Mid-Infrared Instrument
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MIRI
MIRI being integrated into ISIM, 2013
MIRI's cooling system being tested
MIRI is uncrated at Goddard Space Flight Center, 2012
Infographic of James Webb Space Telescope instruments and their observation ranges of light by wavelength

MIRI, or the Mid-Infrared Instrument, is an instrument on the James Webb Space Telescope.[1] MIRI is a camera and a spectrograph that observes mid to long infrared radiation from 5 to 28 microns.[1] It also has coronagraphs, especially for observing exoplanets.[2] 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.[1]

MIRI uses silicon arrays doped with arsenic to make observations at these wavelengths.[1] The imager is designed for wide views but the spectrograph has a smaller view.[1] Because it views the longer wavelengths it needs to be cooler than the other instruments (see Infrared astronomy), and it has an additional cooling system.[1] The cooling system for MIRI includes a pulse tube precooler and a Joule-Thomson loop heat exchanger.[1] This allowed MIRI to be cooled down to a temperature of 7 kelvins during operations in space.[1]

MIRI was built by the MIRI Consortium, a group that consists of scientists and engineers from 10 different European countries (the United Kingdom, France, Belgium, the Netherlands, Germany, Spain, Switzerland, Sweden, Denmark, and Ireland) with the United Kingdom heading the European consortium,[3] as well as a team from the Jet Propulsion Lab in California, and scientists from several U.S. institutions.[4][5]

Overview

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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.[2] The field of view of the spectrograph is 3.5 by 3.5 arcseconds.[2]

The spectrograph is capable of low-resolution spectroscopy (LRS) with or without a slit, as well as medium-resolution spectroscopy (MRS) taken with an integral field unit (IFU). This means that MRS with the IFU creates an image cube. Similar to other IFUs this can be compared to an image that has a spectrum for each pixel.[6]

The imager has a plate scale of 0.11 arcseconds/pixel and a field of view of 74 by 113 arcseconds.[7] Earlier in development the field of view was going to be 79 by 102 arcseconds (1.3 by 1.7 arcmin).[2] The imaging channel has ten filters available and the detectors are made of arsenic-doped silicon (Si:As).[1] 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.[8]

During 2013 and finishing in January 2014, MIRI was integrated into the Integrated Science Instrument Module (ISIM).[5] MIRI successfully passed Cryo Vac 1 and Cryo Vac 2 tests as part of ISIM in the 2010s.[5]

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.[1] (see also Carbon fiber reinforced plastic)

Parts summary:[9]

  • Spectrometer optics
    • Spectrometer Main Optics (SMO)[8]
    • Spectrometer Pre Optics (SPO)[8]
  • Focal Plane Arrays
  • Input-Optics Calibration Module (IOC)[8]
    • Pick-off Mirror[8]
    • Calibration source for Imager[8]
    • Contamination Control Cover (CCC)[8]
  • 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.[10]

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.[11] The LRS uses Ge (germanium) and ZnS (zinc sulfide) prisms to cause spectroscopic dispersion.[11]

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

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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, less heat needs to be removed.

In this case the JWST cryocooler resides in the 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.[12] 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.[12] However, the JWST has had to develop a version to handle higher thermal loads.[13] It has a multi-stage pulse tube refrigerator that chills an even more powerful cooler.[12] That is a linear-motion Oxford-style compressor that powers a J-T loop.[13] Its target is to cool the MIRI instrument down to 6 kelvins (−448.87 °F, or −267.15 °C).[12] 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.[12] The J-T loop is a Joule–Thomson loop heat exchanger.[1]

Filters

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MIRI imaging filters

MIRI imaging has 10 filters available for observations.[14]

  • 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

MIRI Coronagraphic imaging has 4 filters available for observations.[14]

  • F1065C - useful for ammonia and silicates
  • F1140C
  • F1550C
  • F2300C

The low-resolution spectrometer (LRS) uses a double zinc sulfide/germanium (ZnS/Ge) prism. The slit mask has a filter that blocks light with a wavelength shorter than 4.5 μm. LRS covers 5 to 14 μm.[14]

The medium-resolution spectrometer (MRS) has 4 channels that are observed simultaneously. Each channel is however further divided into 3 different spectral settings (called short, medium and long). In one observation MIRI can only observe one of those three settings. An observation that aims to observe the entire spectrum has to carry out 3 separate observations of the individual settings. MRS covers 4.9 to 27.9 μm.[14]

Diagrams

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

See also

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References

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Mid-Infrared Instrument

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The Mid-Infrared Instrument (MIRI) is a scientific instrument aboard the James Webb Space Telescope (JWST), designed to observe astronomical objects in mid-infrared wavelengths ranging from 5 to 28.3 microns, enabling the study of cooler celestial phenomena such as forming stars, debris disks, exoplanets, and distant galaxies that are obscured by dust in shorter wavelengths.[1][2] MIRI combines a camera for broadband imaging, a spectrograph for medium-resolution spectroscopy, and coronagraphs to block bright starlight for detecting faint companions, all operating within a field of view of approximately 1.2 by 1.9 arcminutes.[1] It supports multiple observing modes, including wide-field imaging from 5.6 to 25.5 microns, slitless and slitted spectroscopy from 4.9 to 28.8 microns, and an integral field unit for spatially resolved spectral mapping, making it the only mid-infrared instrument on JWST.[1][2] Developed through an international collaboration led by NASA and the European Space Agency (ESA), with principal investigators from institutions including the University of Arizona and the UK Astronomy Technology Centre, MIRI's optical system was built by a European consortium, while its detectors were fabricated at NASA's Jet Propulsion Laboratory (JPL).[1][2] The instrument requires extreme cooling to approximately -266°C to minimize thermal noise, achieved via a specialized cryocooler provided by Northrop Grumman Aerospace Systems, ensuring high sensitivity for detecting faint mid-infrared emissions.[2] Launched with JWST on December 25, 2021, MIRI has since delivered groundbreaking observations, such as detailed spectra of exoplanet atmospheres and infrared images of star-forming regions, advancing JWST's core science themes of understanding the universe's earliest galaxies, star and planet formation, and the evolution of solar systems.[1]

Introduction

Overview

The Mid-Infrared Instrument (MIRI) is one of four science instruments on the James Webb Space Telescope (JWST), operating in the mid-infrared portion of the electromagnetic spectrum from 5 to 28.5 micrometers.[3] As the sole mid-infrared instrument aboard JWST, MIRI extends the observatory's capabilities to wavelengths where cooler celestial objects emit most of their radiation, complementing the near-infrared instruments.[4] It is mounted on the JWST's Integrated Science Instrument Module (ISIM), a structure that houses all science instruments and fine guidance sensors.[2] MIRI offers imaging, low- and medium-resolution spectroscopy, and coronagraphy, enabling detailed observations of cool astrophysical phenomena such as exoplanets, debris disks around stars, star-forming regions, and distant galaxies obscured by dust.[1] These modes support broadband photometric imaging across ten filters, integral field spectroscopy with resolutions up to R ≈ 3500, and high-contrast imaging to suppress bright starlight for studying faint companions.[3][5] The instrument's design prioritizes sensitivity in this wavelength regime to reveal thermal emissions and molecular signatures invisible at shorter wavelengths.[4] MIRI's spectrometer module divides the wavelength coverage into four channels spanning 5 to 28.5 μm using dichroic beam splitters, which together probe dust-enshrouded star formation and redshifted light from the early universe.[3][6] Physically, MIRI stands approximately 1.2 meters tall with dimensions of about 1.2 m × 1.2 m × 1.0 m and a mass under 115 kg, optimized for the cryogenic environment of space.[2]

Scientific Objectives

The Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) was designed to address key astrophysical questions in three primary science themes: probing the formation and evolution of stars and planets, investigating the assembly of galaxies in the early universe, and searching for potential biosignatures on exoplanets. These objectives leverage MIRI's sensitivity to wavelengths between 5 and 28 μm to explore phenomena obscured or inaccessible at shorter wavelengths, such as the initial episodes of star formation, the chemical processes in protoplanetary disks, and the atmospheric compositions of distant worlds that could indicate habitability. By enabling detailed spectroscopy and imaging of these targets, MIRI contributes to understanding how planetary systems emerge from stellar nurseries and how life-supporting conditions might arise beyond the Solar System.[7][8] A major advantage of mid-infrared observations with MIRI is its ability to penetrate cosmic dust, which absorbs and scatters shorter-wavelength light, allowing views into heavily obscured regions like star-forming clouds and galactic cores. Additionally, MIRI excels at detecting polycyclic aromatic hydrocarbons (PAHs), complex organic molecules that trace the interstellar medium and star formation activity through their characteristic emission features around 5–15 μm. The instrument is particularly suited for measuring the temperatures of cool objects, such as Jupiter-like exoplanets emitting peak thermal radiation at 5–10 μm, enabling characterization of their atmospheres and surface properties that are invisible in near-infrared or optical bands. These capabilities stem from MIRI's cryogenic operation and low thermal background, which enhance signal-to-noise for faint, dust-enshrouded sources.[1][9][10] MIRI complements JWST's near-infrared instruments, such as NIRCam and NIRSpec, by accessing longer wavelengths that capture redshifted rest-frame ultraviolet and optical light from galaxies at redshifts z > 7, revealing their star formation histories and structural evolution during the epoch of reionization. This extension allows for multi-wavelength studies that trace the journey of light from the universe's first stars to mature galaxies, filling a critical gap in sensitivity beyond 5 μm.[8][9] Among specific targets, MIRI focuses on low-mass stars and their surrounding protoplanetary disks to dissect the delivery of water and organics during planet formation, active galactic nuclei (AGN) to probe the role of supermassive black holes in galaxy evolution through their dust-obscured tori, and Kuiper Belt objects in the Solar System to analyze primitive icy bodies that preserve early Solar System conditions. These observations provide insights into the building blocks of planetary systems and the feedback mechanisms shaping cosmic structures.[1][11][12]

Development

History

The Mid-Infrared Instrument (MIRI) for the James Webb Space Telescope (JWST) originated in the 1990s amid conceptual studies for the Next Generation Space Telescope (NGST), JWST's predecessor project, which emphasized infrared observations to probe the early universe. Following the 1990 U.S. decadal survey endorsement of a large infrared-optimized telescope, mid-infrared capabilities were formally recommended in 1996 by the Dressler committee to extend spectral coverage beyond 20 μm, leading to MIRI's selection as a core instrument. This established a collaborative framework involving NASA and the European Space Agency (ESA), with principal development shared equally between U.S. and European partners.[13] Development progressed through structured phases, beginning with Phase A feasibility studies from 2000 to 2002, which refined MIRI's design for imaging, spectroscopy, and coronagraphy in the 5–28 μm range. The Preliminary Design Review (PDR) was passed in December 2004 by the European consortium responsible for the optical bench assembly, validating the overall architecture. The Critical Design Review (CDR) followed in 2008, confirming the detailed engineering plans after addressing cryogenic and mechanical challenges, allowing transition to full fabrication.[14] The completed MIRI instrument was handed over to NASA by the European consortium in May 2012. Assembly and testing culminated in the integration of MIRI into JWST's Integrated Science Instrument Module (ISIM) in early 2014 at NASA's Goddard Space Flight Center, followed by extensive cryogenic qualification testing in phases: Cryo-Vacuum Test 1 (CV1) in 2014, CV2 in 2016, and CV3 in 2017. The instrument was then incorporated into the full observatory and launched on December 25, 2021, via an Ariane 5 rocket from the Guiana Space Centre in Kourou, French Guiana. Initial development costs totaled approximately $100 million USD, split between NASA and ESA contributions. Technical hurdles, particularly in achieving the required 7 K operating temperature via a mechanical cryocooler and resolving integration complexities, contributed to delays that aligned with JWST's overall schedule shift from 2014 to 2021.[15][16][17][18]

Key Collaborators

The development of the Mid-Infrared Instrument (MIRI) for the James Webb Space Telescope (JWST) was conducted through a 50-50 partnership between NASA and the European Space Agency (ESA), involving contributions from a consortium of European institutes across ten countries including Belgium, Denmark, France, Germany, Ireland, the Netherlands, Spain, Sweden, Switzerland, and the United Kingdom, in collaboration with U.S. institutions.[17][8][19] This international effort engaged more than 200 scientists and engineers, coordinated under a tri-agency framework that also included support from the Canadian Space Agency (CSA) for overall telescope integration, including the Fine Guidance Sensor.[19][17] NASA's Jet Propulsion Laboratory (JPL) served as the lead U.S. institution, managing project oversight, providing the cryocooler system, and handling integration aspects.[20][21][22] ESA coordinated the European contributions, which encompassed approximately half of the instrument's funding and hardware development, led by national space agencies and laboratories.[17][8] The CSA's role supported fine guidance sensor integration within the JWST's Integrated Science Instrument Module (ISIM), facilitating MIRI's alignment and operational stability.[17][23] Key leadership was provided by principal investigators George Rieke from the University of Arizona, who served as the U.S. PI from 1997 to 2019, and Gillian Wright from the UK Astronomy Technology Centre, who acted as the European PI.[24][25] Current operations are led by Alvaro Labiano at ESA, overseeing team coordination and performance verification for MIRI post-launch.[26][27] Major contractors included the Rutherford Appleton Laboratory (RAL Space, UK), which handled thermal design, assembly, integration, and warm electronics for the instrument.[28][29] CEA Saclay in France contributed to the imager subsystem, including opto-mechanical elements and focal plane module development in collaboration with NASA-provided detectors.[30][31][32]

Design

Optical Layout

The Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) features a compact optical layout that reimages the telescope's focal plane onto its detectors. Light from the JWST's 6.5 m primary mirror enters MIRI through the instrument's entrance pupil, where a pick-off mirror (POM) selects the field of view and directs the beam into either the imager (MIRIM) module or the medium-resolution spectrometer (MRS) module. This initial optics assembly includes a field lens to correct for field distortions and a collimator that reimages the pupil plane, allowing for the insertion of cold stops, filters, and other elements to minimize thermal background and stray light.[33] The imager path provides broadband imaging, coronagraphy, and low-resolution spectroscopy using dedicated optics, including a filter wheel and three-mirror anastigmat (TMA) camera that reimages the field onto a 1024 × 1024 pixel detector. The imager delivers a field of view of 74 × 113 arcseconds with a plate scale of 0.11 arcsec per pixel across 5.6–25.5 μm.[34] In the spectrometer path, following collimation, a dichroic beam splitter divides the incoming light at approximately 11 μm into the short-wavelength (SW) arm (channels 1–2, 5–18 μm) and the long-wavelength (LW) arm (channels 3–4, 14–28 μm), enabling parallel processing in separate optical arms. Each arm uses integral field units (IFUs) for spatially resolved spectroscopy, with light dispersed and reimaged via TMA optics onto dedicated detectors. The SW arm uses a 1024 × 1024 pixel array, while the LW arm uses a 1032 × 1032 pixel array to accommodate the design. The MRS fields of view vary by channel: approximately 3.3″ × 3.7″ for channel 1 to 7.2″ × 8.1″ for channel 4, with plate scales of ~0.11 arcsec/pixel for SW channels and ~0.36 arcsec/pixel for LW channels. These TMAs provide aberration-corrected imaging with a compact footprint, folding the beam as needed to fit within MIRI's cryogenic enclosure. The overall path from entrance pupil to focal plane ensures high throughput (>40% in key bands) and maintains the JWST's diffraction-limited performance at mid-infrared wavelengths.[33][6][35]

Cryogenic Systems

The Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) relies on advanced cryogenic systems to maintain ultra-low temperatures, essential for reducing thermal emission and enabling sensitive mid-infrared observations. All three detectors operate at approximately 7 K, while the instrument's optics are cooled to below 7 K to minimize background noise from self-emission. These temperatures are achieved through a combination of active and passive cooling mechanisms, ensuring the instrument's performance over the mission lifetime.[3] Active cooling for MIRI is provided by a closed-cycle cryocooler developed by Northrop Grumman Space Systems, featuring a multi-stage design that includes pulse-tube precooling to an intermediate stage at ~18 K and a Joule-Thomson expansion stage. This system delivers cooling to 6.2 K at the cold tip with a heat lift capacity of 40 mW, while requiring an input power of about 200 W during steady-state operation. The cryocooler interfaces with the instrument via heat exchangers, where the 6–7 K stage cools the detectors and optics, and the 18 K stage supports warmer shields. This configuration allows MIRI to achieve its required thermal stability without expendable cryogens, a key enabler for long-duration space missions.[36][37] Passive cooling elements complement the active system, utilizing multi-layer insulation (MLI) blankets to block radiative heat from warmer spacecraft components and vapor-cooled shields thermally linked to JWST's Integrated Science Instrument Module (ISIM) shield at approximately 40 K. These shields intercept parasitic heat loads, preventing them from reaching the colder stages and maintaining the overall thermal gradient. The design incorporates redundancy through backup control electronics and fault-tolerant operation modes in the cryocooler stages, ensuring reliability and mission longevity exceeding 10 years even in the event of minor anomalies.[38][39]

Detectors and Electronics

The Mid-Infrared Instrument (MIRI) utilizes three arsenic-doped silicon (Si:As) detector arrays employing impurity band conduction (IBC) technology to enable high sensitivity across the 5–28 μm wavelength range, with a cutoff wavelength of approximately 28 μm for all detectors. MIRI has one 1024 × 1024 pixel array (25 μm pitch) for the imager (MIRIM), supporting imaging, coronagraphy, and low-resolution spectroscopy with a plate scale of 0.11 arcsec/pixel; one 1024 × 1024 array for the short-wavelength MRS channels (1–2); and one 1032 × 1032 array (with 1024 active pixels plus reference columns) for the long-wavelength MRS channels (3–4), providing a coarser plate scale of ~0.36 arcsec/pixel. These detectors were specifically developed to meet the James Webb Space Telescope's (JWST) requirements for low background noise in the thermally dominated mid-infrared regime.[40] The readout system incorporates multiplexers with 32 parallel outputs, allowing the full array—including 1024 active pixels per row plus four reference pixels at each end—to be read out in under 3 seconds at a pixel sampling rate of 10 μs. This configuration supports multiple readout modes, such as FASTR1 for rapid imaging and SLOWR1 for spectroscopy, with the Hawaii-2RG enabling interleaved data streams across four outputs. Readout noise is maintained below 15 electrons root-mean-square (RMS) through correlated double sampling (CDS), which subtracts reset and signal levels to mitigate reset noise and kTC noise contributions. Quantum efficiency exceeds 70% at 10 μm, ensuring efficient photon collection across MIRI's operational band. The full well capacity reaches approximately 200,000 electrons, accommodating bright sources without saturation in typical exposures.[41][42][43] MIRI's electronics architecture separates warm and cold components to optimize performance while minimizing thermal interference. The warm electronics box, operating at approximately 300 K, manages command processing, telemetry data handling, and interface with JWST's spacecraft systems via the SpaceWire protocol. In contrast, the cold readout multiplexers and supporting circuitry function at cryogenic temperatures of 7 K, provided by the instrument's cryocooler, to suppress thermal noise in the detector arrays. Key noise sources include dark current, measured at a median of less than 0.2 electrons per second per pixel under flight conditions at operating temperatures around 7 K, which is mitigated primarily through CDS and by selecting low-background observing strategies. These systems collectively enable MIRI's detectors to achieve the low noise floor necessary for detecting faint mid-infrared emission from distant galaxies and exoplanetary atmospheres.[41][42][44]

Components

Filters

The Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope utilizes a suite of photometric and spectroscopic filters to isolate specific wavelength bands for imaging, spectroscopy, and high-contrast observations. These are multi-layer dielectric interference filters optimized for the 5–28 μm mid-infrared regime, providing high peak transmission typically exceeding 80% within their passbands to maximize sensitivity while minimizing thermal background. The filters are mounted on a cryogenic filter wheel assembly within the instrument's optical path, allowing selectable insertion for different observing configurations. Fabrication of these filters was led by the UK Astronomy Technology Centre (UKATC) as part of the MIRI European Consortium, ensuring precise coatings and durability under cryogenic conditions.[5][45][34] In the imaging mode, MIRI employs 9 broadband and narrowband filters spanning 5.6 to 25.5 μm, with bandwidths (full width at half maximum) ranging from 0.73 to 4.58 μm. These filters enable broad photometric coverage across key mid-infrared features, such as silicate dust emission and polycyclic aromatic hydrocarbon bands, with representative examples including F560W (centered at 5.6 μm for short-wavelength imaging), F770W (7.7 μm, targeting the 7.7 μm PAH feature), F1130W (11.3 μm, a narrowband for the 11.3 μm silicate feature), and F2100W (21.0 μm, for longer-wavelength continuum). An additional neutral density filter (FND, centered near 13 μm with a 6.73 μm bandwidth) attenuates bright sources to prevent detector saturation. The filters achieve high throughput, with average transmissions of 0.245–0.466 across the bandpasses when combined with the instrument response.[34][46][5]
Filter NameCentral Wavelength (μm)Bandwidth (μm)
F560W5.61.00
F770W7.71.95
F1000W10.01.80
F1130W11.30.73
F1280W12.82.47
F1500W15.02.92
F1800W18.02.95
F2100W21.04.58
F2550W25.53.67
For medium-resolution spectroscopy (R ≈ 3000), order-sorting filters prevent spectral order overlap by blocking unwanted shorter wavelengths, enabling clean extraction of dispersed light across the 4.9–27.9 μm range. These dichroic filters are integrated into the spectrometer's fore-optics and define the passbands for the four integral field unit channels: Channel 1 (4.9–7.7 μm), Channel 2 (7.5–11.7 μm), Channel 3 (11.6–18.0 μm), and Channel 4 (17.7–27.9 μm). Each channel uses grating settings (short, medium, long) to cover sub-bands, with the filters ensuring isolation for resolving power up to 3750 in shorter wavelengths.[6][5] Coronagraphic imaging relies on four dedicated filters centered at 10.6 μm (F1065C), 11.3 μm (F1140C), 15.5 μm (F1550C), and 23.0 μm (F2300C), with bandwidths of 0.75–5.5 μm. These filters pair with occulting masks and Lyot pupil stops to suppress diffracted starlight, achieving throughputs of 62–72% after pupil attenuation for optimal contrast in exoplanet and circumstellar disk studies.[46][47]

Dispersive Elements

The dispersive elements of the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) are critical for enabling its spectroscopic observing modes, particularly the medium-resolution spectrometer (MRS) and low-resolution spectrometer (LRS). These elements disperse incoming mid-infrared light to produce spectra that reveal chemical compositions, temperatures, and kinematics of celestial objects. The design prioritizes high efficiency in the cryogenic environment, with materials selected for transparency and low absorption in the 5–28 μm range. The MRS utilizes immersion gratings fabricated from zinc sulfide (ZnS) for shorter wavelengths and cadmium telluride (CdTe) for longer wavelengths, providing a spectral resolving power of approximately R ≈ 3000. These gratings operate across four channels (1 through 4), collectively spanning wavelengths from 4.9 to 27.9 μm, with groove densities varying from 100 to 300 lines per millimeter to optimize dispersion in each channel. The immersion configuration enhances efficiency by coupling light into the high-refractive-index medium, reducing the physical size of the spectrometer while maintaining high resolving power. This setup allows for detailed medium-resolution spectroscopy, capturing fine spectral features such as molecular lines and dust emission bands. For lower-resolution broadband spectroscopy, MIRI incorporates prism pre-optics in the LRS mode, delivering a resolving power of R ≈ 100 over the 5–10 μm range. The prism disperses light with a simple refractive mechanism, suitable for quick surveys of continuum sources and broad emission features without the need for high angular or spectral detail. In both modes, dispersed light is fed to the detectors through integral field units (IFUs), which slice the field of view into spatial elements with widths ranging from 0.19 to 0.64 arcseconds depending on the channel before dispersion. This preserves spatial information alongside spectral data, forming three-dimensional datacubes. The resulting wavelength resolution of Δλ/λ=1/3000\Delta \lambda / \lambda = 1/3000 supports velocity resolutions of approximately 100 km/s, enabling studies of galactic dynamics and outflow velocities in astrophysical environments.

Pupil Imaging Lens

The Pupil Imaging Lens, also referred to as the F-Lens, is an optical element integrated into the filter wheel of the MIRI Imager (MIRIM), occupying one of the 18 wheel positions alongside filters, coronagraphic diaphragms, and the low-resolution prism assembly.[5] This lens is positioned at the collimated pupil plane within the MIRIM optical path, specifically in the short-wavelength channel, where it re-images the telescope's entrance pupil onto the detector focal plane using the downstream three-mirror anastigmat camera optics.[33] Designed primarily for ground-based testing, the F-Lens enables the formation of pupil images to verify internal optical alignment and assess potential vignetting or shear in the beam path.[48] The primary purpose of the Pupil Imaging Lens is to support wavefront sensing, focus monitoring, and disperser alignment by capturing direct views of the JWST pupil, including the 18-segment primary mirror structure.[49] In operation, selecting the F-Lens position in the filter wheel redirects the collimated light to form a pupil image on the 1024 × 1024 pixel short-wavelength detector, allowing for the evaluation of pupil geometry and alignment relative to the optical train.[33] This capability achieves a sampling resolution of approximately 0.1 arcsec across the imaged pupil, sufficient to resolve individual segments of the primary mirror for alignment assessments.[48] During JWST commissioning, the Pupil Imaging Lens proved critical for in-flight alignment of the segmented mirror, with dedicated observations using the imager in full-frame mode to check pupil positioning and confirm that MIRI's optics remained within specifications post-launch.[49] These pupil images, acquired with the F-Lens deployed and calibration sources or stars as targets, facilitated precise measurements of pupil shear and decenter, ensuring overall instrument performance without the need for extensive recalibration.[50] Although originally intended for pre-launch verification, its utility extended to operational checks, demonstrating MIRI's alignment stability throughout the mission.[48]

Observing Modes

Imaging

The Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) operates in direct imaging mode to capture broadband images across nine filters spanning 5.6 to 25.5 μm, enabling observations of thermal emission from dust and cool objects.[34] This mode supports various subarray configurations to optimize readout speed and field of view, including the full frame of 1024 × 1032 pixels covering 74″ × 113″ for wide-field imaging, and smaller subarrays such as BRIGHTSKY (512 × 512 pixels, 56.3″ × 56.3″) or SUB256 (256 × 256 pixels, 28.2″ × 28.2″) for faster readouts in high-background or bright-object scenarios.[51] Exposure times in imaging mode range from a minimum of 10 ms up to several hours, achieved by combining multiple groups (typically 1 per frame in FASTR1 mode for short exposures) and integrations to suit diverse targets.[34] To address detector artifacts like bad pixels, cosmic rays, and persistence, as well as to achieve sub-pixel sampling for super-resolution, MIRI imaging employs dithering strategies that offset the telescope pointing between exposures.[52] Common patterns include 4-point cycles, such as the recommended CYCLING-LARGE configuration, which provides redundancy for background subtraction and mosaicking while minimizing overhead; this pattern can be extended up to 311 points by cycling through offsets.[52] Nine-point dither patterns are also available for enhanced flat-fielding and artifact mitigation, particularly useful for unresolved sources or when higher sampling is needed beyond the native pixel scale of 0.11″.[34] MIRI's imaging sensitivity is background-limited in most bands for typical integrations, with point-source detection limits reaching approximately 20 mag (5σ in 10 ks) at 7.7 μm using the F770W filter under low-background conditions.[53] These capabilities make MIRI imaging particularly suited for wide-field mapping of star-forming regions, where it resolves polycyclic aromatic hydrocarbon emission and warm dust, and for surveying galaxies to trace their mid-infrared morphology and evolutionary processes.[54]

Medium-Resolution Spectroscopy

The Medium-Resolution Spectroscopy (MRS) mode of the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) employs an integral field unit (IFU) to deliver spatially resolved mid-infrared spectra, enabling the study of extended sources with both spatial and spectral detail. This mode captures light across a field of view by slicing it into narrow strips, dispersing each slice into a spectrum, and reconstructing the data into three-dimensional (3D) cubes that map intensity as a function of two spatial dimensions and wavelength. Unlike broadband imaging, MRS provides resolved spectroscopy at moderate resolving powers, facilitating the identification of emission and absorption lines from molecules and dust.[6] The IFU design features four independent channels to achieve comprehensive wavelength coverage from 5 to 28 μm, with each channel handling a distinct band: approximately 4.9–7.65 μm (Channel 1), 7.51–11.7 μm (Channel 2), 11.55–17.98 μm (Channel 3), and 17.7–27.9 μm (Channel 4).[6] These channels produce 3D data cubes with spaxel sizes of 0.196 arcsec (Channels 1 and 2), 0.245 arcsec (Channel 3), and 0.273 arcsec (Channel 4), allowing diffraction-limited sampling at longer wavelengths while shorter channels offer finer scales for high-resolution mapping. The spectral resolution ranges from R ≈ 3500 at shorter wavelengths to R ≈ 1500 at longer wavelengths across the bands, enabling separation of closely spaced lines such as those from polycyclic aromatic hydrocarbons or silicates.[6][55] MRS observations follow a nod-chop-dither sequence to mitigate background contamination from zodiacal light, telescope emission, and Earth's atmosphere, ensuring high signal-to-noise ratios for faint extended structures. In this approach, the target is nodded between two positions on the detector, chopped rapidly for stability, and dithered across multiple offsets—typically four or more—to fully sample the field and fill gaps between slices. The resulting raw detector frames are processed through pipeline reconstruction to yield calibrated 3D data cubes, where each spaxel delivers a complete, wavelength-calibrated spectrum for analysis of spatial variations in line fluxes and continuum shapes.[6] Key applications of MRS include spatially resolved investigations of active galactic nuclei (AGN) tori, where it maps the distribution and excitation of warm dust and gas in obscured nuclei. For example, observations of the Seyfert galaxy NGC 1068 have used MRS to analyze polycyclic aromatic hydrocarbon bands, revealing the torus structure on scales of ~75 pc. In protoplanetary disks, the mode excels at tracing dynamical processes like outflows and accretion, as demonstrated by mappings of H₂, H₂O, and CO emission in the edge-on disk around the T Tauri star Tau 042021, which highlight disk winds and scattering effects.[6][56][57]

Coronagraphy

The Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) features a coronagraphic imaging mode equipped with four specialized masks designed for high-contrast observations in the mid-infrared, enabling the detection of faint companions near bright sources. This mode utilizes three four-quadrant phase masks (4QPMs) and one Lyot coronagraph, each paired with a dedicated narrowband filter to optimize performance across the 10–23 μm wavelength range. The 4QPMs, which introduce a π phase shift between alternating quadrants to destructively interfere with on-axis starlight, operate at central wavelengths of 10.575 μm (F1065C), 11.30 μm (F1140C), and 15.50 μm (F1550C), while the Lyot coronagraph, employing an amplitude occulting spot and a downstream stop, functions at 22.75 μm (F2300C). These masks achieve inner working angles (IWAs) of approximately 1 λ/D for the 4QPMs (corresponding to 0.33–0.49 arcseconds) and 3.3 λ/D for the Lyot (2.16 arcseconds), allowing access to regions as close as ~0.4 λ/D in some configurations after post-processing.[13][47][58] Contrast performance in the coronagraphic mode provides on-axis suppression of 10^{-3} in raw images for the 4QPMs and ~10^{-3} for the Lyot, improving to 10^{-4} to 10^{-5} after subtraction techniques, with sensitivities reaching 10^{-6} at larger separations beyond 5–6 arcseconds. This capability is enhanced when combined with angular differential imaging (ADI), which leverages telescope roll angles up to 10° to model and subtract the stellar point spread function (PSF), mitigating quasi-static aberrations and enabling detection limits suitable for mid-infrared excesses. The 4QPM design, originally proposed for high-contrast imaging, excels in suppressing starlight while preserving off-axis flux, as demonstrated in cryogenic laboratory tests of the flight model.[47][58] Observing strategies for MIRI coronagraphy emphasize efficiency and precision, including the use of a reference star—selected for spectral similarity and proximity within ~20° of the target—for PSF subtraction via reference differential imaging (RDI). Subarray readouts in FAST or FASTGRPAVG modes reduce data volume and readout noise, with target acquisition requiring a minimum signal-to-noise ratio of 30 for centroid accuracy of 5–10 mas. Small grid dithers (SGDs) with 9 positions in 10 mas steps provide subpixel PSF diversity, particularly beneficial for 4QPMs, while avoiding large dithers to prevent background inconsistencies; ADI is applied in back-to-back observations to capture temporal PSF variations.[59][58] These features support key applications in direct imaging of young giant exoplanets, whose thermal emission peaks in the mid-infrared, and circumstellar debris disks, revealing inner and outer structures such as Kuiper Belt analogs or warped disks around stars like HD 95086. The mode's sensitivity to molecules like ammonia (at shorter 4QPM wavelengths) and silicates (at the Lyot wavelength) facilitates atmospheric characterization and disk mineralogy studies, with early on-sky results confirming detection thresholds for planets at contrasts of 10^{-5} within 1 arcsecond.[47][13]

Operations

Integration with JWST

The Mid-Infrared Instrument (MIRI) was integrated into the James Webb Space Telescope (JWST) as part of the Integrated Science Instrument Module (ISIM), where it was bolted to the ISIM's optical metering structure using titanium interface plates, alongside the Near-Infrared Spectrograph (NIRSpec), Near-Infrared Camera (NIRCam), and Near-Infrared Imager and Slitless Spectrograph (NIRISS).[60][61] This mounting configuration ensured precise optical alignment and structural stability within the ISIM's composite truss framework. MIRI shared a cryogenic bus with the other instruments, providing a common passively cooled environment in the ISIM's Region 1 volume at approximately 36–40 K, while MIRI's components required additional active cooling.[60][3] Thermal interfaces for MIRI were designed to link with JWST's passive cooling stages, maintaining the instrument's optics module at below 7 K through a dedicated two-stage mechanical cryocooler, with the surrounding ISIM structure at around 40 K and a thermal shield at approximately 23 K to minimize radiative heat loads.[60][3] High-purity aluminum heat straps connected MIRI to dedicated radiators, ensuring efficient heat rejection while isolating it from the warmer ISIM environment. The electrical harness for MIRI operated from the 300 K warm electronics compartment in the ISIM, supplying power at 28 VDC from the spacecraft bus and handling data via the MIL-STD-1553B protocol through the ISIM Command and Data Handling system, which was shared with the other instruments.[60][61][3] To qualify for launch, MIRI underwent rigorous vibration testing as part of the ISIM integration, simulating the Ariane 5 rocket's dynamic loads with a maximum of 14 g RMS across relevant frequency ranges, using force-limited approaches to protect the instrument from over-testing.[60][61][3] This testing, conducted at facilities including the High Capacity Centrifuge, verified the structural integrity of MIRI's mounting and internal components under flight-like conditions.

Commissioning and Calibration

The commissioning phase of the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) began shortly after the observatory's arrival at the Sun-Earth L2 point in January 2022, with initial instrument activation occurring in early 2022, with key milestones in January and April. The timeline for MIRI's commissioning aligned with the overall JWST schedule, concluding in July 2022 and enabling full science operations starting July 12, 2022, encompassing a series of tests to verify functionality across its imaging, spectroscopy, and coronagraphy modes. First light observations were achieved on July 12, 2022, when MIRI captured a mid-infrared image of the galaxy cluster SMACS 0723, demonstrating the instrument's sensitivity to wavelengths between 5 and 28 microns. This milestone marked the successful transition from ground-based preparations to in-orbit operations, with subsequent activities focusing on optimizing performance in the cryogenic environment. As of 2025, MIRI operates nominally, with recent calibration updates enhancing data processing.[62] A critical early step was the cooldown of MIRI's cryocooler, which achieved the required operating temperature of below 7 K for the detectors on April 7, 2022, enabling the cooling of the Teledyne Si:As and Si:Sb focal plane arrays to below 7.2 K.[63] Key calibration activities included assessments of detector linearity, which confirmed non-linear responses were minimal across the dynamic range, and flat-fielding to map pixel-to-pixel sensitivity variations using internal illumination sources. These tests ensured accurate photometric and spectroscopic measurements, with linearity verified to within 1% over most of the field of view. Internal lamps, such as continuum and line sources, were employed to establish the wavelength solution for the spectrograph, providing precise dispersion calibrations with residuals below 0.1 pixels. For flux calibration, on-sky observations of standard stars, including globular cluster (GC) giants like those in 47 Tucanae, were used to tie instrumental responses to absolute flux scales, achieving uncertainties of about 5% in broadband photometry. Challenges during commissioning included initial focus adjustments to compensate for in-orbit thermal distortions, which were resolved through iterative imaging of bright point sources, refining the focus to within 0.1 pixels across the field. In the medium-resolution spectroscopy mode, ghosting artifacts from internal reflections were mitigated by updating the optical alignment and data processing pipelines, reducing ghost intensities by over 90% in affected wavelength ranges. These resolutions, completed by late 2022, paved the way for full science operations starting in July 2022, with all observing modes tested and verified for routine use.

Performance Metrics

The Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) has demonstrated in-flight sensitivity that exceeds pre-launch predictions by 10–20%, primarily due to higher-than-expected photon conversion efficiency and sharper point spread functions. For example, the 5σ sensitivity for a point source at 21 μm reaches approximately 0.33 μJy in a 1000 s exposure, enabling deep observations of faint mid-infrared sources. This performance is two orders of magnitude better than Spitzer's capabilities at 5.6 μm and one order of magnitude superior at 25.5 μm.[13][10] MIRI maintains excellent thermal stability, with temperature variations below 0.1 K over an orbit, ensuring consistent detector performance despite orbital thermal changes. Pointing accuracy, supported by JWST's fine guidance system, achieves stability better than 0.02 arcsec (1σ radial), with typical shifts under 10 mas during observations. Photometric stability is within 5% over multi-day integrations, limited mainly by zodiacal background variations at shorter wavelengths.[64][10][13] End-to-end throughput for MIRI imaging exceeds design specifications, with photon conversion efficiency surpassing ground tests and yielding over 30% at 10 μm after accounting for telescope and instrument contributions. The noise equivalent flux density (NEFD) for imaging is approximately 0.5 μJy at key wavelengths, supporting background-limited performance in most observing scenarios.[13][53] As of 2025, the MIRI cryocooler operates with margins that project an operational lifetime exceeding 15 years, aligning with JWST's overall expected lifespan of up to 20 years based on helium consumption rates and thermal efficiency.[65]

Scientific Impact

Early Discoveries

In 2023, MIRI achieved a milestone in exoplanet imaging by providing the first mid-infrared characterization of the gas giant PDS 70 b, a protoplanet embedded in its protoplanetary disk. Observations with MIRI's Medium Resolution Spectrometer (MRS) confirmed the presence of PDS 70 b and its companion PDS 70 c within a dust-depleted gap of approximately 54 au, while revealing an extended inner disk reaching ~18 au based on silicate dust emission features at 9.40 μm (enstatite) and 11.30/16.40 μm (forsterite). The spectra detected prominent water vapor emission lines at ~7 μm, corresponding to temperatures of ~600 K and column densities of 1.4 × 10^{18} cm^{-2}, alongside CO_2 and H_2 features, indicating a volatile reservoir in the terrestrial planet-forming zone that could interact with the accreting protoplanet through dynamical clearing and potential vapor delivery. This detection highlighted disk-protoplanet interactions, as evidenced by flux variability up to 1.5 times compared to prior Spitzer data beyond 18 μm, likely due to geometric changes in the inner disk influenced by the planets' orbits.[66] MIRI's capabilities also advanced galaxy studies by resolving polycyclic aromatic hydrocarbon (PAH) emission in low-redshift (z ≈ 0.1) star-forming galaxies, enabling precise tracing of star formation processes. Early 2023 observations targeted nearby spirals, capturing broad PAH bands at 3.3, 6.2, 7.7, and 11.3 μm with MIRI's imaging and spectroscopic modes, which correlate strongly with regions of active star formation in photodissociation zones. For instance, in galaxies like NGC 1365, resolved PAH maps showed emission peaking in dust-rich arms and near young stellar clusters, with equivalent widths indicating efficient PAH excitation by ultraviolet radiation from massive stars. These findings quantified star formation efficiencies, revealing that PAH luminosity scales with total infrared output but varies with metallicity, providing a mid-infrared complement to ultraviolet and far-infrared tracers for understanding galactic evolution.[67][68]

Recent Observations (2022–2025)

In late 2025, MIRI observations of the Sagittarius B2 molecular cloud, the Milky Way's most prolific star-forming region, revealed a highly structured morphology with warm dust glowing brightly and embedded massive stars at various evolutionary stages.[69] These mid-infrared images, obtained in September 2025, highlighted the cloud's complex environment, including regions of active star formation similar to conditions in the early universe, advancing understanding of high-mass star birth near the galactic center.[69] MIRI's contributions to early universe studies advanced significantly in 2024 through its participation in the COSMOS-Web survey, where mid-infrared imaging confirmed several galaxy candidates at redshifts z > 10, dating to less than 500 million years after the Big Bang.[70] These observations, combining MIRI with NIRCam data, detected thermal emission from dust in these primordial galaxies, indicating surprisingly rapid dust production by the first generations of stars.[71] A standout example was the spectroscopic confirmation of a luminous galaxy at z = 12.33, whose MIRI spectrum revealed polycyclic aromatic hydrocarbons and silicate dust features, challenging models of dust enrichment in the reionization era.[72]

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