MIRI Optics and Focal Plane

The JWST Mid-Infrared Instrument (MIRI) optical light path divides into 2 channels: a spectrometer and an imager. The spectrometer is also configured for integral field unit (IFU) spectroscopy.

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Figure 1 shows the Mid-Infrared Instrument (MIRI) field of view in relation to fields of view of other instruments.

Figure 1. Science instruments fields of view as projected onto the sky

Science instruments fields of view (FOV) as projected onto the sky

The science instruments fields of view as projected onto the sky using the JWST Science and Operations Center convention. For an observatory orientation angle of 0°, the V3 axis is aligned with celestial north and is up in this diagram. The MIRI field of view, in the upper right, is highlighted.

Optically, the instrument is divided into two channels: (1) an imager channel (MIRIM), with one detector array, and (2) a spectrometer channel (MRS) where light is further subdivided into long and short wavelength modules that each have a detector array.

MIRI's pick-off mirror, in front of the JWST optical telescope assembly focal plane, directs the MIRI field of view towards the imager. A small fold mirror adjacent to the imager light path picks off the small (up to 8" × 8") field of view of the spectrometer. A second fold in the spectrometer optical path is used to select either the light from the telescope or from the MIRI calibration system.

Figure 2. MIRI optical architecture

MIRI optical architecture

Overview of the MIRI optical architecture, showing the primary components (detailed components for MIRIM and MRS are given below). The science light path through the MIRI modules are shown in blue. Figure credit: Wright et al. 2015, Figure 2.

Imager (MIRIM)

See also: MIRI ImagingMIRI Low Resolution SpectroscopyMIRI Coronagraphic Imaging

The optics are configured to place the entrance focal plane just outside the MIRIM housing; this allows the focal plane module (which houses the coronagraph masks and low-resolution spectrometer slit) to be bolted directly to the housing in a very simple interface. 

Inside MIRIM, the field of view (FOV) is partitioned into three functional areas on the instrument focal plane: imager, coronagraph, and low-resolution spectrometer.

First, the light is collimated. At the pupil image formed by the collimator, a filter wheel holds the following: filters for both the imager and coronagraphs, a prism assembly for the low-resolution spectrometer, a blank for dark current measurements, and a pupil imaging lens. This entrance focal plane is imaged onto the detector using a three-mirror anastigmat camera with separate areas of the detector being dedicated to the imaging, coronagraphy, and spectroscopy functions. The region of the focal plane for each function is selected by a fold mirror close to the telescope focal plane.

Figure 3. MIRIM optical layout

MIRIM optical layout

M1 forms the pupil where the filters and cold stops are placed. M2 folds the beam. M3–M5 form an anastigmat that re-images the telescope focal plane onto the detector array. (Bouchet et al. 2015, Figure 2)

Medium-resolution spectrometer (MRS)

See also: MIRI Medium Resolution Spectroscopy

The MRS provides diffraction-limited integral field spectroscopy (IFU) over the whole wavelength range from 5 to 28.5 μmThis mode consists of two modules: the spectrometer pre-optics (SPO) and spectrometer main optics (SMO).

The SPO spectrally splits the light into the four spectrometer channels and spatially reformats the rectangular fields of view into slits at the entrance of the SMO. 

Figure 4. Block diagram of the MRS showing main optical functions

Block diagram of the MRS showing main optical functions

The three dichroics needed to divide the spectral band among the four spectrometer channels for one of the three sub-bands are indicated as D1, D2, and D3. (Wells et al. 2015, Figure 1)

Spectrometer pre-optics (SPO)

Inside the SPO, the light is divided into four different wavelength channels using three dichroics.

Each channel has its own dedicated integral field unit (IFU) and the spectra from each of the four channels occupy half of one of the two MRS detectors.

Each channel is split, by an additional dichroic chain, into three sub-bands that are observed sequentially by rotation of just two mechanisms that carry both the wavelength sorting dichroics and the dispersion gratings in a very compact and efficient configuration. 

Figure 5. Layout of the dichroic and fold mirrors for all channels in the spectrometer pre-optics (SPO)

Layout of the dichroic and fold mirrors for all channels in the Spectrometer Pre-Optics (SPO)

The position of the input pupil and fold mirror are labelled "IN." The locations of blocking filters (BF), light traps (LT), powered mirrors (SM) and dichroics (D) are shown. After the light has been divided into the appropriate spectral ranges, it is output to the integral field units (IFU) for the four spectrometer channels. (Wells et al. 2015, Figure 3)

Spectrometer main optics (SMO)

The SMO consists of two arms which perform the following three functions: (1) collimation of the output beams of one of the four IFUs, (2) dispersion of the collimated beam with diffraction gratings, and (3) imaging of the resulting spectrum onto one-half of one of the two focal plane arrays.

One of the two spectrometer arms includes the two short wavelength channels (1 and 2); the other has long wavelength channels (3 and 4). Each spectrometer arm uses six gratings (to allow for any combination of two wavelength channels and three sub-band exposures). The dispersed beams are imaged by three-mirror-anastigmat (TMA) camera systems (M1–M2–M3). Folding flats reflect the channel 1 and channel 4 beams such that the combined (channel 1 + 2) and (channel 3 + 4) beam pairs are imaged onto opposite halves of the detectors.

Figure 6. Cross-section of the camera symmetry plane in the spatial direction

Cross-section of the camera symmetry plane in the spatial direction

Light arrives at the gratings (G1A, G2A, G3A, G4A) from the IFU output collimating mirrors. (Wells et al. 2015, Figure 8)

Integral field units (IFUs)

See also: JWST Integral Field Spectroscopy

The optical path through the IFUs begins with the four toroidal mirrors, which comprise the anamorphic pre-optics (APO) module. The APO re-images the input focal plane (8" × 8") onto the image slicer mirror. Light exits the IFU through individual pupil masks for each beam, then through individual slitlets. Reimaging mirrors behind the slitlets relay the beam to the input of the appropriate spectrometer.

Figure 7. Optical trace of the MIRI IFU

Figure 8. MIRI IFU output slits


Boccaletti, A. et al. 2015, PASP, 127, 633 
The Mid-Infrared Instrument for the James Webb Space Telescope, V: Predicted Performance of the MIRI Coronagraphs

Bouchet, P. et al. 2015, PASP, 127, 612 
The Mid-Infrared Instrument for the James Webb Space Telescope, III: MIRIM, The MIRI Imager

Wright, G. et al. 2015, PASP, 127, 595 
The Mid-Infrared Instrument for the James Webb Space Telescope, II: Design and Build

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