The most popular design of telescopes for astronomical research is the reflecting telescope. First developed in the 17th century, the typical reflecting telescope uses a curved primary mirror to focus incoming light, and a secondary mirror to direct that light to an eyepiece or sensor. There are many variations on the design, but the underlying principle is the same: light is focused largely by reflection, rather than refraction as in a lens-based Galilean refracting telescope, which allows them to avoid both the chromatic aberration common to lenses and the weight required to create very large ones. Reflecting telescopes have a long history in astronomy and astrophysics, from William and Caroline Herschel to the Hubble Space Telescope and beyond.

 reflecting telescope mosaic: A, diagram of a typical reflecting telescope, after Pearson Scott Foresman; B, diagram of a Schmidt-Cassegrain reflecting telescope, after Griffinjbs; C, photograph of an 18th century astronomical reflecting telescope built by astronomer William Herschel; D, photograph of the Hubble Space Telescope

We have two models in our collection of how reflecting telescopes work. Demonstration L7-14 models the behaviour of light in a reflecting telescope on our optical board, which uses real optical elements to create a viewable two-dimensional ray diagram.

Demonstration L7-14: Light is focused by a large concave mirror and then directed towards an observer by a smaller mirror

We also have a static model, demonstration E2-54, which shows the construction of a typical reflecting telescope with strings to represent the paths of light rays through the device. The two are best used in combination to show students how this system lets us observe distant objects.

 Demonstration E2-54: a plastic and string model of a reflecting telescope

You can experiment with this at home as well, with this simulation from JavaLab. You can adjust the angle of the incoming light and see how it reflects off the primary mirror and forms an image at the secondary mirror, and use an eyepiece lens to focus it on an observer. Try it out at