Optical design of the Discovery Channel Telescope

The Discovery Channel Telescope (DCT) is a joint venture between Discovery Communications and Lowell Observatory. The telescope will have a 4.2-meter clear aperture, active primary mirror working at F/1.9. Two observing stations are presently planned; a Ritchey-Chretien focus some two meters behind the vertex of the primary mirror and a prime focus featuring a wide-field optical corrector (WFOC) with a two-degree field of view. The Ritchey-Chretien focus will be used for a variety of optical and near infrared imaging and spectroscopic instrumentation while the prime focus will be largely used as a survey tool to search for near-earth and Kuiper belt objects, for example. In order to take advantage of sub-arc second seeing at the DCT site, a stringent set of requirements has been placed on the two foci. The requirements are for the full-width, half-maximum (FWHM) image of a point source to be less than 0.20 arc second at the Ritchey-Chretien focus over a 21 arc minute field and less than 0.27 arc second at prime focus in each of six filter bands including a very broad band for survey purposes. This paper describes the optical design of the field correctors at the two foci. Particular attention is paid to the WFOC. This state of the art device poses a number of optical challenges which are discussed here, as well as mechanical challenges which are discussed elsewhere.