MIRI (Mid-Infrared Instrument)


MIRI, or Mid-Infrared Instrument, is an instrument on the James Webb Space Telescope. MIRI is a camera and a spectrograph that observes mid to long infrared radiation from 5 microns to 28 microns. It also has coronagraphs, especially for observing exoplanets.
Whereas most of the other instruments on Webb can see from the start of Near-infrared, or even as short as orange visible light, MIRI can see longer wavelength light where the other instruments leave off. MIRI uses silicon arrays doped with arsenic to make observations at these wavelengths. The imager is designed for wide views but the spectrograph has a smaller view. Because it views the longer wavelengths it needs to be cooler than the other instruments, and it has an additional cooling system. The cooling system for MIRI includes a Pulse Tube precooler and a Joule-Thomson Loop heat exchanger. This allows MIRI to be cooled down to a temperature of 7 kelvins during operations in space.

Overview

The spectrograph can observe wavelengths between 4.6 and 28.6 microns, and it has four separate channels, each with its own gratings and image slicers. The field of view of the spectrograph is 3.5 by 3.5 arcseconds.
The imager has a plate scale of 0.11 arcseconds/pixel and a field of view of 74 by 113 arcseconds. Earlier in development the field of view was going to be 79 by 102 arcseconds. The imaging channel has ten filters available and the detectors are made of arsenic-doped silicon. The detectors have a resolution of 1024x1024 pixels, and they are called Focal Plane Modules or FPMs.
During 2013 and finishing in January 2014, MIRI was integrated into the Integrated Science Instrument Module. MIRI successfully passed Cryo Vac 1 and Cryo Vac 2 tests as part of ISIM in the 2010s. MIRI was developed by an international consortium.
MIRI is attached to the ISIM by a carbon-fiber and plastic hexapod structure, which attaches it to the spacecraft but also helps thermally isolate it.
Parts summary:
Most of MIRI is located in the main ISIM structure, however the cryocooler is in region 3 of ISIM which is located in the Spacecraft Bus.
The Imager module of MIRI also includes the a Low Resolution Spectrometer that can perform long-slit and slitless spectroscopy from 5 to 12 μm light wavelength. The LRS uses Ge and ZnS prisms to cause spectroscopic dispersion.

Cryocooler

To allow mid-infrared observations within the JWST, the MIRI instrument has an additional cooling system. It works roughly similar to how most refrigerators or an air-conditioner works: a fluid is brought down to a cold temperature in the warm section, and sent back to the cold section where it absorbs heat, then it goes back to the condenser. One source of heat is the left-over heat of the spacecraft, but another is the spacecraft's own electronics, some of which are close to the actual instruments to process data from observations. Most of the electronics are in the much warmer spacecraft bus, but some of the electronics needed to be much closer, and great lengths were taken to reduce the heat they produce. By reducing how much heat the electronics make on the cold side, then less heat needs to be removed.
In this case the JWST cryocooler resides in the Spacecraft Bus and it has lines of coolant the run to the MIRI instrument, chilling it. The cryocooler has a heat radiator on the spacecraft bus to emit the heat it collects. In this case the cooling system uses helium gas as the refrigerant.
The JWST cryocooler is based originally on the TRW ACTDP cryocooler. However, the JWST has had to develop a version to handle higher thermal loads. It has a multi-stage pulse tube refrigerator that chills an even more powerful cooler. That is a linear-motion Oxford-style compressor that powers a J-T loop. Its target is to cool the MIRI instrument down to 6 kelvins. The ISIM is at about 40 K and there is a dedicated MIRI radiation shield beyond which the temperature is 20 K. The J-T loop is a Joule–Thomson loop heat exchanger.

Diagrams

Region 3 is inside the Spacecraft Bus of JWST