The Keck Low-Resolution Imaging Spectrometer

The Low Resolution Imaging Spectrometer (LRIS) for the Cassegrain focus of the Keck 10-m telescope on Mauna Kea is described. It has an imaging mode so it can also be used for taking direct images. The field of view in both spectrographic and imaging modes is 6 by 7.8 arcmin. It can be used with both conventional slits and custom-punched slit masks. The optical quality of the spectrograph is good enough to take full advantage of the excellent imaging properties of the telescope itself. The detector is a cooled back-illuminated Tektronics Inc. 2048 X 2048 CCD which gives a sampling rate of 4.685 pixels per arcsec. In the spectrographic mode the spectrograph has a maximum efficiency at the peak of the grating blaze of 32%-34% for the two lowest resolution gratings and 28% for the 1200 g mm^(-1) grating. This efficiency includes the detector but not the telescope or the atmosphere.