11/7/2005 submitted Conference: Astronomical Telescopes and Instrumentation 2006 Ground-based and Airborne Instrumentation (Iye, McLean) Title: Infrared Multi-Object Spectrograph of MOIRCS Primary Author: Chihiro Tokoku Co-Authors: Ryuji Suzuki, Koji Omata, Masahiro Konishi, Tomohiro Yoshikawa, Tetsuo Nishimura Abstract: Design, development, operation and test observation results of MOS mode of MOIRCS are described. MOIRCS (Multi-Object Infrared Camera and Spectrograph) is one of the second-generation instruments for the Subaru Telescope and provides imaging mode with 4' x 7' field of view and multi-object spectroscopy (MOS) mode for the wavelength range from 0.85 to 2.5 micrometers. To achieve infrared MOS up to K-band, MOS mode uses cooled multi-slit masks and a mask exchange system in cryogenic environment. The masks are housed in a vacuum dewar attached to the MOIRCS main dewar and separated by a large gate valve. The mask dewar is equipped with its own cryogenic cooler and a vacuum pump and can store nineteen masks. The masks are made of thin aluminum film. Slits are cut with a laser. For slits cutting coordinates, the software corrects the effects of thermal contraction. Masks are cooled down to below 130 K in the mask dewar and transported to the focal plane in the main dewar through the gate valve with a linear motion manipulator. Interlock is equipped on the mask exchange system to secure the cryogenic instrument from accident. Replacing masks can be done in a daytime without breaking vacuum of the main dewar by isolating the mask dewar by closing the gate valve. The basic idea of acquisition is to iterate taking sky images through alignment holes on mask until rotation and offset between alignment stars and alignment holes become small enough. MOIRCS/MOS mode will be open to public in late 2006. Keywords: near infrared, multi-object spectroscopy, multi-slit mask Principal Author Biography: Chihiro Tokoku is a junior research astronomer of the Subaru Telescope. She received the B.S. in Physics from Tokyo Metropolitan University in 1998, and the M.S. in Astronomy from Tohoku University in 2001. Her research interests are observational study of large scale structure and galaxy evolution in the early universe. She is currently working for commissioning of MOIRCS and preparing for opening to public use.