MIRIM is the imager of the Mid Infrared Instrument (MIRI), one of the three scientific instruments on the James Webb Space Telescope (JWST). MIRIM will provide imaging between 5.6μm and 25.5μm, low resolution spectroscopy (LRS) between 5 and 10μm, and coronagraphy at 10.65μm, 11.4μm, 15.5μm and 23μm.
The Optical bench Assembly of MIRIM Flight Model (FM) has been integrated and tested between 2008 and 2009 at CEA (Saclay, France). The tests consist in characterisation of optical performances at all wavelengths and in all three modes (imaging, spectroscopy and coronagraphy), using a test bench (or Ground Support Equipment - GSE) that has been developed for this purpose. The GSE comprises a helium cooled cryostat for the instrument itself, a proto-IR focal plane module (with JPL sensor chip and CEA electronics and housing), a warm telescope simulator that delivers a JWST-like beam, and computers and software for running automatic test procedures. It is designed to allow a large set of performance verifications, such as high-resolution PSF measurements, characterisation of coronagraphs, response to monochromatic line or resolving power of the spectroscopic mode, some of them being unique along the test program of the instrument.
After a short description of the test equipment, this paper focuses on the tests results. A full assessment of performances is given. When applicable, performances are cross checked with requirements.
Imaging mode and coronagraphy had already been validated on optically representative models along the MIRIM development plan, especially with the Engineering and Test Model (ETM) of MIRIM, early 2008. The FM test campaign allowed us to confirm that the flight model behaves as expected in these two modes. We also tested for the first time, and validated, the low-resolution spectroscopy mode.