Gemini infrared multi-object spectrograph: calibration system

The Gemini Infrared Multi-Object Spectrograph (GIRMOS) is a four-channel adaptive-optics-assisted integralfield spectrograph being designed for the Gemini 8-meter telescopes. Deployed behind the Gemini-North Adaptive Optics (GNAO) system, it will provide spatially-resolved spectra over the 0.9-2.4 um wavelength range for four fields simultaneously. Its multi-object adaptive optics will provide additional correction of the target fields, beyond that achieved by the GNAO system, enabling integral-field spectroscopy with near-diffraction-limited resolution and unprecedented sensitivity. A parallel imaging channel will view the field of regard and provide a simultaneous imaging capability. The primary science objectives include mapping chemical abundances, star formation and kinematics in high-redshift galaxies, and studies of stellar populations, star formation and supermassive black holes in nearby galaxies. In order to support the science programs, GIRMOS requires a system that enables photometric, spectroscopic and astrometric calibration. The GIRMOS Calibration System (CAL) serves this purpose, uniformly illuminating the spectroscopic and imaging channels with both continuous and narrow-line light for flat-field and wavelength calibration. In order to replicate the light path through the instrument as closely as possible, the CAL optical system matches both the pupil position and the focal ratio of the beam delivered to the instrument by GNAO. CAL also includes a metrology system, employing focal-plane masks, to permit precise calibration of the positions of the pick-off arms of the object selection system, and to map optical distortion and instrument flexure. This paper summarizes the key requirements of the CAL system, presents its conceptual design and discusses its expected performance.