Gemini Infrared Multi-Object Spectrograph: Preliminary Design Overview

The Gemini Infrared Multi-Object Spectrograph (GIRMOS) is an adaptive optics-fed multi-object integral field spectrograph with a parallel imaging capability. GIRMOS implements multi-object adaptive optics (MOAO) for each of its spectrographs by taking advantage of the infrastructure offered by Gemini upcoming wide-field AO facility at Manua Kea. The instrument offers the ability to observe four objects simultaneously within the Gemini-North AO (GNAO) system's field-of-regard or a single object by tiling the four fields that feed light to four separate spectrographs. Each integral field spectrograph has an independent set of selectable spatial scales (0.025 '', 0.05 '', and 0.1 ''/spaxel) and spectral resolution (R similar to 3,000 and 8,000) within an operating band of 0.95 - 2.4 mu m. These spatial scales correspond to indvidual spectrograph fields of view of 1 x 1 '', 2 x 2 '', and 4 x4 '', respectively. GIRMOS's imager offers Nyquist sampling of the diffraction limit in H-band over a 85 x 85 '' imaging field. The imager can function in a parallel data acquisition mode with just minor vignetting spectroscopic pick-offs when they are deployed. We discuss the key science drivers and the derived instrument requirements. GIRMOS is being designed from the ground up to be an AO survey instrument by maximizing its etendue and throughput for narrow field and multi-object observations, as well as minimizing acquisition and AO setup times. Design optimizations have been made to minimize overheads and calibrations, as well as reduce the impact of pointing drift for long exposures. As a result, GIRMOS will offer unparalleled survey efficiency by reaching up to ten times more survey throughput over existing single-object spectrographs. Finally, we also present the preliminary design of the instrument, particularly its architecture, overall optical design, and key subsystems. GIRMOS is being developed in close collaboration with the GNAO facility and will be commissioned as its first light instrument.