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JWST is a world class space observatory that will add profound knowledge of the universe that Hubble began. Within the observatory there are four scientific instruments and a precision guider. During the commissioning phase,1 the primary instrument that will be used to align the mirrors and measure pupil shear will be the Near InfraRed Camera (NIRCam). The NIRCam instrument has two modules each including a focal plane positioned at the final image plane of the instrument. NIRCam2 can also use its focal planes to image the OTE pupil by using a deployable lens known as the Pupil Imaging Lens (PIL). When the PIL is deployed, NIRCam can image the pupil with the focal plane and the data can be analyzed to determine the amount of pupil shear between the Optical Telescope Element (OTE) and NIRCam. At various points in the commissioning process, the PIL is deployed and the alignment between NIRCam and the OTE is measured. Other commissioning activities measure the relative photometry and infer whether the signal is vignetted. Like NIRCam, MIRI can measure pupil shear of the system. MIRI, the Mid-Infrared Instrument comprises both a wide-field imager (5-28μm) and a medium resolution spectrograph (R 1500 to 3500)3 Unlike NIRCam which uses a deployable lens to measure the pupil, the Imager channel uses a filter wheel located at the pupil plane between the MIRI pick-off mirror and the imager detector. It has 18 filters and mask elements which can be selectively commanded into the science path. One of these elements, called the ALignment Lens (AL) or Filterwheel Lens (FLENS) projects a shadow onto the MIRI imager focal plane. The OTE shadow through the FLENS allows MIRI to serve as a cross-check to NIRCAm on pupil shear. This paper details the MIRI pupil shear analysis process used by the JWST team throughout commissioning.
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