COSMIC: A multiobject spectrograph and direct imaging camera for the 5 meter Hale telescope prime focus

We describe the design, construction, and operation of the Carnegie Observa tories Spectroscopic Multislit and Imaging Camera (COSMIC) for the prime fo cus 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 als o be used as a direct imaging camera with a 9.75 arcmin square field of vie w. The wavelength coverage extends from 350 nm to almost I pm; the detector is a thinned, back-illuminated SITe 2048 x 2048 CCD with high quantum effi ciency and excellent cosmetics. Multislit aperture masks are produced photo graphically, with spectra of up to similar to 50 objects fitted on a single row of a slit mask. The instrument exhibits very little flexure and uses a n active thermal control to maintain focus over a wide range of ambient tem perature. 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 Angstrom of slightly better than 24% of light falling on the 5 m mi rror. COSMIC is optimized for faint-object imaging, down to Gunn r = 26 mag , and multiobject spectroscopy, down to r = 23 mag, with typically 30 objec ts per spectroscopic exposure.