James Webb Space Telescope

The James Webb Space Telescope (JWST) is a space telescope serving as an observatory operating mainly in the infrared, developed by NASA with the participation of the European Space Agency (ESA) and the Canadian Space Agency (CSA). Larger and more expensive as a space telescope at launch, the JWST is designed to continue the work of the Hubble Space Telescope, but make its observations in longer wavelengths. It was launched on December 25, 2021, and the first scientific-quality image produced by the telescope is published in July 2022.

General data
Organization	• NASA• ESA• CSA
builder	• Northrop Grumman (formerly TRW) • Ball
Program	Origins
estate	Infrared astronomy
Status	Operational
Launch	December 25, 2021 of the Guiana Space Center
Pitcher	Ariane 5 ECA
Duration	5.5 years (primary mission)
Website	jwst.nasa.gov

Specifications
Mass at launch	~ 6,173 kg

Orbit
Orbit	Heliocentric
Location	Lagrangian point L2

Telescope
Type	Anastigmatic with three mirrors
Diameter	6.50 m
Area	25 m2
Focal length	131.4 m
Angular resolution	0.1 arcseconds
Wavelength	Orange to mid-infrared (0.6 to 28 μm)

Main instruments
NIRCam	Near-infrared imager
NIRSpec	Near-infrared spectrograph
MIRI	Mid-infrared spectro-imager
NIRISS	Near-infrared imager

JWST observations focus on the near and mid-infrared, while including part of the spectrum in the visible range (wavelengths ranging from 0.6 to 28 μm). By its resolution, its collecting surface and the spectral band covered, it far surpasses Hubble for infrared observation, but, unlike the latter, it can observe neither ultraviolet nor all visible light. Despite the large size of its primary mirror (6.5 m in diameter compared to 2.4 m for Hubble), its mass of 6,200 kg is half that of its predecessor. Its resolving power reaches 0.1 arcseconds and it can collect an image nine times faster than Hubble. The JWST carries four instruments: the NIRCam camera operating in the near-infrared, the MIRI spectro-imager in the mid-infrared, the NIRSpec spectrograph in the near-infrared, and the NIRISS spectro-imager, also in the near-infrared.

The angular and spectral resolutions of its instruments, its unprecedented capabilities in the mid-infrared and spectroscopy (multi-object modes and field integral) will be used to deepen our knowledge in the main fields of astronomy: reionization period and formation of the first stars and galaxies after the Big Bang, formation and evolution of planetary processions and composition of the atmosphere of exoplanets. The data collected will help explain the genesis and role of supermassive black holes in galaxies, clarify the process of planet formation, determine the proportion of planets that can host life and provide information about the mysterious dark energy.

Work on the JWST began in 1989, but the project underwent many evolutions and vicissitudes due to the technological challenges it raised (foldable primary mirror, deployable heat shield) and budget overruns. The project came close to cancellation in 2011. For NASA alone, its manufacturing cost, which was estimated at three billion US dollars at the end of the general design phase in 2005, finally reached about ten billion USD. The launch date, initially set for 2013, is regularly pushed back until the end of 2021. In 2002, the project was named after NASA’s second administrator, James E. Webb, who contributed greatly to the success of the Apollo program.

The telescope was launched by an Ariane 5 rocket on December 25, 2021, from the Kourou base in French Guiana, and placed, after a one-month transit, in orbit around the L₂ Lagrangian point of the Sun-Earth system, located 1.5 million kilometers from Earth, on the opposite side of the Sun. Following a six-month commissioning phase, including the particularly delicate deployment of its heat shield and mirrors, the five-year scientific mission to meet the objectives assigned to the JWST telescope begins. The observation time is divided, by a scientific commission, between the teams that contributed to the project and researchers from all over the world, through an annual evaluation of the contribution of their proposals. The JWST carries reserves of propellant (necessary to maintain its position at the Lagrange point) which should allow it to remain in operation for at least ten years.

Context

The radiation emitted by celestial bodies (planets, stars, galaxies, asteroids…) in the infrared is an important source of information for understanding the processes at work in space. But molecules in Earth’s atmosphere largely block this type of radiation, preventing any in-depth observation from Earth’s soil. As a result, infrared astronomy experienced significant growth from the 1980s onwards, thanks to the development of space telescopes, which made it possible to overcome the obstacle constituted by the atmosphere. Infrared astronomy became the source of many discoveries, including the formation of stars and planets, primordial galaxies and cold objects located in galaxies.

The American space agency, NASA, plays a major role in the development of infrared space telescopes, thanks to its enormous financial means, and its mastery of the necessary technologies, partly resulting from military work on detectors. She developed the IRAS infrared telescope, a pioneering instrument that transmitted its first images in 1983. In the early 1990s, when the community of astronomers was consulted by the space agency on the characteristics of the successor to the Hubble telescope, the spearhead of astronomy at NASA, its choice fell on a telescope optimized for infrared observation. It is indeed in this spectral domain that we hope to find answers to many questions raised by the latest advances in the fields of astronomy and cosmology.

History of the project JWST: from first sketches to detailed specifications (1989-2009)

Key project milestones
1989	Early studies
1995	Sketch of the JWST with eight-meter mirror diameter
2000	First definition of needs
2001	Mirror diameter reduced to six meters
2002	Selection of manufacturers
2004	Start manufacturing mirrors and instruments
2004	Detailed specifications
2005	Selection of the Ariane 5 launcher
2008	JWST project approved
2010	Validated architecture
2011	Mirror manufacturing completed
2017	Assembly and testing
2021	Launch of the mission

The first studies relating to the James Webb Space Telescope were initiated by NASA in 1989, even before the launch of the Hubble Space Telescope (1990) of which it was to be the successor. It took another 20 years (1989-2009) for the technical architecture and scientific objectives to be set and for the American space agency to decide to develop this project with its extraordinary characteristics and cost.

First sketches (1989-1994)

In 1989, the director of the Space Telescope Science Institute, the center in charge of the operations of the Hubble Space Telescope, initiated a reflection on the telescope that would take over around 2005. The report resulting from the work, organized with the support of NASA, proposes that the space agency study a telescope eight meters in diameter, observing in the near-infrared thanks to a passive cooling system. The problems encountered by Hubble shortly after its launch (1990), the decrease in NASA’s budget and the change in the presidency of the United States temporarily put an end to the study of the new telescope. Studies were relaunched in 1993.

At NASA’s request, the Association of Universities for Research in Astronomy (AURA) created the HST and Beyond Committee to define the characteristics of Hubble‘s successor that was to enter service in the first decades of the following century. In 1995, the committee proposed extending Hubble‘s lifespan by five years (until 2010) and outlined the characteristics of its successor: it was to include a mirror four meters in diameter. The scientific objectives of the future telescope are the study of the process of formation of galaxies, stars, planets and life, with a focus on the early Universe. The telescope called Hi-Z must circulate in a heliocentric orbit of 1 × 3 astronomical units. NASA commissioned one of its institutions, the Goddard Space Flight Center (traditionally responsible for astronomical missions at NASA), to conduct a feasibility study.

Feasibility study and detailed definition of requirements (phase A: 1995-2001)

Daniel Goldin, NASA’s new administrator in 1995, as part of his “Faster, better, cheaper” policy, is urging the astronomer community to make bold choices, while looking for technologies to lower the cost. In response, scientists opted for an eight-meter-diameter telescope, which seemed necessary to study the most distant galaxies characterized by a redshift of one to five or more. They propose an innovative concept called Next Generation Space Telescope (NGST) comprising an eight-meter mirror, deployed in space and placed in orbit around the L₂ Lagrangian point, with a baffle-free optics, cooled passively thanks to a multi-layer sun visor.

In June 1997, NASA selected TRW and Ball Aerospace to identify possible technical architectures and make an initial assessment of the cost of the project. The feasibility study concludes that it is possible to build such a telescope at a cost of USD 500 million, provided that the whole, including the instruments, is developed by the same company. However, the latter condition is inapplicable. The report The Next Generation Space Telescope: Visiting The Time When Galaxies Were Young defines a reference architecture for the telescope and provides the elements for the US space agency to launch tenders from industry. In 1999, NASA selected two companies, Lockheed Martin and TRW, to conduct a study (Phase A) including preliminary design analysis and cost evaluation.

The foundations for NASA’s collaboration with the Canadian Space Agency and the European Space Agency (ESA) for instrument development were laid at this time. In parallel, simulations carried out subsequently make it possible to specify the necessary scientific instrumentation. It is now envisaged to observe galaxies with a redshift of 15, which requires being able to observe in the mid-infrared. These simulations highlight the need for spectroscopy, because the instruments located on Earth cannot support this aspect of observation (as is done for Hubble), because of the absorption of light radiation, by the atmosphere, of the infrared spectral band observed by the future telescope.

From 1997 to 2000, a working group representing the astronomer community, the Science Working Group, worked to define the main scientific objectives that the future telescope should be able to meet and the instrumentation that would enable it to achieve them. A wide-field near-infrared camera, a multi-object near-infrared spectrograph and a mid-infrared imaging spectrograph are selected. The first technical studies are carried out to develop new onboard technologies: low-mass mirrors, wavefront detection and control system, infrared detectors and actuators. At the end of 2000, a detailed analysis showed that the cost of the telescope exceeded the budget planned until then by several hundred million US dollars. The launch is not possible before 2008, given the length of the mirror development cycle. To reduce the cost, the diameter of the primary mirror was reduced in 2001 to six meters.

Builder selection and overall design (Phase B: 2002-2008)

Contributions from different actors
Component	Country	Industrial	Lead laboratory
Platform	US	Northrop Grumman
Optics	US	Ball
Heat shield	US	Northrop Grumman
Instrument NIRCam	US	Northrop Grumman	University of Arizona
Instrument NIRSpec	European Union	Airbus
Instrument MIRI	US	JPL	University of Arizona
MIRI Instrument – Optics	European Union	Airbus	University of Edinburgh
Instrument NRISS	Canada	Honeywell
Fine pointing SDF	Canada	Honeywell
Ariane 5 ECA launcher	European Union	Arianespace
NIRSpec micro shutters	US	Goddard
MIRI refrigerator	US	Northrop Grumman

In August 2002, NASA selected the manufacturer of the space telescope for the general design phase (phase B): TRW’s proposal, associated for the optical part with Ball Aerospace, was chosen. In the same year, TRW was absorbed by Northrop Grumman in a hostile takeover bid and became Northrop Grumman Space Technology. The Jet Propulsion Laboratory (JPL) is selected for the development of the MIRI (Mid-Infrared Instrument) instrument.

In June 2002, the development of the NIRCam (Near-InfraRed Camera) is entrusted to a team from the University of Arizona. The launcher that will place the telescope into orbit is selected: the Ariane 5 ECA rocket, funded by the European Space Agency, is chosen instead of the Atlas V rocket, initially planned but of lower capacity. The development of the NIRSpec instrument and the optical part of the MIRI instrument are entrusted to Europe, while the FGS/NRISS instrument is to be developed by Canada. In exchange for these participations, European and Canadian scientists are allocated an observation time of 15% and 5% respectively.

In September 2002, the telescope was renamed the James Webb Space Telescope (JWST), in honor of this administrator who headed NASA between 1961 and 1968 at the time of the Apollo program. He played a major role in the success of this project. In May 2021, a petition, signed by 1,200 people, including at least four astronomers, challenges the tribute. He is accused of his participation, as Under-Secretary of State in the Truman administration (1949-1952), in the hunt for homosexual employees of the American administration, as well as the exclusion of a NASA employee during his legislature for the same reason. NASA replied in October that it had carried out extensive research on the subject in its archives and those of the government and had not found a reason to change the name of the space telescope.

During this phase of the project, the characteristics of the space telescope become clearer while continuing to evolve. The area of the mirror is reduced from 29.4 to 25 m² while the number of elements of the primary mirror is reduced from 36 to 18. NASA chooses beryllium as the material for the manufacture of this 6.5 m diameter mirror. The cryostat developed by Europe, which was to maintain the temperature of the detectors of the MIRI instrument, is abandoned in favor of a mechanical refrigerator developed under American supervision (JPL).

In 2004, the telescope entered a detailed specification phase that eventually lasted four years. Costs are reassessed at the end of this phase. The development of the most complex parts of the telescope (the instruments and the 18 segments of the primary mirror), which require a long development phase or which employ technologies that are not fully mature, begins as soon as March 2004, even before NASA had given its approval for the construction of the telescope. In August 2006, the NIRCam (Near-InfraRed Camera) and MIRI (Mid-Infrared Instrument) instruments pass the critical review of definition, which allows the realization of flight models to begin.

From January 2007 to December 2008, commissions, both internal to NASA and external, review the design and planning of the project. In July 2008, the ISIM structure, in which the instruments are housed, is delivered to the Goddard Space Flight Center for a series of tests. They must verify that it is able to withstand the acceleration forces during launch and then the thermal environment of the space, while keeping the instruments in a precise position in relation to the optical part.

At the end of 2008, the US space agency, based on the various reviews carried out over the past two years, concluded that the design of the space telescope had reached a sufficient level of maturity to be able to launch its manufacture. The project moves into Phase C (detailed definition) which precedes Phase D (construction). The project is part of the Origins program, which brings together NASA’s aerial and space astronomical missions whose objective is to study the origins of the Universe.

Scientific objectives of the James Webb Space Telescope

The James Webb Space Telescope is designed to contribute to the themes at the heart of modern astronomy:

• search for the first stars and galaxies that appeared in the Universe after the Big Bang;
• determine how galaxies evolve, from their formation to the present day;
• observe star formation from the earliest stages to the formation of planetary systems;
• measure the physical and chemical characteristics of planetary systems, including the Solar System, and look for the components necessary for the appearance of life in the atmospheres of exoplanets.

Study of the first stars and galaxies

The oldest known event in our Universe is the Big Bang, which took place about 13.6 billion years ago. The matter, which then takes the form of a soup of protons, neutrons and electrons at very high temperatures, cools to form hydrogen ions and a small amount of helium (during primordial nucleosynthesis), then, after electron capture, neutral atoms (during recombination, early dark ages). The first stars and galaxies begin to form several hundred million years after the Big Bang (the precise time interval is not known).

The radiation from these first stars reionizes the ambient gas of hydrogen and helium (reionization). Light from some of these early stars and galaxies is likely reaching Earth. But, due to the expansion of the Universe, our galaxy is moving away at an increasing speed from its source and this light is strongly redshifted, by the Doppler effect. As a result, light that has been emitted in the visible or ultraviolet spectrum can only be observed in the near or mid-infrared, i.e. in the part of the spectrum for which the telescope has been optimized. Thanks to its spatial resolving power and spectral coverage, the JWST should be able to observe objects that appeared up to 100 to 250 million years after the Big Bang.

The JWST should help answer the following questions:

• when and how did the reionization of the Universe occur?
• What were the causes of reionization?
• What were the characteristics of the first galaxies?

The JWST is to study the first galaxies by making long-term observations in the near-infrared, followed by low-resolution spectroscopic analyses and mid-infrared photometric measurements. To study reionization, near-infrared spectrometry will be required.

Formation and evolution of galaxies

Scientists are trying to determine how this matter has organized itself and how it has changed since the Big Bang, by studying the distribution and behavior of matter at different scales from the particle, at the subatomic level, to galactic structures. Galaxies structure matter in the Universe on a large scale. They provide clues to the nature and history of the Universe. With this in mind, the JWST telescope should answer the following questions:

• Spiral galaxies (including ours) have not always had this shape. They were formed over several billion years and are the result of the sequence of several processes, including the collision between smaller galaxies. The hypothesis, which remains to be confirmed, is that all giant galaxies underwent at least one major merger, while the Universe was six billion years old;
• The most distant galaxies (and therefore the oldest) have a very different structure from recent galaxies. They are small and collected, with very dense regions, where new stars form. The transition from this shape to that of spiral galaxies is not explained;
• The process of formation of the first galaxies is unknown, as are the factors that led to the diversity of galaxy shapes observed today;
• Astrophysicists have discovered that supermassive black holes are located at the center of most galaxies. But the nature of their relationship with the galaxies that host them is unknown. It is not fully understood whether the mechanisms behind star formation are internal to the galaxy or are related to interaction or merger with another galaxy.

Formation of stars and planetary systems

Protoplanetary systems and stars are born in huge clusters of gas and dust that block visible light emitted by these processes. On the other hand, the infrared radiation emitted is not intercepted by dust clouds and it is thus possible to observe the formation of stars and planets inside these clusters. The JWST should make it possible to examine these radiation-soaked regions with unparalleled finesse.

Fifty years ago, astronomers were unaware that new stars were continuing to form in the universe. The process of generating stars by collapsing clouds of dust and gas is still poorly understood. The same applies to the interactions between young stars, in the regions where they form (the “star nurseries”). Finally, the discovery of planetary systems with very different characteristics of our solar system has upset theories about how planets form. With its ability to observe in the infrared, the JWST must contribute to answering the following questions:

• How do clouds of gas and dust collapse to form stars?
• Why do most stars form in groups?
• How exactly are planetary systems formed?
• How do stars evolve and how do they eject the heavy elements they have produced at the end of their lives and which are recycled by the next generation of stars and planets?

Study of planetary systems and search for elements conducive to life

Since the early 2000s, thousands of exoplanets have been discovered, some of which have a diameter close to Earth and are at a distance from their star that theoretically allows the presence of water in the liquid state, which therefore fulfills one of the important conditions for the appearance of life. One of the main objectives of the JWST is the study of the atmospheres of exoplanets in order to determine whether the constituents allowing the appearance of life (water vapor, oxygen…) are present in solar systems other than ours.

To achieve this objective, the JSWT will use the transit method: this consists of performing a spectral analysis of the star’s light as the exoplanet interposes itself between it and the space observatory. When this event occurs, the amount of light received from the star decreases and its spectral composition is changed if it passes through the exoplanet’s atmosphere. Analysis of the spectrum of infrared radiation received will reveal absorption lines, which will make it possible to deduce the molecular composition of the exoplanet’s atmosphere.

The JWST should also be used to study the planets of our solar system, as its sensitivity and resolution allow it to complement the information collected by existing observatories (terrestrial, space and space probes). The JWST will observe Mars, the giant planets, the dwarf planets (Pluto and Eris) and the small bodies of the Solar System, but, on the other hand, will not be able to observe Venus or Mercury, too close to the Sun. It will make it possible to discover new small celestial bodies: dwarf planets, Kuiper belt objects, and asteroids. Observations will include trace organic materials in Mars’ atmospheres and the seasonal cycles of giant planets. The JWST will provide spectral data on small bodies that ground-based observatories are unable to produce.

The JWST must contribute to answering many questions on this theme, including:

• What are the components of protoplanetary disks that contribute to planet formation?
• Do planets form in place or do their orbits shift?
• What is the impact of giant planets on smaller planets?
• Are there planets located in the habitable zone of their star, where liquid water (and possibly life) exists?
• how did life develop on Earth?
• Was there life on Mars?

Technical architecture of the James Webb Space Telescope

The general architecture of the James Webb Telescope

The architecture resulting from the objectives pursued is particularly ambitious and complex because it introduces several technical innovations. Its main features are as follows:

• To achieve the set objectives, the telescope is optimized for observing infrared radiation rather than visible light. Infrared makes it possible to observe distant galaxies despite their redshift, to examine the formation of stars despite the presence of dust and to study objects, the majority of which have a very low temperature. The observable wavelength range is between 0.6 and 28 micrometers;
• for the infrared detectors to function despite thermal emissions from the telescope and its instruments, the assembly must be kept in a temperature range below 55 Kelvins (around 40 K, or −233.15 °C);
• the minimum duration of the mission is set at 5.5 years in order to meet the objectives;
• To keep the mid-infrared detectors (above 5 μm) at a sufficiently low temperature, a mechanical cooling system is adopted. Unlike cryogenic liquid cooling, its operation is not limited in time and it reduces the mass of the telescope;
• The angular resolution chosen requires a large diameter telescope (6.5 m) that cannot fit under the fairing of existing launchers (maximum outer diameter of about 5 m), which requires launching the telescope with its primary mirror folded. To achieve a perfect optical surface, the various components of the mirrors are designed in such a way that they can be adjusted once the telescope is in orbit;
• To maintain the temperature of the infrared detectors within the set range, the telescope has a heat shield of unprecedented size (22 × 12 meters). This shield is composed of several spaced layers of metalized fabric, a material responsible for blocking infrared rays from the Sun, Earth and Moon, and intercepting stray light. The heat shield thus passively maintains the temperature of the detectors at 37 K, which makes it possible to obtain very good performance in the near and mid-infrared. The large size of this heat shield requires to launch it, too, in the folded position and therefore also to unfold once in orbit;
• the telescope is anastigmatic with three curved mirrors, allowing a wide field of view (2.2 × 4.4 arc minutes), minimizing the main optical aberrations;
• The spectrometric capabilities of the telescope are particularly important, with multi-object and integral field modes;
• the JWST is positioned at the Lagrangian point L₂ of the Sun-Earth system, which has several advantages. The telescope, although located outside the Earth’s gravity field, maintains a constant distance from Earth, allowing data to be transmitted at a constant high rate. On the other hand, since the telescope is located 1.5 million kilometers from Earth, the heat flux from it is lower than if it were in orbit around our planet, like Hubble. Finally, the Sun and Earth are aligned here, allowing the heat shield to protect the telescope from these two heat sources. The counterpart is that, unlike the Hubble telescope which circulates in a low orbit, the JWST is too far from Earth for a crew to intervene in case of technical failure;
• the space observatory carries consumables (propellants) that allow observations to be made for at least ten years, with objectives that should be achieved after five years;
• The total mass is approximately 6,173 kilograms at launch. This is limited by the maximum capacity, for the chosen orbit, of the heavy launchers available at the time of the telescope’s design.

The main innovations include the main mirror (low mass, in-orbit deployment, segment adjustment system), the heat shield (low mass, complex in-orbit deployment), the MIRI detector cooling system (mid-infrared) and the NIRSpec micro-shutters based on MEMS technology.

James Webb Telescope instruments

The JWST carries four instruments that exploit the radiation collected by the optical portion of the space telescope, each designed to fulfill several of the objectives of the JWST mission:

Key features of the instruments
Instrument	Micrometer spectral band	Image Pixel size field of view	Spectroscopy Mode, resolution	Other features
NIRCam	0,6 – 5	Field of view: 2.2 × 4.4 arc minutes Pixel: 32 and 65 (>2.4 micrometers) milliseconds of arc		19 wide and narrow filters Coronagraph
NIRSpec	0,6 – 5		Multi-object mode: 100 observable objects on 9 minarc 2, spectral resolution up to 2,700 Full-field mode: 900 spectra on 3″ × 3″ field of view Slit mode: 3 slots with spectral resolution up to 2,700
MIRI	5-28,5	Field of view: 74 × 113 arc seconds Pixel: 110 milliseconds of arc	Full-field mode: 3″ × 3″ field of view and spectral resolution 1,500 Low resolution 100 between 5 and 11 micrometers	Coronagraph 10.65, 11.4, 15.5 and 23 micrometers
NIRISS	0,6 – 5	Field of view: 2.2 × 2.2 arc minutes	Spectral resolution 150 (0.8-2.25 micrometers)Spectral resolution 700 (0.7-2.5 micrometers)	Interferometer 3.8, 4.3 and 4.8 micrometers Two sets of filters

NIRCam

The NIRCam camera is the main instrument for providing near-infrared images (0.6 to 5 μm) that allows to overcome dust (star and planetary system in formation). It is equipped with a coronagraph to photograph exoplanets whose light is very faint compared to their star, by masking the latter. The instrument should make it possible to take photos and spectra of young exoplanets and their atmospheres and to analyze hot dust and molecular gases from young stars and protoplanetary disks.

NIRSpec

NIRSpec (Near-InfraRed Spectrometer) is a versatile instrument operating in the near-infrared from 0.6 to 5.3 μm. In addition to conventional slit spectroscopy, it has a multi-object mode thanks to a matrix of programmable micro-shutters (MSA) that allows to simultaneously realize the spectrum of 100 selected objects in a field of 3.6 × 3.6 arc minutes. Each object is observed via an aperture corresponding to a field of 0.20 × 0.45 arcseconds. The spectral resolution can be 100, 1,000 or 2,700. It is thus optimized for the observation of very distant, faint galaxies, by allowing the observation of several objects in parallel during very long exposure times. It also makes it possible to produce spectra in “field integral”.

MIRI

MIRI (Mid InfraRed Instrument) is the only instrument observing in the mid-infrared from 5 to 28 μm. This instrument provides both images and spectra (spectro-imager). MIRI’s resolution is 0.11 arcseconds per pixel, for a maximum field of view of 74 × 113 arcseconds. Four observation modes are possible: images, coronagraphy, low-resolution spectroscopy (spectral resolution of 100) between 5 and 11 μm and “field integral” spectroscopy over a field of view of 3 × 3 arcseconds, with a spectral resolution of about 1,500.

NIRISS/FGS

NIRISS (Near Infrared Imager and Slitless Spectrograph) is a secondary instrument associated with, but independent of, the FGS fine guidance system. It is a spectro-imager for making spectra and images. The only instrument equipped with an aperture mask, it has the unique ability to take images of a single, shiny object, with an angular resolution greater than that of all other instruments.

Comparison with Hubble and Spitzer

The JWST compared to Hubble and Spitzer
Characteristic	JWST	Hubble	Spitzer
Commissioning	2021-	1990-	2003-2020
Wavelengths	0.6–28 micrometers Near and mid-infrared	0.1–2.5 micrometers Ultraviolet, visible and near-infrared	3.6–180 micrometers Mid and far infrared
Size	22 × 12 m	Long. 13.2 m × ∅ 4.2 m	Long. 4.45 m × ∅2.1 m
Mass	6.2 t	11 t	0.95 t
Orbit	Lagrangian point L2	Low Earth orbit	Heliocentric orbit
Angular resolution	0,1″	0,1″	1,5″
Field of view
Spectroscopy	“Multi-objects” “Field integral”

For infrared astronomy, the James Webb Telescope follows on from Spitzer, NASA’s large space telescope that was placed into orbit in 2003 and whose mission ended in 2020. Due to its exceptional capabilities, it is considered the successor (but not the replacement) of the Hubble Space Telescope launched in 1990 by NASA and still in operation in 2021. The James-Webb combines a very large aperture with image quality characterized by low diffraction and sensitivity over a wide infrared spectrum. No terrestrial or space observatory has its characteristics.

Hubble’s diameter is much smaller and it can only observe in the infrared up to 2.5 micrometers, compared to 28 μm for JWST. On the other hand, Hubble covers ultraviolet and some visible light that JWST cannot observe. The Spitzer mirror has a much smaller diameter (83 cm) and it is much less sensitive and has a much lower angular resolution. In spectroscopy, the James Webb telescope has, thanks to its multi-object and field integral modes, capabilities absent from Hubble and Spitzer.

Its characteristics allow it to observe all galaxies whose redshift is between 6 and 10 and to detect the light of the first galaxies that appeared after the Big Bang, whose redshift is about 15. The James Webb Telescope is designed to be complementary to future large ground-based observatories such as the Thirty Meter Telescope in wavelengths up to 2.5 μm. It is superior to them beyond this wavelength, because terrestrial observatories are handicapped by thermal emissions from the atmosphere.

The real replacement of the Hubble telescope, capable of observing in the same wavelengths (from ultraviolet to near-infrared) is, in 2021, at the study stage and should not be launched before 2035/2040. Two projects were proposed to NASA in 2019: Habitable Exoplanet Observatory (HabEx), specialized in the observation of exoplanets relatively close to the solar system, and Large UV/Optical/Infrared Surveyor (LUVOIR), which takes up the architecture of the JWST (segmented mirror, large sun visor), but with a diameter increased to 8 or 16 meters. The French Academy of Sciences made an evaluation of these projects in 2021 and recommends the development of the LUVOIR project, in a smaller version (6.5-meter mirror) which, thanks to its resemblance to the JWST, would reduce costs and delays while reducing risks.

The James Webb Telescope performance

The James Webb Space Telescope has a resolving power of 0.1 arcseconds, for a wavelength of 2 micrometers. This ability makes it possible to distinguish a football placed at a distance of 550 km. It is roughly equivalent to that of the Hubble Space Telescope, which has a mirror with a much smaller diameter (2.75 times smaller). But it makes its observations in shorter wavelengths (about 0.7 micrometers). However, at the same mirror size, the resolving power is all the greater the shorter the wavelength.

Construction of the James Webb space telescope (2009-2021)

Construction of the space telescope began in 2009 when the project was approved by NASA. Its cost was then set at 4.964 billion US dollars, with a planned launch date in June 2014. The project quickly falls behind schedule and the budget explodes. The reasons for this slippage are multiple: initial underestimation of the cost, organizational problems, development of new technologies, the complexity of testing the complete system, incomplete assembly procedures at the main contractor, COVID-19 pandemic. Finally, the characteristics of the space telescope are not degraded, but delivery is postponed to 2021 and the cost of the project more than doubles.

Component manufacturing and testing (2009-2016)

In March 2010, JWST passes the critical design review, the objective of which is to ensure that the space telescope fulfills all the scientific and technical objectives set out in the specifications. In November 2011, the realization of the segments of the primary mirror is completed. These, after polishing, were covered with a thin layer of gold and passed a cryogenic test to ensure their behavior when exposed to the cold of space.

The Goddard Space Center receives in January first two scientific instruments — the Mid InfraRed Instrument (MIRI) spectrometer, operating in the mid-infrared, delivered by the European Space Agency, and the Near Infrared Imager and Slitless Spectrograph (NIRISS), supplied by the Canadian Space Agency — as well as the Fine Guidance Sensor and Near Infrared Imager and Slitless Spectrograph (FGS), delivered by the same agency. Ball delivered the first three segments of the primary mirror to the Goddard Center, while Northrop Grumman and his partner ATK completed the fabrication of the central part of the structure supporting the primary mirror.

End February 2012 Construction of the two moving parts of the primary mirror mount is completed, while the last two scientific instruments, the NIRCam (Near-InfraRed Camera) and the NIRSpec (Near InfraRed Spectrograph), are delivered respectively by the University of Arizona and the European Space Agency. The construction of the platform, which brings together all the support equipment, is completed in 2014. Grumman is making a scale 1 engineering model of the heat shield to test its folding and deployment.

In the same year, the Integrated Science Instrument Module (ISIM), in which the four scientific instruments were assembled, successfully underwent a series of thermal tests that verified the performance and behavior of the associated electronics. In October 2015, the optical part of the telescope (OTE, for Optical Telescope Element), comprising the 18 segments of the primary mirror, the support structure and the secondary mirror, is assembled. In March 2016, the optical part, the ISIM and the scientific instruments are in turn assembled. The manufacture of all components is completed in 2016.

Final assembly of the James Webb Telescope and integration tests (2017-2021)

At the end of 2016, all components (instruments, electronic equipment, moving parts) were tested individually, including the segments forming the primary mirror. The project is entering an expensive and complex phase of verifying the operation of the entire telescope. Because of its size, the James Webb Space Telescope cannot be fully tested assembled under conditions similar to those it will experience in space (vacuum of space, absence of gravity, temperature). But, unlike Hubble and despite the very high cost of this operation, the project managers decided to check, under realistic conditions (apart from the absence of gravity), the entire optical chain (from the primary mirror to the instruments), to avoid an anomaly similar to that which affected the Hubble primary mirror.

In May 2017, the optical and instrument assembly is transported by ship to the Johnson Space Center in Houston, Texas. There, optical tests are carried out in vacuum chamber A of the space environment simulator. Operators are able to adjust the primary mirror with the required precision, given the presence of gravity, and obtain images with the expected resolution. At the beginning of 2018, the committee in charge of the progress review noted delays affecting in particular the heat shield and the propulsion system. To overcome the remaining problems, NASA postponed the launch, scheduled for May 2019, in the month of May 2020, then in March 2021. Then the heat shield, platform, ISIM and optics at Northrop Grumman are conducted to the Redondo Beach site in California for final assembly and integration testing.

The Covid-19 pandemic, which hit the United States in the first half of 2020, is disrupting the pace of work of the teams. In June 2020, the launch date, expected in March 2021, is postponed to the end of October because of an anomaly affecting the fairing of the Ariane 5 rocket. In July 2021, the space telescope successfully completes integration tests at Northrop Grumman. It is installed in a container benefiting from a controlled environment and transported by road to the port of Seal Beach (California) forty kilometers away. There, he is embarked aboard the cargo ship MN Colibri, (ro-ro ship chartered by Arianespace for the transport of Ariane launchers and satellites between Europe and the Kourou base), for a fifteen-day journey through the Panama Canal, to the port of Pariacabo (French Guiana), not far from the Kourou base, where it arrives on October 13.

Soaring costs and postponed launch date

During the definition phase of the project, the estimated cost of developing the space telescope ranges from USD 1 billion to USD 3.5 billion, with a launch date ranging from 2007 to 2011. In 2006, the cost of development was reassessed at USD 4.5 billion and the launch date was pushed back to 2013. Half of the increase in the budget is attributed to the postponement of the launch date, including a one-year delay for the choice to use the European Ariane 5 launcher and another ten months due to the reduction in the budget of NASA’s scientific programs in 2006 and 2007, under President Bush. For one-third, the additional cost is due to late changes in requirements. In April 2009, the project is approved and the budget is set at USD 4.964 billion, with an expected launch date in June 2014.

Evolution of the cost (US share) and launch date
Estimated year	Launch Date	Cost billion USD
1997	Feasibility studies
1997	2007	0.5
1998	2007	1
1999	2007 to 2008	1
2000	2009	1.8
2002	2010	2.5
2002	General design
2003	2011	2.5
2005	2013	3
2006	2014	4.5
2009	Start of development
2009	2014	4.5
2011	2018	8.7
2013	2018	8,8
2017	Integration Testing
2018	March 2021	9.66
2020	October 2021

In the following years, the construction cost was reassessed several times and the launch date was regularly pushed back. In 2010, following initial budget shifts and project delays, the U.S. Senate Committee on Space Affairs requested that the project be reviewed by an independent commission. The report highlights many problems of management, cost estimation and communication. Following this, NASA reviewed the planning of the project.

Its cost increases to USD 8.835 billion including operational management (the European Space Agency’s participation of USD 650 million is not included in this sum) and the launch date is postponed to October 2018. During the summer of 2011, the cancellation of the project was considered by some representatives of the US Congress. Finally, the project escaped cancellation, but NASA was ordered to communicate, on a monthly basis, the evolution of the progress of the project and its cost. However, the budget share of the space agency’s astronomy program absorbed by this project now penalizes other projects, raising protests within the community of astronomers.

In September 2017, NASA announces a further postponement of the launch date, now set for June 2019. The causes of this change are complications encountered at the time of integration of the various components of the space telescope, as well as various technical problems. In March 2018, the US space agency announces, following an analysis of the risks affecting the achievement of the project’s deadlines, a new delay in the launch date, postponed to May 2020.

In June 2018, the cost of the space telescope is reassessed at $ 9.66 billion and the launch postponed to May 2020, then on March 30, 2021. In July 2020, the US space agency announces a further postponement of seven months (i.e. for October 2021), caused by problems encountered in integration tests and the ongoing Covid-19 pandemic. A final postponement is announced in September 2021, following a fairing problem encountered by the Ariane 5 launcher. The new launch date is now set at the end of December 2021.

In October 2021, the total cost of the space telescope is estimated at USD 9.7 billion, including USD 8.8 billion for the development of the telescope (2004-2021) and USD 861 million for operations during the five years of the primary mission (2022-2026). Taking inflation into account, this amounts to about USD 10.8 billion in 2020. This amount does not take into account the participation of the European Space Agency (€700 million, or USD 800 million) or the Canadian Space Agency (C$200 million, or USD 150 million).

This places the James Webb Space Telescope among the most expensive science projects in history, close to CERN’s Large Hadron Collider and its predecessor, the Hubble Space Telescope. Although JWST has greatly hampered other space astronomy projects, consuming for twenty years a third of the envelope allocated to this field to NASA, almost the entire community of astronomers believes that the investment is justified. The Hubble telescope, which in its time had suffered cost and time overruns of the same order of magnitude, is now almost unanimous, as its role in the progress of astronomy over the last thirty years has been important. The JWST telescope has the potential to contribute to similar scientific breakthroughs.

Conduct of the James Webb Telescope mission

The James Webb Telescope was launched on December 25, 2021by an Ariane 5 rocket, from the Kourou base in French Guiana. It is placed, after a transit of one month, in orbit around the Lagrangian point L₂ of the Sun-Earth system, located 1.5 million kilometers from the Earth, on the side opposite the Sun. Following a six-month commissioning phase, including a particularly delicate deployment of its heat shield and mirrors, the five-year scientific mission to meet the objectives assigned to the JWST telescope begins. The JWST carries propellant reserves that should allow it to remain in operation for at least ten years.

Launch of the James Webb Telescope

The James Webb Space Telescope is launched on December 25, 2021, from the Kourou space center in French Guiana by an Ariane 5 ECA rocket. The launch preparation campaign that takes place on the site lasts 55 days. At the end of this phase, the space telescope is placed under the fairing of the launcher of which it occupies practically the entire interior volume, 16.19 m high and a diameter of 4.57 m. The launch window of the James Webb Space Observatory has few constraints and the launch can take place 270 days a year. The daily launch window has a variable duration of up to 90 minutes and is usually between 11:45 and 14:00 UTC, corresponding to the end of the morning/midday in local time.

The launch of the James Webb Space Telescope has its characteristics imposed by its characteristics. To prevent any residual air pockets from causing the fragile heat shield to tear when the fairing is opened, the twenty-eight vents located in it, which ensure a gradual depressurization as the launcher ascents, have been modified. Several measures have also been taken to eliminate any prolonged exposure of the primary mirror to the Sun, which can distort its structure.

The launch takes place around noon so that, during its ascent, the Sun illuminates the nose of the launcher and, at the separation of the telescope, its rear part. The launcher’s orientation law has been modified (roll control) to avoid exposing the segments of the primary mirror directly to the Sun and creating a hot spot. Notwithstanding these few adaptations, the flight profile differs little from that of a large telecommunications satellite destined for geostationary orbit. The space telescope, with its mass of 6.2 t, less than the Ariane 5 geostationary transfer orbit (GTO) injection capacity, can be easily placed on its trajectory to the L₂ Lagrangian point, as it requires only a small excess of speed compared to the GTO orbit.

206 seconds after liftoff, while the rocket was at an altitude of 115 km, both halves of the fairing were jettisoned and the JWST telescope began transmitting telemetry to ground controllers. The separation of the JWST from the second stage of the launcher occurs at an altitude of 1,400 km, about thirty minutes after liftoff.

The James Webb Telescope transit to the Lagrange point

The space observatory then begins its journey to its destination, the Lagrangian point L₂, 1.5 million kilometers from Earth. The launcher placed the space telescope on a trajectory that takes it directly to its target. The JWST will gradually leave the Earth’s gravitational field, whose influence diminishes until it cancels out at the Lagrangian point L₂. Moving thanks to the impulse given by the launcher, it sees its speed decrease as it moves away from the Earth. During this transit, the space telescope is oriented so that the primary mirror is not exposed to the Sun because this would cause a distortion of its geometry fatal to the mission.

The speed communicated by the launcher is intentionally slightly too low for the JWST to reach its goal. The apogee of the orbit on which the telescope is placed by the Ariane rocket is 500,000 kilometers, while it would have to be 1.5 million kilometers to reach the Lagrange point. The first trajectory correction, the most critical because it must provide the extra speed to reach L₂, is made between 12.5 and 20 hours after launch. It requires running JWST’s small liquid propellant rocket engines for several hours. A second maneuver is performed 2.5 days after launch, just before the start of the heat shield deployment. The last one is carried out 29 days after launch and aims to insert the JWST into an optimal orbit around the Lagrangian point L_2.

The James Webb Space Telescope deployment

Deployment process
J = 25 Dec.	Deployed item
j	Solar panels
j	1st trajectory correction
D + 1	Medium and high-gain antennas
D + 2	2nd trajectory correction
D + 3	Heat shield pallets
D + 3	Optics-supporting tube (DTA)
D + 5	Moment compensating flap
D + 5	Protective coating
D + 6	Side beams
D + 7	Tensioning the heat shield
D + 8	Spacing of shield layers
J – 10	Secondary mirror
D + 11	Instrument radiator
D + 12-13	Primary mirror
D + 15-24	Mirror adjustments
D + 29	3rd trajectory correction
D + 29	Insertion into orbit around L2

This section summarizes the steps for deploying the telescope, which can be followed live on the NASA website.

During the transit, which lasts about a month and ends around January 24, 2022, the different moving parts of the telescope (mirror, heat shield, antennas, solar panel) are gradually deployed. No scientific mission has so far required such a complex sequence of operations of this type. In space, mechanical movements always present a risk because the absence of gravity does not allow them to be reproduced during tests carried out on Earth, while the behavior of the mechanisms is modified under these conditions. This phase of deployment is therefore critical. If not completed, it could lead to a complete failure of the mission.

Immediately after the separation of the launcher, the solar panels providing the energy are unfolded. The next day, the support of the large and medium gain antennas is unfolded in turn, allowing the high-speed link with the Earth. Other deployment operations begin only 2.5 days after launch and last several days. The first is to unfold the DTA (Deployable Tower Assembly) telescopic mast that solidifies the heat shield, on the one hand, with the optical part and the instruments, on the other hand. The deployment of this mast, consisting of two telescopic tubes, makes it possible to move the part of the JWST that must be kept at low temperature away from the heat shield.

In the following days, the deployment of the latter begins, which is the most delicate operation: commands are sent to execute sequences of operations that activate in stages 139 cylinders, eight motors and thousands of other components in order to unfold and tighten the five layers of the heat shield. These operations are carried out in three stages: the two pallets serving as support for the heat shield rotate to form a right angle with the primary mirror, then the layers of the shield are unfolded in the direction of the width and finally, they are separated vertically from each other. The entire process is broken down into many steps to allow ground engineers to control their proper execution. Procedures are provided for if an anomaly is encountered.

Electronics are redundant; shaking or rotating movements can be printed with the space telescope to facilitate the unfolding of the coatings; A deployment step can be run again. All these procedures have been extensively tested on the ground. Once the heat shield is deployed, 10 days after launch, the beams supporting the secondary mirror rotate to place it in its final position. The radiator of the ISIM module containing the instruments is then deployed.

On the following days, the lateral segments of the primary mirror are aligned with the central segments. Between D+15 (15 days after the launch date) and D+24, the positions of the 18 segments composing the primary mirror and the secondary mirror are adjusted in several steps. It is anticipated that if only one of the segments could not be adjusted (due to actuator failure), the primary mirror could still fulfill the objectives assigned to the mission in this degraded configuration.

In its operational orbit

Once there, the space observatory enters an orbit around the Lagrangian point L₂. Now, the JWST revolves around the Sun by permanently holding the Earth between the Sun and it (approximately). Normally, being at a greater distance from the Sun than the Earth, JWST should orbit the Sun more slowly than the Earth (according to Kepler’s laws). But objects near point L₂ undergo combined gravitational influences from the Sun and Earth, forcing an orbit around the Sun synchronous with that of the Earth.

JWST is not precisely at point L 2, which is not stable: it is simpler and more stable to insert it into orbit around the virtual point L₂. The plane of its orbit is perpendicular to the Earth-Sun axis and the plane of the ecliptic. It travels this orbit in six months at a speed of about 1 km/s. Its distance from the Lagrange point varies between 250,000 and 832,000 km, while that with the Earth oscillates between 1.5 and 1.8 million kilometers. Its maximum excursion above the plane of the ecliptic is 520,000 km.

The orbit is calculated so that the space telescope is never in the projected shadow of the Earth in order to avoid interrupting its only source of energy via its solar panels. This orbit is unstable and the radiation pressure exerts an asymmetric torque on the heat shield, which eventually saturates the reaction wheels responsible for compensating it and moves the space telescope away from Earth. To desaturate the reaction wheels and rectify its orbit, the space telescope uses its propulsion approximately every 21 days.

Commissioning

Commissioning does not take place until six months after launch, as it requires the optical assembly and instruments to be lowered to a temperature compatible with infrared observations and calibrated. The temperature of the JWST begins to gradually decrease after launch. Three weeks later, the part of the telescope located in the shadow of the heat shield (optics and instruments) reaches its target temperature (40 K, −233.15 °C). It takes one hundred days, from the date of launch, for the MIRI detector to reach its nominal temperature (7K, −266.15 °C) thanks to its mechanical cooling system.

One week after insertion into orbit around the L₂ Lagrangian point, the NIRCam instrument has dropped sufficiently in temperature to be used for mirror alignment. The operators first make sure that the image reaches the NIRCam camera. Using a wavefront control process that relies on the FGS fine guidance system and NIRCam camera, controllers on Earth align the segments of the primary mirror and the secondary mirror one after the other thanks to the cylinders that secure them with their support. They adjust the curvature (primary mirror) and inclination of the mirrors to achieve the desired performance of the image that forms on the focal plane of the space telescope.

Then begins a period of testing and calibration of the instruments (MIRI…) which is expected to last six months. On February 11, 2022, NASA announces that the telescope has almost completed Phase 1 of alignment, with each segment of the primary mirror having located, imaged and virtually centralized the target star HD 84406. Phase 1 of the lens alignment ends on February 18, 2022. The long process of commissioning optics and instruments ends in early July 2022. During this phase, the 17 operating modes of the scientific instruments were verified. The different filters, prisms and mechanisms were individually tested before being used in an operational configuration.

Better-than-expected performance

Data collected during the six-month commissioning phase that ends in early July 2022 demonstrate that the performance of the space telescope is almost always higher than that foreseen by the specifications.

The elements forming the optics are better aligned than expected: the point spread function is more precise and contains a greater amount of energy, and the optical performance is more stable over time. The fine guidance system is more precise. The mirrors are smoother than expected, which reduces stray radiation in near-infrared and reduces celestial background noise in wavelengths smaller than five micrometers.

The noise of the detectors is of the same order as that observed during ground tests, presenting a higher proportion of cosmic rays, as expected in interplanetary space. Overall, these results result in significantly improved instrument sensitivity in most observation modes and, in many cases, tens of percent higher performance. The telescope will generally be able to observe farther than expected. In addition, thanks to the precision of the launch and correction maneuvers during the transit to the Lagrangian point L₂, it is confirmed that the space telescope has enough propellant (necessary for orbit corrections and desaturation of the reaction wheels) to operate 20 years instead of the planned 10 years.

The four scientific instruments have demonstrated that they can obtain an electromagnetic spectrum of an exoplanet (by the transit method) with an accuracy greater than 100 ppm. The space telescope has demonstrated its ability to track an object in the Solar System moving at a speed of up to 67 thousandths of an arcsecond per second. The space telescope has managed to detect galaxies with a luminous flux of less than a few nanojanskys and to observe objects as bright as Jupiter.

The telescope could also observe ancient supernovae. According to NASA’s Mike Engesser, the NIRCam camera detected in July 2022 an unknown and extremely bright star in a distant galaxy. This would be the first supernova observed by the James Webb. This opens up a new field, because the nature of the older supernovae, exploded in the first hundreds of millions of years after the Big Bang, would be quite different from that of currently known stars, population I and population II, and these supernovae could be those of hypothetical population III stars.

First scientific images

Marking the end of the development and calibration phase, the first image of scientific quality, made on June 7, 2022, was released by NASA on July 11. Made with the NIRCam instrument (near-infrared camera) with an exposure time of 12 hours and 30 minutes using six filters, it shows the deep field, that is to say, some of the most distant areas of the Universe, and is centered on the SMACS 0723 galaxy cluster located 4.2 billion light-years away. Although the area of the overcast sky extends over only two arc minutes, there are hundreds of galaxies. The most distant, located more than 13 billion light-years away, are among the first to appear after the Big Bang (about a billion years after it.

This image demonstrates the superiority of the telescope over Hubble in observing very distant galaxies. To make a similar image, Hubble, whose mirror is much smaller and whose observations are regularly interrupted by the passage of the telescope behind the Earth, must accumulate several weeks of observation. Moreover, Hubble, unlike James Webb, can only observe a small portion of the infrared spectrum, while light from the most distant galaxies reaches us in this spectral band.

Four other images, demonstrating the telescope’s capabilities in the various astronomical observation domains that are part of James Webb’s objectives, were released on July 12, 2022, by NASA.

• The image of the Carina Nebula, obtained by combining observations made using NIRCam (near-infrared) and MIRI (mid-infrared), is much more detailed than those made by Hubble and allows to observe the first phases of star formation thanks to the capabilities of the James Webb in terms of spatial resolution and sensitivity.
• The group of galaxies known as Stephan’s Quintet, observed by James Webb in the near and mid-infrared, makes it possible to observe in an unprecedented way the interactions between galaxies belonging to the same group. The image produced makes it possible to distinguish the propagation of shock waves produced by one of the galaxies colliding with the group of galaxies as well as certain stars individually.
• The image of the planetary nebula NGC 3132, taken in different wavelengths using the NIRCam and MIRI instruments, has made it possible to highlight certain chemical components.
• The telescope produced a particularly detailed electromagnetic spectrum of the exoplanet WASP-96b. This gas giant discovered in 2013 has a surface temperature of more than 1,000 °C. Hubble had detected the possible presence of water vapor in its atmosphere. The James Webb telescope confirmed this one, but also that of haze and clouds.

First scientific images of the James Webb Telescope

Operation of the James Webb Telescope

Observable region of the sky

The entire sky cannot be observed at a given moment, because it is imperative that the detectors and the optical assembly are completely sheltered from the radiation of the Sun and the Earth by the heat shield. The telescope is free to rotate 360° around the direction of the Sun, as the impact of solar radiation on the heat shield remains unchanged. On the other hand, given the size and shape of the heat shield, the angle between it and the direction of the Sun (solar elevation) must be between -5° and 40°. Because of this constraint, the observable area at a given moment represents about 40% of the celestial vault (80% for Hubble).

JWST’s orbit around the Sun allows it to make observations of the entire sky over the course of a year for at least 100 days. In the region of the ecliptic poles, between 85 and 90°, the observation can be continuous. Celestial objects closer to the Sun than the Earth (Venus, Mercury, asteroids circulating in this area) can never be observed. The space telescope can also oscillate slightly around the telescope’s axis, from 3 to 7° depending on the solar elevation.

Conduct of observations of the James Webb Telescope

The James Webb Space Telescope Control Center is hosted by the Space Telescope Science Institute (STScI), located in Baltimore, Maryland. This organization is managed by the Association of Universities for Research in Astronomy (AURA) on behalf of NASA. STScI is also responsible for selecting and scheduling observations. It fulfills the same role for the Hubble telescope. Exchanges between Earth and the space telescope take place via the large satellite dishes of NASA’s Deep Space Network, located in Goldstone, California, Madrid, Spain and Canberra, Australia. The TDRS satellites, the Malindi station in Kenya and the ESOC control center in Germany are also used to maintain a permanent link with the space telescope.

The observations are programmed well in advance and are transmitted in the form of sequences of operations to take place during about twenty days (time between two orbit corrections), without the intervention of ground controllers. If an observation cannot be executed (difficulty of pointing …) The space telescope scheduler automatically performs the next observation. The expected availability rate (proportion of time actually spent on observations) is over 70%. The sequence of scheduled observations can be interrupted within 48 hours to study an unexpected astronomical event such as the appearance of a supernova, a gamma-ray burst or a collision between two celestial bodies in the solar system.

The scientific data collected by the detectors are recorded non-destructively in the mass memory, every 20 to 200 seconds, to limit possible data loss due to cosmic rays (the exposure time can be much longer and, at the Lagrangian point L₂, the pixel corruption rate is 5 to 10% over a period of 1,000 seconds). Commands are transmitted through the S-band control center, while data is transmitted in Ka-band. It is planned to transmit up to 232 gigabytes of data per day (mass storage capacity), during daily communication sessions lasting three hours.

The accuracy of the telescope’s pointing, required to make an observation, depends on the instrument used. It is between 5 and 7 arc seconds and 5 milliseconds of arc. The pointing is based on guide stars that are selected in a region close to that observed and that are listed in the FGS instrument catalog. The latter is responsible for locating and keeping the telescope pointed at its target, continuously measuring the position of the guide stars and providing, in case of deviation, instructions to the attitude control system. The latter uses reaction wheels to correct pointing errors. The pointing accuracy is 0.10 arc seconds and the pointing stability is between 6.2 and 6.7 milliseconds of arc (depending on the instrument), for an exposure time of 1,000 seconds.

Orbital corrections of the James Webb Telescope

Unlike ground-based observatories that are confronted with atmospheric disturbances and deformations resulting from gravity, the James Webb telescope is affected only by small temperature variations requiring only spaced corrections. Every other day, the wavefront is checked using the NIRCam instrument. Adjustments to the mirrors, necessary to take account of their deformations, will be made every two weeks at most and should not consume more than 1 to 2% of the observation time.

Data archiving

All data collected by JWST is stored in the Mikulski Archive for Space Telescopes ( MAST), which makes it available to researchers and the public. This system archives astronomical data collected in the ultraviolet, visible and near-infrared, by ground-based and space observatories managed by NASA (Pan-STARRS, Kepler, TESS, Hubble).

Observation selection process

The mission of the Space Telescope Science Institute is to manage the operation of the telescope in orbit, evaluate, select and program observations, collect data, distribute and archive them. As with NASA’s other major space observatories, 10% of the observation time over the lifetime of the instrument is allocated to astronomers who participated in the realization of the instruments (Guaranteed Time Observer or GTO), or 4,020 hours for the first three observations cycles spread over 30 months. Over the same period, 10% of the observation time remains at the discretion of the STScI (Director’s Discretionary Time or DD), while 80% of the time is allocated to astronomers around the world (Guest Observer or GO).

The latter, in order to be able to use the telescope, submit their observation proposals to a committee composed of two hundred astronomers as well as representatives of the space agencies involved in the development of the JWST. The committee selects the most relevant proposals, taking into account the overall objectives of the mission. The observations of the first annual cycle should be in line with the objectives of the Early Release Science Program, defined to quickly obtain the greatest possible scientific feedback and accurately measure the capabilities of the instruments. The proportion of time allocated to the OWG will be higher for this first cycle (between 25 and 49%).

For the first cycle of observations (June 2022 – June 2023), 6,000 hours were offered to astronomers around the world as part of the Guest Observer (see above): 3,500 hours of short observations, 1,500 hours of medium duration and 1,000 hours of long duration + reserves. Of the 1,084 proposals, 266 were selected, including 89 from European countries and 10 from Canada (the country is the country of the lead proponent of the observation).

70% of observations are spectroscopy and 30% are imaging (the opposite proportion of Hubble). The observation time is distributed among the instruments as follows: NIRSpec (40.8%), MIRI (28.1%), NIRCAM (24.4%) and NIRISS (6.7%). The theme of the observations roughly reflects the objectives assigned to the telescope: the study of galaxies and the intergalactic medium (32%), exoplanets and protoplanetary disks (23%), stellar physics (12%), stellar population and interstellar medium (11%), supermassive black holes (9%), large-scale structure of the Universe (7%) and Solar System (6%).

Life span

To meet scientific objectives, JWST was designed to operate for at least five and a half years. Unlike infrared observatories that preceded it, such as Herschel, its lifespan is not limited by the amount of cryogenic liquid available, because its detectors are cooled mechanically (for MIRI, Mid InfraRed Instrument) or passively. The only limiting factors are the wear of the electronic or mechanical components and especially the exhaustion of the propellants used to keep the telescope in its orbit, because it is not completely stable. JWST carried enough propellant to remain in its orbit for at least ten years.

Like most space telescopes, but unlike Hubble (until the retirement of the US Space Shuttle), JWST cannot be repaired and its instruments cannot be replaced, as its remoteness prevents human intervention. Indeed, there is currently no module allowing the survival of a crew during the minimum two months of a mission and allowing a return to Earth.

At the end of May 2022, a small meteor hit one of the telescope’s mirrors, which it pushed out of its axis, but without irreversible damage. It is already the fifth meteor and the largest to hit the telescope since its deployment.

Detailed technical specifications

Once in orbit, the James Webb Space Telescope is 8 m high, 21.2 m long and 14.2 m wide. Its launch mass is about 6,173 kg. It consists of four sub-assemblies divided between “warm side” and “cold side”:

• the platform (or bus), located on the “hot side”, brings together all the support functions: control and maintenance of the orbit, power supply, storage of data collected by the instruments and communications with the Earth and between the equipment of the observatory;
• The heat shield separates the “warm side” from the “cold side”. Its role is to protect the most sensitive parts of the telescope (optics and instruments) from infrared radiation from the Sun, Earth and Moon, as well as from the platform;
• the optical part of the OTE telescope (Optical Telescope Element), located “cold side”, collects the radiation of the celestial bodies using several mirrors and sends it back to the scientific instruments;
• the four instruments in the Integrated Science Instrument Module (ISIM), also placed “cold side”, analyze the collected radiation and produce electromagnetic images and spectra.

Platform

The James Webb Observatory platform brings together the equipment that supports the operation of the space telescope. It is attached to the illuminated face of the heat shield, near the center of mass of the spacecraft. It contains a lot of electronics that generate heat. It is for this reason that it was fixed on the “hot side” of the heat shield. The platform has the shape of a parallelepiped 3.5 × 3.5 m side and about 1.5 m high.

Its central part is occupied by a conical structure made of carbon fiber reinforced plastic, 2.5 m in diameter at the base; attached to the rocket and which, during launch, supports the weight of the heat shield and the optical part. At the base of the platform (opposite the heat shield) is the main propulsion system of the space telescope. The antennas are attached under this module, while the radiators and solar panels are fixed on the sides.

The main subsystems of the platform are:

• the electrical energy production system that relies on fixed solar panels. These form a 5.9 m long wing, fixed on the platform at an angle of 20° to the plane of the heat shield. The solar panels, which are permanently illuminated, produce at least 2,000 W throughout the life of the space telescope;
• The attitude control system maintains telescope pointing with an accuracy of 0.01 μrad relative to the reference position provided by the FGS. To act on orientation, it uses six reaction wheels (including two spare ones) which, when saturated, are discharged by small rocket engines. Determination of orientation and motion is provided by two-star finders, solar sensors, and gyroscopes.
• The telecommunications system transmits data collected by instruments and telemetry informing ground monitoring of the status of the space telescope (uplink). In return, he receives instructions from the controllers. The exchanges are made via a high-gain parabolic antenna of 60 cm in diameter operating in the Ka-band and a medium-gain antenna of 20 cm in diameter operating in the S-band. Both are fixed on a common steerable platform. The width of the beam emitted by the high-gain antenna is about the size of the Earth’s surface, at its reception, so the pointing of the antenna must be regularly changed as JWST moves around the Lagrange point. To ensure that the vibrations produced by the movements of the antenna do not disturb the observations, these adjustments are made during a pause in the observations, every 10,000 s. The medium-gain antenna makes it possible to transfer telemetry, with a minimum throughput of 40 kilobits per second, to any visible earth station. The high-gain antenna is used to transmit scientific data at a default rate of 3.5 megabytes per second. In case of passage in survival mode, exchanges are made via two omnidirectional antennas;
• The Data and Command Management (C&DH) system, which is based on an onboard computer, receives and interprets the operations to be performed, rebroadcasts them, collects and stores scientific data before transmitting it to Earth. While waiting for transmission, the data is stored in a mass storage type solid-state recorder (SSR) with a capacity of 65 gigabytes;
• The space telescope has two propulsion systems. Two redundant pairs of SCAT (Secondary Combustion Augmented Thrusters) liquid propellant rocket engines, with a thrust of 22 newtons (about 2.2 kilograms-force), are used for orbit corrections. They burn hypergolic propellants (hydrazine and nitrogen peroxide). Eight pairs of lower thrust (4.4 N) monopropellant rocket engines (MRE) monopropellant (hydrazine) are used to control the orientation of the telescope and desaturate the reaction wheels. 301 kg of hydrazine and 133 kg of nitrogen peroxide, which allow for at least 10.5 years of operation, are stored in tanks housed in the platform. Helium is also carried to pressurize propellants before they are injected into rocket engines.
• the thermal control system, which keeps the entire platform within the intended temperature range, thanks to multi-layer insulators and four radiators deployed in orbit on either side of the platform;
• finally, the platform also houses three of the four floors of the refrigerator that keeps the MIRI instrument detector at a temperature of 7 K. This equipment has been placed “hot side” because it is itself a heat generator.

James Webb Telescope heat shield

The heat shield is a structure in the shape of an elongated hexagon 22 meters long and 12 m wide. Its role is to isolate the optical part and the instruments from the heat fluxes coming from the Sun, the Earth and the Moon. While its face facing the Sun is permanently exposed to radiation from the star, Earth and Moon and brought to a temperature of 300 to 383kelvins (27 to 110 °C), it maintains the optical part and the scientific instruments, without any active refrigeration device, at a temperature of 40K (−233°C), necessary for the operation of the infrared detectors and the geometric stability of the telescope. Of the 200,000 watts of power received, the heat shield allows only one watt to pass through.

On the hot side is the platform containing the telescope easements (telecommunications, attitude control, propulsion system, etc.), which is itself a source of infrared. Located on the other side of the heat shield, the cold part includes the telescope and scientific instruments. Instrument detectors are kept at an even lower temperature: for MIRI detectors, through a mechanical refrigeration system that lowers their temperature to 7 K (−266 °C) and for detectors in other instruments, through passive devices that maintain their temperature at 39K (−234 °C).

The heat shield consists of five spaced layers of metalized polymer that reflect heat back into space. The material used is extremely thin to limit its mass: 0.05 mm for the layer facing the Sun and 0.025 mm for the others. Going from the outer layer to the inner layer, each layer is cooler than the previous one. The fabric used is a kapton polyimide that remains stable over a very wide temperature range (between −269 °C and +452 °C).

All layers receive a 100-nanometer-thick aluminum coating, responsible for reflecting the heat flow. The two hottest layers also receive a 50-nanometer silicon coating, which allows electrical charges to circulate (heat shield grounding). The size and position of the heat shield is calculated so that only the innermost layer is visible to the telescope, regardless of the part of the sky observed by the telescope (within the limit of the region of the sky defined as observable, taking into account thermal stresses).

The heat shield is fixed on two rectangular clerestory paddles, as long as this one, but much less wide to fit under the fairing. These are folded along the telescope body for launch, then lowered into orbit. A set of beams and cables as well as 107 actuators allow the heat shield to be deployed in space. Six vertical beams attached to these pallets serve as an anchor point for the five layers of coating, allowing them to be tensioned and spaced. The latter varies from about thirty centimeters at the edge, to about 13 cm in the center of the heat shield. To limit the risks in case of failure, all electronics are redundant; however, the mechanisms are not.

Optical part

The optical part OTE (Optical Telescope Element) consists of an anastigmatic system with three mirrors, with a focal length of 131.40 m for an aperture of f/20. This type of telescope uses three curved mirrors that provide a wide field of view while minimizing the main optical aberrations. The optics are composed of a primary mirror 6.5 meters in diameter, a secondary mirror 74 centimeters in diameter and a tertiary mirror. The optical part also contains the structure supporting the mirrors and a thermal regulation system including radiators.

Primary mirror

The primary mirror is of the segmented type, with a diameter of about 6.5 m and a mass of 705 kg. The mirror is just under three times the diameter of the Hubble telescope (2.4 m) and its collecting area is 25.4 m² (Hubble 4.525 m ²). Too large to fit under the fairing of the launcher, it is composed of 18 hexagonal elements 1.3 m wide, which allow it to be folded into three parts for launch, then deployed once in space. The segments of the primary mirror are attached to a rigid structure made of carbon composite material. Each segment is made of beryllium.

Beryllium was chosen because it is a strong, lightweight metal with an extremely low coefficient of thermal expansion at temperatures encountered in space (between 30 and -80 K). It has been successfully used by the Spitzer and IRAS infrared space telescopes. The beryllium mirror has a thickness of 1 mm, which limits the total mass of the primary mirror to 705 kg compared to 1 t for the Hubble glass mirror. Each segment has a mass of 20 kg (40 kg with actuators). Each segment is manufactured so that it takes the desired shape once in space and subjected to a temperature of K.

Each segment has six actuators to adjust its position and orientation, and a seventh to change its radius of curvature. These controls allow for an accuracy greater than 10 nanometers. The structure that carries the primary mirror also serves as a support for the ISIM module containing the instruments. The assembly (support, mirrors and ISIM) has a total mass of 2,400 kg. In order to maintain the accuracy of the curvature of the primary mirror, which has a direct impact on the resolution of the telescope, this structure is designed not to deform by more than 32 nanometers at a temperature of −240 °C.

The surface of the primary mirror, like that of other JWST mirrors, is covered with a thin layer of gold (thickness of 100 nm, or 48.25 g for the entire mirror). Gold has the property of optimally reflecting the part of the electromagnetic spectrum observed by JWST instruments: the red of the visible spectrum and the infrared invisible to our eyes. On the other hand, it reflects very poorly the blue of the visible spectrum. The gold layer, very fragile, is in turn covered with a thin layer of glass. It is this thin layer of gold that gives the characteristic golden color to the surface of the mirrors.

The surface area of the primary mirror, 5.5 times larger than Hubble’s, allows the telescope to collect an image nine times faster than its predecessor. The resolving power of the telescope reaches 0.1 arcseconds in the infrared range (0.6 to 27 micrometers wavelength). Unlike Hubble, it does not allow to observe the light spectrum in the ultraviolet and visible.

James Webb Telescope’s secondary mirror

The secondary mirror is a circular convex mirror with a diameter of 0.74 meters that concentrates the light from the primary mirror and reflects it back to the tertiary mirror. It is suspended above the primary mirror, by a tripod-shaped structure, folded along the primary mirror for launch. The orientation of the beryllium mirror can be adjusted using six actuators according to six degrees of freedom.

Other elements of the optical part

The rest of the optical part (after optics) includes the fixed tertiary mirror and a mobile fine-pointing mirror (FSM). The tertiary mirror is of aspherical concave type and elongated in shape (0.73 × 0.52 m). It returns the collected radiation to the FSM, while correcting aberrations, so as to provide a quality image over the entire field of view. The WSF is a flat mirror that stabilizes the image during scientific observations. When they take place, its position is constantly adjusted in two dimensions to counter the movements of the telescope detected by the attitude control system. A mask at the edge of the WSF reduces stray radiation.

Instruments

The telescope is equipped with three main instruments and one secondary instrument, which are assembled in a structure attached to the back of the primary mirror bracket and form the ISIM (Integrated Science Instrument Module). ISIM also includes, at some distance from the instruments, radiators that remove heat from the instruments to keep their temperature low, electronic equipment to control the instruments, an ISIM-specific control and data management system, ICDH (ISIM Command and Data Handling), and the mechanical cryogenic cooler used to lower the temperature of the Mid InfraRed Instrument (MIRI).

NIRCam Camera

NIRCam (Near-InfraRed Camera) is a wide-field camera operating in the near-infrared of 0.6 to 5 micrometers. The camera has two virtually identical subsets that cover adjacent portions of the sky separated by 44 arcseconds. The field of view of each of these modules is 2.2 × 2.2 arcminutes. One of the two instruments covers wavelengths between 0.6 and 2.3 μm (short wave), the other between 2.4 and 5 μm. Light from the short-wave instrument arrives at four detectors (2 × 2) of 2,040 × 2,040 pixels each, while that from the second instrument arrives at a single detector of 2,040 × 2,040 pixels.

The resolution is 0.032 arcseconds per pixel for the first set of detectors and 0.065 arcseconds for the second. Filters allow you to select particular wavelengths. The shortwave instrument has five filters selecting wide bands (R~4), four medium (R~10) and three narrow (R~100). The second instrument has three wide, eight medium and four narrow filters. The instrument has a coronography mode to be able to make images of very faint objects, close to very bright sources, such as exoplanets or debris disks. The instrument can also perform fast imaging on small areas, as well as slitless spectroscopy on the 2.4–5 μm spectral band with an R resolution of about 1,700. NIRCam is being developed by a team at the University of Arizona and Lockheed Martin’s Center for Advanced Technology.

NIRSpec spectrometer

NIRSpec (Near-InfraRed Spectrometer) is a multi-object spectrometer operating in the near infrared of 0.6 to 5.3 μm. It is optimized for observing very distant, faint galaxies and many compact sources.

Three observation modes are available:

• NIRSpec has a multi-object mode thanks to a matrix of programmable micro-shutters (Micro-Shutter Assembly MSA) that allows to simultaneously realize the spectrum of 100 selected objects in a field of 3.6 × 3.6 arc minutes. Each object is observed via an aperture corresponding to a field of 0.20 × 0.45 arcseconds. The spectral resolution can be 100, 1,000 or 2,700;
• slit spectroscopy. This mode remains available when the Micro Shutter Assembly (MSA) is used (no overlay). The instrument has a 0.4″ × 3.8″ slot, three 0.2″ × 3.3″ slots and a 1.6″ wide × 1.6″ opening. The spectral resolution can be 100, 1,000 or 2,700;
• “Field integral” spectroscopy over a field of view of 3 × 3 arcseconds. The spectral resolution can be 100, 1,000 or 2,700. The field is divided into 30 images of 0.1 × 3 arcseconds. The corresponding aperture is closed when this mode is not in use.

To avoid confusion that could be generated by the overlapping of spectra, the observable spectral band (0.6 to 5.3 μm) is divided into three sub-bands, selected by a filter, which must be observed separately.

From a technical point of view, NIRSpec consists of 16 mirrors (two telescope coupling mirrors, three mirrors for each of the three TMAs, one mirror for reflection between the MSA and the collimator, two focusing mirrors and two mirrors used during calibration), as well as a set of eight filters and seven interchangeable dispersive elements. The luminous flux passes through a first filter which allows either to select the spectral band to be observed (>0.7 μm, >1 μm, >1.7 μm, >2.9 μm), or to perform pointing operations towards the target (clear filter), or to perform calibration operations (mirror).

After passing through the slits or the MSA matrix, the radiation passes through diffractive optics that is selected according to the wavelength and spectral resolution that one wishes to favor. The focal plane contains two mercury-cadmium telluride infrared photodetectors of 2,048 × 2,048 pixels, sensitive to wavelengths from 0.6 to 5 μm and developed by Teledyne Imaging Sensors. They are separated by an interval of 17.8 arcseconds that leads to a hole in the spectrum (this spreads over both detectors). The NIRSpec instrument, which measures 1.9 meters in its largest dimension, has a mass of 200 kg.

The MSA matrix consists of a grid consisting of four quadrants each subdivided into 365 cells on the x-axis (direction of spectral dispersion) and 171 cells in the y-direction, or 248,000 cells in total (62,000 per quadrant). Each cell, which measures 100 × 200 μm (the thickness of a few hairs), is closed using a movable door. Two electrodes are attached, on the one hand, to the door closing the cell and, on the other hand, to the partition on which it can be folded.

By applying a charge of the opposite direction to the two electrodes of a given cell, its opening is triggered. A movable magnetic arm makes it possible to act on all doors. These microsystems use MEMS technology. One of the limitations of the MSA is that only one star can be observed on each row parallel to the x-axis, as its spectrum uses the entire width of the detector. The star must also be centered in the cell. To observe all the stars in a given area, it is, therefore, necessary to make several observations preceded each time by a modification of the pointing of the telescope.

NIRSpec is provided by the European Space Agency and its development is overseen by the European Space Technology Centre (ESTEC) in the Netherlands. The main supplier is Airbus Defence and Space‘s facility in Ottobrunn, Germany. The detectors and micro shutter system are provided by NASA’s Goddard Space Center.

MIRI camera/spectrometer

MIRI (Mid InfraRed Instrument) is a spectro-imager comprising a camera (MIRIM) and a spectrometer (MRS) that operates in the mid-infrared (5 to 28 μm). The instrument will make it possible to take photos and spectra of young exoplanets and their atmospheres, to identify and characterize the first galaxies in the Universe and to analyze hot dust and molecular gases from young stars and protoplanetary disks. Four modes of observation are possible:

• Realization of images through ten filters. MIRI’s resolution is 0.11 arcseconds per pixel, for a maximum field of 74 × 113 arcseconds. Several smaller fields will also be available (7 × 7, 14.1 × 14.1, 28.2 × 28.2, 56.3 × 56.3 arcseconds) to allow a short exposure time (taking images of luminous objects or luminous environment);
• Coronography: its role is to attenuate or suppress the flow of a very bright object (a star, for example) in order to observe its nearby faint environment (an exoplanet, for example). In this observation mode, the field of view is 15 × 15 arcseconds and the angular resolution is 0.11 arcseconds. The coronagraphs include three 4QPM (Four-Quadrant Phase Masks) monochromatic phase masks and a Lyot mask. The three-phase masks operate at 10.65 μm, 11.4 μm and 15.5 μm respectively, while the Lyot mask operates at 23 μm. Since the angular separation between a star and its planetary system is very small, the use of classical Lyot pellet coronagraphs is not suitable. A new generation of four-quadrant phase coronagraphs, known as “4QPM”, has been developed by LESIA;
• low-resolution spectroscopy (spectral resolution of 100) between 5 and 11 μm;
• “Field integral” spectroscopy on a field of 3 × 3 arcseconds with a spectral resolution of about 1,500.

The Mid InfraRed Instrument (MIRI) is provided by the European Space Agency. It is built by a consortium of laboratories from ten European countries, coordinated by the Edinburgh Observatory in Scotland. MIRI consists of two distinct parts. The first subset, the imager/coronagraphs/low-resolution spectro-called MIRIM, developed and built under the aegis of CNES in France by the Department of Astrophysics of CEA-Saclay, with the participation of LESIA (Paris Observatory), the Institute of Space Astrophysics (IAS) and the Laboratory of Astrophysics of Marseille (LAM). The second sub-assembly, the medium resolution spectrograph with an integral field (IFU) feature, called “MRS”, built by the Rutherford Appleton Laboratory (RAL) under the aegis of the English Science and Technology Facilities Council (STFC). The RAL ensures the integration of all instrument and test components.

MIRI consists of three detectors, each with one million pixels: one for the MIRIM imager and two for the MRS spectrometer. These detectors are identical in their design. These are arsenic-doped chips, each with 1,024 × 1,024 pixels. In the observed wavelengths, the detector is particularly sensitive to thermal emissions from the telescope and the 40 K temperature of the telescope is insufficient. To be able to operate, it is cooled to 7 K by a particularly efficient mechanical cryo-cooler, developed under the supervision of the Jet Propulsion Laboratory (JPL).

It cools helium in four stages using, for the first three, pulsation tubes exchanging heat by thermoacoustics. The first three floors of the refrigerator are housed in the platform (on the hot side of the space telescope). A pipe, 10 m long and 2 mm in diameter brings the fluid, whose temperature has been lowered to 18 K, into the enclosure of the MIRI instrument. There, the temperature of helium is further lowered by the Joule-Thomson effect to 6 K. The development of this equipment had to overcome two problems: suppressing the generation of vibrations by the pumps used to compress the gas and preserving the temperature of helium in the long pipe to the instrument’s detectors.

NIRISS Near Infrared Imager

NIRISS (Near Infrared Imager and Slitless Spectrograph) is a secondary instrument associated with, but independent of, the Fine Guidance System (FGS). It is a spectro-imager for making wide-field spectra in the 1 to 2.5 μm band with a spectral resolution R of about 150, spectra on a single object in the 0.6 to 2.8 μm band using a gray with a spectral resolution R of about 700.

It also allows interferometry spectra using a non-redundant mask (NRM) in the spectral band from 3 to 4.8 μm. The instrument can also make images on a wide spectrum (1 to 5 μm) and an optical field of 2.2 × 2.2 arc minutes. The instrument has two sets of filters for selecting narrow spectral bands. The radiation reaches the focal plane on a mercury-cadmium telluride detector with 2,048 × 2,048 pixels. The instrument is provided by the Canadian Space Agency. The main manufacturer is Honeywell (formerly COM DEV).

Other payload instruments: FGS fine guidance system

The FGS (Fine Guidance System) is a fine guidance system that performs three functions:

• provide images of the entire telescope field, with the aim of finding the region of the sky to be studied at the beginning of a new observation campaign;
• Identify, in the region visualized by the telescope, a guide star listed in the catalog recorded in its memory. Once the guide star is identified, it is centered in a window of 8 × 8 pixels, then the orientation of the telescope is changed to position the guide star in an area of the pre-specified window so that the portion of the sky observed is in alignment with the telescope axis;
• Provide the attitude control system with measurements to keep the space telescope’s pointing at the guide star, with an accuracy of one milliarcsecond, by taking an image 16 times per second.

Technically, the FGS consists of a first mirror deriving the incident radiation (POM pick-off mirror) and a set of three mirrors (three-mirror assembly) collimating this radiation towards a mirror which focuses it on a detector located in the focal plane. It includes a mercury-cadmium telluride infrared photodetector of 2,048 × 2,048 pixels, sensitive to wavelengths of 0.6 to 5 μm. Its sensitivity is 58 μJy (microjansky) for a wavelength of 1.25 μm. The instrument does not have a shutter or optical filter. The FGS is provided by the Canadian Space Agency. Its main manufacturer is Honeywell (formerly COM DEV).

References (sources)

« Frequently Asked Questions (FAQ) – Webb’s Orbit », JWST, NASA; « Webb Orbit », NASA; Article James-Webb (télescope spatial), Wikipedia; Eric Berger, « James Webb Space Telescope will absolutely not launch in March », Ars Technica, 2020; Simon Lilly, « The Next Generation Space Telescope (NGST) », University of Toronto, 1998.