STScI Logo

James Webb Space Telescope
MIRI Imager

The imager module provides broad and narrow-band imaging (5-27 µm), coronagraphy, and low-resolution (R~100, 5-10 µm) slit spectroscopy using a single 1024x1024 pixels Raytheon Si:As sensor chip assembly (SCA) with 25 µm pixels.

MIRI Functional Block Diagram

As shown in the image above, MIRI is composed of three primary components and associated assemblies namely, Optical bench Assembly (OBA), Cooler and Flight Software

The image below shows the focal plane arrangement of the various elements of the MIRI imager. The region on the left is the clear aperture available for imaging. The gray region on the right marks the mechanical frame that supports the coronagraphic masks and the slit for the low-resolution spectrometer. The coronagraphic masks include three four-quadrant phase masks (4QPM) and one opaque spot for a Lyot coronagraph. The coronagraphic masks have a square field of view of 26"x26" and are optimized for particular wavelengths (10.65, 11.5, 16 and 24 µm). The imager's only moving part is an 18-position filter wheel. Filter positions are allocated as follows: 10 filters for broad-band imaging(there are two F2550W filters, one for redundancy), one neutral density filter used only for Target Acquisition, a lens in one slot that is used ONLY for instrument testing, 4 filter and diaphragm combinations for coronagraphy, 1 ZnS-Ge double prism for the low-resolution spectroscopic mode and 1 opaque position. The imager will have a pixel scale of 0.11" and a total field of view of 113"x113"; however, the field of view of its clear aperture is 84"x113" because the coronagraph masks and the LRS are fixed on one side of the focal plane.

MIRI Focal Plane

The MIRI imager's focal plane with sizes in pixels and mm.