The Keck-I Cassegrain atmospheric dispersion corrector

We describe the design and construction of the Atmospheric Dispersion Corrector (ADC) for the Keck-I Cassegrain focus. This is a "linear" or "longitudinal" ADC with fused silica prisms slightly over 1-meter in diameter. It is designed to operate at zenith distances up to 60 degrees and over a 20 arcminute field-of-view with negligible impact on image quality and throughput, and to provide dispersion compensation from 0.31 to 1.1 microns. During the design phase, it was realized that the LADC design effectively displaces the optical axis of the telescope as the prisms separate, leading to (a) a tilting of the focal surface, and (b) a change in telescope pointing. Both effects can have significant consequences, particularly for off-axis instruments, and should be carefully considered in selecting this ADC design. We also discuss in some detail the broad-band anti-reflection coatings, which consist of silica Sol-gel over MgF2. The Keck ADC is currently undergoing final assembly and testing at the UCO/Lick Observatory Instrument Labs, and will be commissioned in late 2006.