Optical Systems for Spectroscopic Telescopes

Astronomical spectrographs are usually mounted at one of three focal positions on a reflecting telescope: (1) the Newtonian focus, at the top of the telescope tube, with only one reflection from the primary mirror, where the spectrograph must be small and light, and designed for low resolution on faint objects; (2) the Cassegrain focus, behind the cell of the primary mirror, with a total of two reflections, where the spectrograph can be appreciably larger, to give intermediate resolution; and (3) the coude focus, at a fixed location below the end of the polar axis, with three, four or five reflections, where there is no limit to the size of the instrument, and where resolution is limited only by the brightness of the object and the light efficiency of the system. This is the ideal location for image tubes, Fabry-Perot interferometers, and equipment for Fourier spectroscopy.