The Mid-Infrared Instrument (MIRI), on-board the James Webb Space Telescope (JWST), was designed to produce a diffraction-limited Point Spread Function (PSF) at the detector image plane in the 5 to 28 micron wavelength range. For the MIRI Medium-Resolution Spectrometer (MRS), a PSF broadening of 60% down to 10% is observed in the 5 to 28 micron range. Additionally, 20% of the light is scattered into the wings as an extended component on the detector. The same PSF systematics manifest in the MIRI Imager and Low-Resolution Spectrometer (LRS) data. We use physical optics propagation to propagate a uniform wavefront from the JWST pupil to the MIRI Imager detector plane. The camera F-number and variation of incidence angle across the detector allow us to reproduce the detailed features of the cruciform, as well as an observed bending in the cruciform arms that changes across the detector. This presents a significant leap for PSF-weighted photometry. The model can be extended to the LRS and potentially to the MRS, although the optical path of the latter is much more complex to model.
The direct characterization of exoplanetary systems with high contrast imaging is among the highest priorities for the broader exoplanet community. As large space missions will be necessary for detecting and characterizing exo-Earth twins, developing the techniques and technology for direct imaging of exoplanets is a driving focus for the community. For the first time, JWST will directly observe extrasolar planets at mid-infrared wavelengths beyond 5 μm, deliver detailed spectroscopy revealing much more precise chemical abundances and atmospheric conditions, and provide sensitivity to analogs of our solar system ice-giant planets at wide orbital separations, an entirely new class of exoplanet. However, in order to maximise the scientific output over the lifetime of the mission, an exquisite understanding of the instrumental performance of JWST is needed as early in the mission as possible. In this paper, we describe our 55-hour Early Release Science Program that will utilize all four JWST instruments to extend the characterisation of planetary mass companions to ∼15-20 μm as well as image a circumstellar disk in the mid-infrared with unprecedented sensitivity. Our program will also assess the performance of the observatory in the key modes expected to be commonly used for exoplanet direct imaging and spectroscopy, optimize data calibration and processing, and generate representative datasets that will enable a broad user base to effectively plan for general observing programs in future cycles.
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