Optical Design For The 5-28 Mu M Ngst Infrared Imager Of Miri

MIRI is the mid infrared instrument planned for the NGST. Working in the 5-28 gm band, it includes 3 units: a spectrograph, an imager and a calibration facility. We describe here the optical design of the MIRI imager channel as it is at the end of the phase A study. The MIRI imager provides 3 observing modes: an imaging mode with a field of view of 1.3 arcmin x 1.7 arcmin and a Pixel Field Of View of 0.1 arcsec/pixel, a coronagraphic mode and a low resolution spectroscopic mode for point sources, between 5 mum and 10 mum, with a spectral resolution R=lambda/Deltalambda around 100.