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James Webb Space Telescope
Mid Infrared Instrument

The integral field spectrograph obtains simultaneous spectral and spatial data on a relatively compact region of sky. The spectrograph field of view is next to that of the imager so that accurate positioning of targets will be possible by locating the image with the imager channel and off-setting to the spectrograph. The spectrograph uses four image slicers to produce dispersed images of the sky on two 1024x1024 SCAs to provide R~3000 integral field spectroscopy over a λ=5-27 µm wavelength range with a goal of λ=5-28.3 µm. The four integral field units (IFUs) divide the spectral window as follows: (1A) 4.96-7.77 µm, (1B) 7.71-11.90 µm, (2A) 11.90-18.35 µm, and (2B) 18.35-28.30 µm.

As in the figure above, the IFUs provide four simultaneous and overlapping fields of view, ranging from 3.6″x3.6″ to ~7.6″x7.6″ with increasing wavelength. Slice widths for each image slicer in order of increasing wavelength are 0.18″, 0.28″, 0.39″, and 0.64″ and set the angular resolution perpendicular to the slice. The pixel sizes in order of increasing wavelength are 0.19″, 0.19″, 0.24″ and 0.27″ and set the angular resolution along the slice. The number of slices decreases from 30 to 12 with increasing wavelength. The spectral window of each IFU channel is covered using three separate gratings (i.e. 12 gratings to cover the four channels). Each grating is fixed in orientation and can be rotated into the optical path using a wheel mechanism (there are two wheel mechanisms which each hold 3 pairs of gratings). The optics system for the four IFUs is split into two identical sections (in terms of optical layout). One section is dedicated to the two short wavelength IFUs and the other handles the two longer wavelength IFUs. There is one SCA for each section. The two sections share the wheel mechanisms (each mechanism incorporates three gratings for one of the channels in the short wavelength section and three gratings for one of the channels in the long wavelength section).

Sensitivity: The MIRI limiting sensitivity in imaging at 10 and 20µm (with S/N=10 in 10,000 seconds) are, respectively, AB=24.53 and AB=22.15. The limiting flux in 10,000 seconds for a resolving power of R=2400 and at 9.2µm is 3.4 10-18erg cm-2s-1.

Medium Resolution Spectrograph

As shown schematically in the figure below, the image slicers in the MIRI IFU dissect the input image plane. The dispersed spectra from two IFU inputs are placed on one detector side-by-side. The spatial information from the IFU is spread out into two adjacent rows with the information from each slice separated by a small gap.

  • The Spectrometer has two arms, each with its own Focal Plane Module.
  • Each spectrometer arm is further divided into 2 channels.
  • Two mechanisms present gratings and dichroics which cover the full 5 to 28 micron wavelength range by rotation between three positions.