MOSFIRE: a multi-object near-infrared spectrograph and imager for the Keck Observatory

Abstract

MOSFIRE, the multi-object spectrometer for infra-red exploration, is a near-IR (0.97-2.45 micron) spectrograph and imager for the Cassegrain focus of the Keck I telescope. The optical design provides imaging and multi-object spectroscopy over a field of view (FOV) of 6.14' x 6.14' with a resolving power of R~3,270 for a slit width of 0.7 arc seconds (2.9 pixels along dispersion). The detector is a 2.5 micron cut-off 2K x 2K H2-RG HgCdTe array with a SIDECAR ASIC for detector control. A special feature of MOSFIRE is that its multiplex advantage of up to 46 slits is achieved using a cryogenic Configurable Slit Unit (developed in collaboration with the Swiss Centre for Electronics and Micro Technology) reconfigurable under remote control in <5 minutes without thermal cycling. Slits are formed by moving opposable bars from both sides of the focal plane. An individual slit has a length of ~7.1 arc seconds but bar positions can be aligned to make longer slits. A single diffraction grating in two positions along with order-sorting filters gives essentially full coverage of the K, H, J and Y bands using 3rd, 4th, 5th or 6th order respectively. The grating and a mirror are mounted back-to-back, and when the bars are retracted from the FOV MOSFIRE becomes a wide-field imager. A piezo tip-tilt mirror following the field lens is used to provide flexure compensation at the 0.1 pixel level. Two large CCR heads allow the instrument to reach operating temperature in ~7 days. MOSFIRE is currently in construction.