Cassegrain Telescope

This Demonstration shows the simplest version of a Cassegrain reflecting telescope, invented by Laurent Cassegrain, ca. 1672. The primary paraboloidal mirror focuses incoming light toward a hyperboloidal mirror suspended on the axis of the telescope's cylinder. The focal length is determined by the curvature of the paraboloid. The reflected light from the secondary mirror is directed through an aperture in the center of the primary mirror into an eyepiece. The telescope has captured an image of the comet Hale–Bopp (photo by Jeff Bryant).