FIELD CORRECTORS FOR WIDE-FIELD CCD IMAGING WITH RITCHEY-CHRETIEN TELESCOPES

The increasing availability of large format CCDs has made it possible to construct imaging arrays covering a 5 to 10 cm square format at mod erate cost. In an era of 8 to 10 m telescopes, these large imaging arr ays may extend the scientific usefulness of 1 to 2 m class telescopes, particularly if excellent image quality can be maintained across the large field of view. Many of these telescopes have Ritchey-Chretien op tics with dominant off-axis image aberrations of field curvature and a stigmatism. We briefly review the literature of one and two element re fractive correctors for Ritchey-Chretien telescopes, and present varia nts optimized for CCD imaging. We describe a new family of two element field correctors that can provide excellent images (less than or equa l to 0.2 '') over a field diameter exceeding 0.5 degrees. These new co rrectors have five appealing features: (1) they are compact, (2) they use only spherical surfaces, (3) they are constructed from fused silic a, with high transmission over the entire optical band, (4) ghost imag es are well controlled, and (5) one of the corrector elements can serv e as the dewar vacuum window to minimize the number of glass-air surfa ces. We present two element corrector designs for the 1.2 m telescope at the Whipple Observatory and the 2.4 m Hiltner Telescope at the Mich igan-Dartmouth-M.I.T. Observatory, and describe how to design such a c orrector with a modern optical design program. (C) 1997 American Astro nomical Society.