The Basics of Reflector Telescopes

One of the most popular telescopes among scientists and amateur stargazers is the reflector. Reflectors are a type of optical telescope that use mirrors to reflect light and produce the image. Reflectors can be compared to refractors, which use lenses to produce the image.

How Reflectors Work

As stated above, reflectors use one or more curved mirrors in order to reflect light and produce their image. A curved primary mirror will be placed in a tube where it creates an image at the focal plane. Depending on the telescope, secondary mirrors may be added to modify different characteristics or to redirect the light.

The mirrors can be ground into a spherical or parabolic shape. Both shapes come with advantages and disadvantages to the viewer. You choose the shape of the mirror based on your observation preferences.

Reflectors differ from refractors because they use mirrors wile refractors use lenses. Mirrors allow for the light to be reflected from the mirror. Lenses, on the other hand, focus on and intensify the light that comes through the end of the tube.


The disadvantages of both shapes come in the form of aberrations. Aberrations are slight distortions from the original image. In total, there are six main optical aberrations: spherical aberration, coma, astigmatism, curvature of field, distortion, and longitudinal chromatic aberration. Reflection causes aberration because light disperses or separates whenever it passes through a prism or lens.

If they are not taken into consideration, aberrations can distort measurements or observations. So, it is important to take your scope’s aberrations into consideration.


The first reflector was made in 1668 by Isaac Newton. His telescope became known as the Newtonian telescope, which is known for its paraboloid primary mirror and flat secondary mirror. Since the 17th-century, many other reflector designs have been made:

  • Gregorian: The Gregorian telescope features a concave secondary mirror. This mirror design was published in 1663 by James Gregory, but Gregory did not make a physical telescope.
  • Cassegrain: The Cassegrain telescope has a primary parabolic primary mirror and a hyperbolic secondary mirror. There are many types of Cassegrain telescopes.
  • Off-Axis Designs: There are several telescope designs where the secondary mirror is either eliminated or moved. The purpose of this is to avoid obstructing the incoming light.
  • Liquid-Mirror Telescopes: Liquid-Mirror Telescopes use a rotating mirror that consists of a liquid metal. As the tray spins, the water creates a paraboloidal surface.