The Mid-Infrared Instrument (MIRI) is the coldest and longest wavelength (5-28 micron) science instrument on-board the James Webb Space Telescope observatory and provides imaging, coronography and high and low resolution spectroscopy. The MIRI thermal design is driven by a requirement to cool the detectors to a temperature below 7.1 Kelvin. The MIRI Optics Module (OM) is accommodated within the JWST Integrated Science Instrument Module (ISIM) which is passively cooled to between 32 and 40 K. Thermal isolation between the OM and the ISIM is therefore required, with active cooling of the OM provided by a dedicated cryostat, the MIRI Dewar. Heat transfer to the Dewar must be minimised to achieve the five year mission life with an acceptable system mass.
Stringent cleanliness levels are necessary in order to maintain the optical throughput and the performance of thermal control surfaces. The ISIM (and MIRI OM) is launched warm, therefore care must be taken during the on-orbit cooldown phase, when outgassing of water and other contaminants is anticipated from composite structures within the ISIM. Given the strong link between surface temperature and contamination levels, it is essential that the MIRI thermal and contamination control philosophies are developed concurrently.
The MIRI is the mid-IR (5-28μm) instrument for NGST and provides for imaging, cororographic, high- and low-resolution spectroscopic capabilities. Unlike to the other instruments on NGST, the MIRI must be cooled - to reduce the thermal background from the optics and because the detectors require an operating temperature of about 7k.. In this paper we summarise the science goals, the proposed overall opto-mechanical concept, the thermal design aspects, the detectors and the expected sensitivity of the instrument.
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