Development of large high-performance lenses for astronomical spectrographs

The advent of new large telescopes and the promise of ever larger CCDs and CCD arrays has spawned the need for spectrograph cameras with unprecedented high-performance characteristics. During the last decade, familiar catadioptric cameras gave way to lenses which offered less obstruction, better transmission, wider spectral coverage and larger field of view. These developments culminated in a series of successful multi-aspheric camera lenses with elements as large as 225 mm in clear diameter such as the LRIS lens for the Keck 1 Low Resolution Imaging Spectrograph. Advances in optical glass technology and in the production of CaF2 ingots some 400 mm in diameter and 140 mm thick have brought even larger, faster lenses within reach. Optical designs now exist and in some cases construction is under way of a generation of camera and collimator lenses with elements as large as 360 mm in clear diameter and focal lengths as short as (305 to 381) mm. These optics will service a variety of spectrographs with collimated beam diameters in the (150 to 205)-mm range and anamorphic factors in the (1.0 to 1.5) range. Specific optical design examples of aspheric lenses are presented including the DEIMOS camera (for Keck 2), the IMACS short camera (for Magellan 1), the BINOSPEC camera and the BINOSPEC collimator (for converted MMT). All-spherical lenses are presented including the ESI camera (for Keck 2), the HRS camera (for Hobby Eberly) and the IMACS long camera (for Magellan 1).