magnifying glass. Think of the focused light coming in from the first lens as a bug, and think of the eyepiece magnifier as a basic magnifying glass that we look at the bug with. That’s it in a nutshell.


Reflector Telescopes
A reflector telescope uses two mirrors instead of two lenses. Isaac Newton developed this telescope to combat chromatic aberration (a rainbow seen around some objects viewed with a refractor telescope). A mirror used to gather light doesn’t suffer from this effect. Light from an object enters the telescope tube and is reflected off a curved mirror at the end of the tube. A second, small, flat mirror in the middle of the tube reflects this image to the eyepiece. There are potential problems associated with the mirrors. Firstly, some light is always lost in the reflection; good quality telescopes can usually gather 90% of the light coming in. Secondly, the mirror might not be a perfect curve, so the image being reflected will not come to a perfect point. This results in a dragging effect; a point could be seen as a line or cross. Also, the mirrors need to be cleaned and realigned from time to time.

About 

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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