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.
The Mid Infra Red Instrument (MIRI) is one of the four instruments onboard the James Webb Space Telescope (JWST),
providing imaging, coronagraphy and spectroscopy over the 5 - 28 μm band. To verify the optical performance of the
instrument, extensive tests were performed at CEA on the flight model (FM) of the Mid-InfraRed IMager (MIRIM) at
cryogenic temperatures and in the infrared. This paper reports on the point spread function (PSF) measurements at 5.6 μm,
the shortest operating wavelength for imaging. At 5.6 μm, the PSF is not Nyquist-sampled, so we use am original technique
that combines a microscanning measurement strategy with a deconvolution algorithm to obtain an over-resolved MIRIM
PSF. The microscanning consists in a sub-pixel scan of a point source on the focal plane. A data inversion method is used
to reconstruct PSF images that are over-resolved by a factor of 7 compared to the native resolution of MIRI. We show that
the FWHM of the high-resolution PSFs were 5 - 10 % wider than that obtained with Zemax simulations. The main cause
was identified as an out-of-specification tilt of the M4 mirror. After correction, two additional test campaigns were carried
out, and we show that the shape of the PSF is conform to expectations. The FWHM of the PSFs are 0.18 - 0.20 arcsec,
in agreement with simulations. 56.1 - 59.2% of the total encircled energy (normalized to a 5 arcsec radius) is contained
within the first dark Airy ring, over the whole field of view. At longer wavelengths (7.7 - 25.5 μm), this percentage is
57 - 68 %. MIRIM is thus compliant with the optical quality requirements. This characterization of the MIRIM PSF, as
well as the deconvolution method presented here, are of particular importance, not only for the verification of the optical
quality and the MIRI calibration, but also for scientific applications.
The present paper describes the different steps leading to the Flight Model integration of the Mid-Infra Red IMager
Optical Bench MIRIM-OB which is part of the scientific payload of the JWST. In order to demonstrate a space
instrument capability to survive the challenging space environment and deliver the expected scientific data, a specific
development approach is applied in order to reduce the high level of risks. The global approach for MIRIM-OB, and the
principal results associated to the two main models, the Structural Qualification Model for vibration and the Engineering
and Test Model for optical performance measured in the infra red at cryogenic temperature will be described in this