Uncrating the Webb's MIRI Instrument (7310210944)

MIRI, or Mid-Infrared Instrument, is an instrument on the

James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope which conducts infrared astronomy. As the largest optical telescope in space, its high resolution and sensitivity allow it to view objects too old, distant, or faint for the Hubble Spa ...

. MIRI is a camera and a

spectrograph An optical spectrometer (spectrophotometer, spectrograph or spectroscope) is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify mate ...

that observes mid to long infrared radiation from 5 to 28

micron The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...

s. It also has

coronagraph A coronagraph is a telescopic attachment designed to block out the direct light from a star so that nearby objects – which otherwise would be hidden in the star's bright glare – can be resolved. Most coronagraphs are intended to view ...

s, especially for observing

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

s. 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. MIRI uses

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...

arrays doped with

arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, but ...

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 (see

Infrared astronomy Infrared astronomy is a sub-discipline of astronomy which specializes in the observation and analysis of astronomical objects using infrared (IR) radiation. The wavelength of infrared light ranges from 0.75 to 300 micrometers, and falls in betwee ...

), and it has an additional cooling system. The cooling system for MIRI includes a Pulse Tube precooler and a Joule-Thomson Loop

heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...

. This allowed MIRI to be cooled down to a temperature of 7

kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phys ...

s 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 The plate scale of a telescope connects the angular separation of an object with the linear separation of its image at the focal plane If focal length f is measured in mm, the plate scale in radians per mm is given by angular separation ''θ'' and ...

of 0.11

arcsecond A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The na ...

s/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 (1.3 by 1.7

arcmin A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The na ...

). The imaging channel has ten filters available and the detectors are made of arsenic-doped silicon ( Si: As). The detectors (one for the imager, and two for the spectrometer) each 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 (ISIM). 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. (see also Carbon fiber reinforced polymer, Carbon fiber reinforced plastic) Parts summary: *Spectrometer optics **Spectrometer Main Optics (SMO) **Spectrometer Pre Optics (SPO) *Focal Plane Arrays *Input-Optics Calibration Module (IOC) **Pick-off Mirror **Calibration source for Imager **Contamination Control Cover (CCC) *CFRP hexapod *Imager *Image slicers *Deck 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 (James Webb Space Telescope), spacecraft bus.NASA JWST Integrated Science Instrument Module (ISIM
- Accessed December 12, 2016] The imager module of MIRI also includes the Low Resolution Spectrometer that can perform long-slit and slitless spectroscopy from 5 to 12 μm light wavelength. The LRS uses Ge (germanium metal) and ZnS (zinc sulfide) prisms to cause spectroscopic dispersion. Commissioning is complete as of the following dates: * Imaging, 06/17/2022 * Low resolution spectroscopy, 06/24/2022 * Medium resolution spectroscopy, 06/24/2022 * Coronagraphic imaging, 06/29/2022

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 (James Webb Space Telescope), spacecraft bus and it has lines of coolant that 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 James Webb Space Telescope's 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 (−448.87 °F, or −267.15 °C). The ISIM is at about 40 K (due to the sunshield) and there is a dedicated MIRI radiation shield beyond which the temperature is 20 K. The J-T loop is a Joule–Thomson effect, Joule–Thomson loop

Filters

MIRI has 10 filters available for observations. * F560W - Broadband Imaging * F770W - PAH, broadband imaging * F1000W - Silicate, broadband imaging * F1130W - PAH, broadband imaging * F1280W - Broadband imaging * F1500W - Broadband imaging * F1800W - Silicate, broadband imaging * F2100W - Broadband imaging * F2550W - Broadband imaging * F2550WR - Redundant filter, risk reduction * FND - For bright target acquisition * Opaque - Darks

Diagrams

File:JWST MIRI block diag large.jpg, Diagram of MIRI and its cooler, showing the connections between the different systems related to their location.
Region 3 is inside the spacecraft bus (JWST), spacecraft bus of JWST File:JWST MIRI noCryostat large.gif, Color-coded and labeled diagram of the MIRI instrument without cryocooler File:ISIM 3 logical region.jpg, Diagram highlighting ISIM, which shows the location of the MIRI cyrocooler (color-code blue in ISIM Region 3) in the spacecraft bus, on the other side of the heat shield from the instrument.

References

External links

ESA - MIRI - the mid-infrared instrument on JWSTPresentation on MIRI's coronographs
(.pdf)
The Mid-Infrared Instrument for JWST, II: Design and Build - Wright, et al
(long paper on Miri)

{{Jwstnav Spectrographs Space imagers James Webb Space Telescope instruments