MIRI Spectrometer Optical Design

MIRI (the Mid InfraRed Instrument) is one of the focal plane instruments of the James Webb Space Telescope. The instrument comprises a camera and a spectrometer module. The instrument plays the following key roles in the JWST science program.- Discovery of the "first light".- Assembly of galaxies: history of star formation, growth of black holes, production of heavy elements.- Formation of stars and planetary systems.- Evolution of planetary systems and conditions for life.The MIRI spectrometer covers the spectral range from 5 -28.3 mum with a spectral resolution better than 2000. The spectral module is an imaging spectrometer with a field of view greater than or equal to 3 arcsec. The spectrometer consists of 4 spectral channels that share 2 detectors of 1024 x 1024 pixels each. In a single measurement one third of the wavelength range of the channels is imaged onto the detectors. By changing gratings the whole spectral range is covered in 3 measurements.In this paper the optical design of the MIRI spectrometer is described.