Cassegrain telescope

Cassegrain telescope. A schematic diagram of the optical arrangement. A reflecting telescope in which the image is brought to a focus just behind a central hole in the primary mirror (see illustration). The design was proposed in about 1672 by Jacques Cassegrain (1652-1712), professor of physics at Chartres in France, some four years after Isaac Newton constructed the first reflecting telescope. It employs a convex secondary mirror, rather than the flat one in Newton's own design. Cassegrain did not build a telescope himself and it was some years before his idea was put into practice. Today, the Cassegrain focus is popular and widely used in both modest amateur instruments and large professional telescopes.