How Reflector Telescopes Work

What is a Reflector Telescope

A reflector telescope uses mirrors to reflect the images seen by you when you look into the telescope. Were you to use a refractor telescope, you would directly view the object of your study. A refractor telescope uses lenses, rather than mirrors. So when you use a reflector telescope, the object you view is actually the mirror image of that object, except it is also upside-down.

The mirrors in a reflector telescope collect light. Distant objects, like any object, can be seen because you can see light. The primary mirror in a reflector telescope collects the light of the distant object. The light goes to the secondary mirror, which then sends the light to the eyepiece, which is a lens. This lens then "spreads" the light out, resulting in a magnification of the upside-down and reversed image. So when you look through a reflector telescope at the moon, you can see the craters, because light from the moon has been collected and magnified.

The primary mirror in a reflector telescope is curved; it is a concave shape that creates a focus of light on the secondary mirror. Were it to be flat or convex, the light would scatter outward. In that case, the secondary mirror could not capture it, and the image would lose its focus. It was Sir Isaac Newton who worked out the exact formula for the shape of the primary mirror that would result in this essential focus of light. Newton built the first practical reflector telescope in 1669.