COSMIC: A Multiobject Spectrograph and Direct Imaging Camera for the 5 Meter Hale Telescope Prime Focus

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Abstract
We describe the design, construction, and operation of the Carnegie Observatories Spectroscopic Multislit and Imaging Camera (COSMIC) for the prime focus of the Hale 5 m telescope at Palomar Observatory. COSMIC is a reimaging grism spectrograph with a 13.65 arcmin square field of view, which can also be used as a direct imaging camera with a 9.75 arcmin square field of view. The wavelength coverage extends from 350 nm to almost 1 μm; the detector is a thinned, back‐illuminated SITe 2048 × 2048 CCD with high quantum efficiency and excellent cosmetics. Multislit aperture masks are produced photographically, with spectra of up to ~50 objects fitted on a single row of a slit mask. The instrument exhibits very little flexure and uses an active thermal control to maintain focus over a wide range of ambient temperature. In direct mode COSMIC is typically used with Kron‐Cousins, Gunn, and narrow bandpass filters. The instrument achieves throughputs of greater than 50% for direct imaging and, in spectroscopic mode, a peak efficiency at 5500 Å of slightly better than 24% of light falling on the 5 m mirror. COSMIC is optimized for faint‐object imaging, down to Gunn r = 26 mag, and multiobject spectroscopy, down to r = 23 mag, with typically 30 objects per spectroscopic exposure.

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