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 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 I pm; the detector is a thinned, back-illuminated SITe 2048 x 2048 CCD with high quantum efficiency and excellent cosmetics. Multislit aperture masks are produced photographically, 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 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 Angstrom 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.