The Ohio State Multi-Object Spectrograph (OSMOS) is a new, wide-field imager and multi-object spectrograph for the 2.4-m Hiltner Telescope at the MDM Observatory. OSMOS has an all-refractive design that reimages a 20 arcminute diameter field-ofview onto the 4064x4064 MDM4K CCD with a plate scale of 0.273 arcseconds per pixel. Approximately an 18.5 ′ square region of this field illuminates the detector and is available for spectroscopy, although with reduced wavelength coverage near the edges of the field. Slit masks, filters, and dispersers are all mounted in a series of six-position aperture wheels. These mechanisms rotate between positions in only a few seconds and consequently the instrument may be rapidly reconfigured between imaging and spectroscopic modes. At present a low-resolution triple prism (R ∼ 60 − 400) and a moderate resolution VPH grism (R ∼ 1600) are available.

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We describe an instrument that is capable of taking simultaneous images at one optical (UBVRI) and one near-infrared (JHK) wavelength. The instrument uses relatively simple optics and a dichroic to image the same field on to an optical CCD and an HgCdTe array. The mechanical and thermal design is similar to previous instruments built by our group and the array controllers are based on the same architecture. The instrument has been in use for the past four years on the CTIO/Yale 1m telescope in Chile and has an excellent operational/reliability record. A number of notable science results have been obtained with the instrument; especially interesting are several photometric monitoring projects that have been possible, since the instrument is available every night on the telescope.

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We describe the design, construction and measured performance of the Kitt Peak Ohio State Multi-Object Spectrograph (KOSMOS) for the 4-m Mayall telescope and the Cerro Tololo Ohio State Multi-Object Spectrograph (COSMOS) for the 4-m Blanco telescope. These nearly identical imaging spectrographs are modified versions of the OSMOS instrument; they provide a pair of new, high-efficiency instruments to the NOAO user community. KOSMOS and COSMOS may be used for imaging, long-slit, and multi-slit spectroscopy over a 100 square arcminute field of view with a pixel scale of 0.29 arcseconds. Each contains two VPH grisms that provide R~2500 with a one arcsecond slit and their wavelengths of peak diffraction efficiency are approximately 510nm and 750nm. Both may also be used with either a thin, blue-optimized CCD from e2v or a thick, fully depleted, red-optimized CCD from LBNL. These instruments were developed in response to the ReSTAR process. KOSMOS was commissioned in 2013B and COSMOS was commissioned in 2014A.