The Low Resolution Imaging Spectrometer (LRIS) on Keck I provides imaging and spectroscopy from the ultraviolet (3100A) to near-infrared (1 micron). Unfortunately, atmospheric refraction, acting much like a prism, results in a displacement of an incoming beam that depends on wavelength and zenith distance (see Figure 1 below). This atmospheric dispersion, even at modest zenith angles above 60 degrees, can significantly degrade image quality, especially when broadband filters are used, or can seriously reduce slit throughput if the position angle of the slit is not perpendicular to the horizon. To correct for this problem, an atmospheric dispersion corrector (ADC) has been designed for LRIS. Expected first light for the ADC is November 2006.
