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Cassegrain Telescope - Definition and Applications

The Cassegrain telescope is an astronomical reflecting telescope, in which the light is incident on a large concave paraboloid mirror, and reflected onto a smaller convex hyperboloid mirror. This reflected light is reflected again through a hole in the concave mirror to finally form the image.

The reflecting principle used in the working of this telescope is incorporated in most modern optical telescopes. This telescope was invented by a Frenchman named Nicolas Cassegrain. Cassegrain designs are also used in the construction of satellite communication systems, such as earth stations and radio telescopes.

Construction of Cassegrain Telescope

Symmetrical Cassegrain telescopes are made up of two mirrors aligned about the optical axis. The primary mirror contains a hole in the centre. This hole facilitates the reflected light to reach the eyepiece, camera, and a light detector. In asymmetrical Cassegrain telescopes, the mirrors are capable of being tilted, thus there is no need for a hole in the primary mirror.

The classic Cassegrain telescope consists of a parabolic reflector (primary mirror) and a hyperbolic mirror (secondary mirror). The parabolic shape of the mirror helps in gathering light, similar to the way that a refractor gathers light using an objective lens. The hyperbolic secondary mirror reflects light, such that it is focussed onto the hole in the primary mirror. The eyepiece and the other instruments are located behind the telescope. The folded optical path in this telescope makes it compact and portable.

Different Cassegrain Designs

The various types of Cassegrain designs are classic Cassegrain, Ritchey-Chretien, Off-axis configuration, and Dall-Kirkham. These are briefly discussed below:

  • Classic Cassegrain – This design consists of a parabolic primary mirror and a hyperbolic secondary mirror. The secondary mirror reflects light back to the hole in the primary mirror
  • Ritchey-Chretien – This Cassegrain configuration consists of two hyperbolic mirrors. The image from this configuration is free of comatic and spherical aberrations
  • Dall-Kirkham – This configuration was created by Horace Dall. It consists of a concave elliptical mirror (primary) and a convex spherical mirror (secondary)
  • Off-axis Configuration – This configuration of the Cassegrain makes use of tilted mirrors, in order to avoid the shadow on the primary mirror

Applications of Cassegrain Telescopes

Given below are some of the main applications of Cassegrain telescopes:

  • In astronomical observatories
  • Photographic observations

Sources and Further Reading

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