Development and test programme of the wheel mechanisms for the Mid Infra-Red Instrument (MIRI) of the James Webb Space Telescope (JWST)

RESUME In 2012 NASA will launch the JWST as successor of the Hubble Space Telescope. Four scientific instruments will investigate the near and mid infra-red range between 0.6 and 28.3 microns. The Mid Infra-Red Instrument (MIRI) allows for broad and narrow band imaging with low resolution spectroscopic and coronagraphic capabilities as well as for integral field medium resolution spectroscopy above 5 microns. For this large variety of astronomical observing modes the instrument is equipped with three wheel mechanisms for positioning various elements like filters, dichroics, gratings, prisms and coronagraphic masks in the optical beam. Prime requirements for these mechanisms are high reliability under cryo-vacuum conditions (~ 7K), long lifetime (> 5years), high vibration capabilities (~ 45G), high positioning accuracy (~ 5arcsec) and low heat dissipation (< 1mW). To reach these goals simultaneously in a short timeframe, MPIA has transferred experience and heritage from its previous involvements in ESA missions like ISO and HERSCHEL into early development models (DMs), which are presently in the assembly phase at MPIA. Their detailed design and qualification programme are in the focus of this paper. 1. JAMES WEBB SPACE TELESCOPE In 2012 the James Webb Space Telescope (JWST, formerly Next Generation Space Telescope, NGST, see [1]), a joint venture between NASA, ESA and CSA, will be launched on an ARIANE 5 carrier. After a journey of more than 100 days as far as 1.5 Mio km away from the earth in the anti-solar direction, the 6600kg observatory will enter a trajectory to reach its final orbit around the second Lagrange point L2. The successor of the Hubble Space Telescope (HST) comprises a 6.5m primary mirror, see Fig. 1, passively cooled to about 45K and hosts 4 focal plane instruments: NIRCam (visible/near infra-red camera, 0.65μm, NASA contribution), FGS (near infra-red tunable filter camera, 1.24.8μm, CSA contribution) NIRSpec (near infra-red multi-object spectrograph, 0.6-5μm, ESA contribution), MIRI (mid infra-red camera and spectrograph, 5-28μm, 50% NASA / 50% ESA contribution, see [2]). While NIRCam, FGS and NIRSpec will work at a temperature of 35K passively reached by the use of radiators on the instrument’s outer walls, MIRI comprises an independent cryo cooler system for active cooling of the optical bench to about 7-18K and of the detectors down to 6K. JWST is conceived for a mission lifetime of 5 years in space, with a goal of 10 years. Fig. 1: The James Webb Space Telescope: A primary mirror of 6.5m diameter and 25m surface (pieced together by 18 monolithic beryllium hexagons) collects the light for the four scientific instruments mounted on the Integrated Science Instrument Module (ISIM, behind the M1). This structure also carries the cold head of MIRI’s cryo cooler and the Fine Guidance Sensors (FGS, Canadian contribution) for guiding and highly stabilized pointing of the observatory. A huge (tennis court size) multi-layer radiation shield blocks the direct sunlight. The spacecraft bus and the sunlight collectors are located below this shield facing towards the earth on a collateral orbit around the sun. (Source: Northrop Grumman Space Technology, NGST). 2. MID-INFRARED-INSTRUMENT: MIRI MIRI features a modular optical design in conjunction with an iso-thermal all aluminium construction, see Fig. 2. Optically the instrument is divided into an imager channel and two spectrometer channels. All channels end up in a 1kx1k Si:As detector array. The imager channel covers the range 5 to 27μm with 0.11arcsec pixels and a 1.3x1.7arcmin field of view (FOV). Additionally coronagraphy is possible at four wavelengths in this range as well as low resolution spectroscopy (double-prism + slit) with R~100. The spectrometer channel provides medium resolution integral field spectroscopy with R~3000 between 5 and 28.3μm and a 3.5x3.5 – 7x7 arcsec FOV. Only recently, due to a mass budget problem MIRI’s solid hydrogen dewar concept has been cancelled and exchanged by a Joule-Thompson cooler system (with a Stirling or pulse tube pre-cooler). Basic changes for the MIRI Optics Module are the implementation of a cold stage at 18K (e.g. for harness thermal sinking purposes) and an important temperature shift for the detector cold end from 6.9K to 6.0K allowing for a drastic reduction in dark currents. Fig. 2: The MIRI Optical Module (OM). The three wheel mechanisms are located within MIRIM (imager) and SPO (spectrometer).

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