<title>COHSI: the Cambridge OH Suppression Instrument</title>

Abstract

One of the main limitations to the study of faint objects in the near-infrared (1 to 2 micrometer) is the luminous, varying sky background from very narrow OH emission lines originating in the Earth's upper atmosphere. This source of background contributes 95% to 98% of the total sky counts in the J & H atmospheric windows. We present the optical layout of the Cambridge OH suppression instrument. COHSI is designed to deliver OH suppressed, R equals 500, spectroscopy for both J & H spectral bands simultaneously providing an integral field mode and a multi-object mode. COHSI also has an OH suppression imaging mode. A modular approach has been selected for COHSI with the instrument consisting of three components. The first section consists of simple re-imaging lenses and a lens array interfacing the telescope to a set of optical fibers. This decouples the design of COHSI's main components from the telescope allowing COHSI to be easily used with different telescopes and making it free from flexure problems. The second section of COHSI is the OH suppression 'filter' itself. The size of this section is significantly smaller than in other similarly planned instruments. The third and final module of COHSI is the cryogenic low-resolution imaging spectrograph.