The Keck-I Cassegrain ADC

摘要

We briefly describe the design, construction and performance of the recently-commissioned Atmospheric Dispersion Corrector (ADC) for the Keck-I Cassegrain focus. This is a 'longitudinal' ADC with fused silica prisms slightly over 1-meter in diameter, 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. It provides dispersion compensation from 0.31 to 1.1 microns. The sol-gel-bascd antireflection coatings were a major technological challenge, and we encountered some previously unrecognized performance consequences of the LADC design which should be considered before adopting this design.