The Webb Space Telescope

Eighteen Mirrors Acting as One

The primary mirror is about 6.5 m across, made of 18 hexagonal segments. Each is 1.32 m across the flats, made of beryllium and coated with about 100 nm of gold. All 18 together carry about 48 g of gold.

Behind each segment are seven actuators: six form a hexapod that sets its position and tilt, and one in the middle sets its curvature. The smallest step an actuator takes is 7.7 nm.

Once aligned, the whole telescope’s wavefront error in the near-infrared camera is about 65–70 nm rms. The requirement was 150 nm or less.

18 segments · 1.32 m across flats · 7 actuators each · 7.7 nm steps · wavefront error ≈ 65–70 nm

Eighteen Stars Become One

In February 2022 the segments had just left their launch locks, each pointing slightly differently. Aimed at the star HD 84406, the camera saw 18 images of it, one from each segment.

The team worked out which image came from which segment, then tilted the segments one by one until all 18 images lay on one spot. At that stage the 18 beams only add their brightness.

Then they moved each segment forward or back until the crests of the 18 light waves lined up. The light now adds as waves, and the image draws into one sharp core. Fine phasing was finished on 11 March, and the whole telescope was aligned by 23 April.

identify → stack → coarse phase → fine phase · images computed by diffraction on this page (2 µm)

Why Infrared

The universe is expanding, and light from distant galaxies is stretched on its way here. The ultraviolet and visible light of the first galaxies arrives as infrared.

Infrared light also passes through dust. Stars form inside dust clouds that hide them in visible light but not in infrared.

Webb sees light from 0.6 to 28 µm. Human eyes see 0.4 to 0.7 µm.

observed wavelength = emitted × (1 + z) · Lyman-α at 121.6 nm reaches 1.34 µm at z = 10

Five Layers Against the Sun

Infrared radiation is heat radiation. A warm telescope would glow in the infrared and drown the faint light of distant galaxies, so the mirrors must be colder than 50 K (about −223 °C).

The sunshield does this. About 21 m long and 14 m wide, the size of a tennis court, it is five layers of aluminised polyimide film (Kapton). The outer layer is 0.05 mm thick, the other four 0.025 mm. The layers are a few centimetres apart in the middle and about 25 cm at the edges, where the heat escapes to space.

Over 200 kW of sunlight falls on it, and less than one watt gets through. The Sun-facing side reaches about 383 K (110 °C); the shaded side can fall to about 36 K (−237 °C).

The mid-infrared instrument, MIRI, must be colder still. A closed-cycle helium cryocooler takes it to about 7 K.

layer 1 ≈ 383 K → layer 5 ≈ 36 K (NASA model values) · the middle three layers have no published temperatures

L2, 1.5 Million km Out

Webb does not orbit the Earth. It circles the Sun–Earth L2 point, on the far side of the Earth from the Sun, about 1.5 million km away: some four times the Moon’s distance.

From there the Sun, the Earth and the Moon are always on the same side, so one sunshield blocks all three.

Webb loops around L2 on a halo orbit, about once every six months, between 250,000 and 832,000 km from it. It entered this orbit on 24 January 2022 with a 297-second burn. The launch was so accurate that the propellant saved will last more than 20 years.

≈ 1.5 million km from Earth · halo orbit about 6 months · Earth, Moon and Webb drawn enlarged

Cut Open

Behind the primary is the Integrated Science Instrument Module (ISIM). Four instruments sit on a composite frame, wrapped in insulation, with radiators on top facing cold space.

Light reflects from the primary to the secondary 7.2 m in front, back through the hole in the middle of the primary, into the aft optics, and on to the instruments.

Below the sunshield, on the warm side, is the spacecraft bus: solar array, antenna, electronics, propellant and the compressor of MIRI’s cryocooler. The cold side and the warm side are joined only by a telescoping tower.

cold side: telescope and instruments | warm side: the spacecraft bus · instrument shapes are schematic

Four Instruments

Near-Infrared Camera, NIRCam, 0.6–5 µm: the main imager, and the wavefront sensor used to align the segments. Built by the University of Arizona and Lockheed Martin.

Near-Infrared Spectrograph, NIRSpec, 0.6–5.3 µm. Its quarter of a million microshutters let it take spectra of a hundred or more objects at once. Built for ESA by Airbus.

Mid-Infrared Instrument, MIRI, 4.9–27.9 µm: a camera and spectrographs, working at about 7 K. Its optics come from a European consortium; its detectors and cryocooler from the United States.

The Fine Guidance Sensor, FGS, and the Near-Infrared Imager and Slitless Spectrograph, NIRISS, share one package from the Canadian Space Agency. FGS keeps the telescope locked on its target; NIRISS is often used on exoplanets.

NIRCam 0.6–5 µm · NIRSpec 0.6–5.3 µm · MIRI 4.9–27.9 µm · NIRISS 0.6–5 µm

Folded for Launch

The primary is 6.5 m wide and the sunshield 21 m long, but the Ariane 5 fairing is only 5.4 m across the outside. Webb had to be launched folded.

Three segments on each side of the primary folded back, the secondary’s tripod folded in front of the mirror, and the five layers were packed onto two pallets that stood up against the telescope.

It lifted off from Kourou in French Guiana on 25 December 2021, weighing about 6,200 kg.

At launch it had 344 single-point failures, almost all of them in the deployments: items whose failure alone could keep the mission from meeting its basic requirements.

Ariane 5 · VA256 · 2021-12-25 12:20 UTC · fairing 5.4 m outside diameter

Two Weeks of Unfolding

About half an hour after launch the solar array opened. The next day the high-gain antenna swung out.

On day 3 the forward and aft pallets were lowered. On day 4 the tower pushed the telescope 1.22 m away. On day 5 the momentum flap opened and the sunshield’s covers were released. On day 6 the two mid-booms slid out, drawing the five layers out sideways.

On days 9 and 10 the layers were tensioned one by one. On day 11 the secondary’s tripod swung out and latched; on day 12 the aft radiator opened; on days 13 and 14 the two wings of the primary unfolded and latched.

Then the 18 segments and the secondary were moved off their launch locks (finished 19 January). On day 30, 24 January, Webb entered its halo orbit around L2.

each step on the day NASA reported it · the model moves faster than the real thing

What It Has Seen

The first full-colour images were released on 12 July 2022: the deep field of the galaxy cluster SMACS 0723, the “Cosmic Cliffs” of the Carina Nebula, the Southern Ring Nebula, Stephan’s Quintet, and a spectrum showing water in the atmosphere of the exoplanet WASP-96 b.

The same year it made the first clear detection of carbon dioxide in an exoplanet’s atmosphere, WASP-39 b’s.

It has confirmed many very distant galaxies. The most distant confirmed by spectrum so far is MoM-z14, at redshift 14.44: we see it as it was about 280 million years after the Big Bang.

first images 2022-07-12 · CO₂ on WASP-39 b at 4.1–4.6 µm · MoM-z14 z = 14.44

Taken Apart

The telescope, the secondary’s tripod, the instrument module, the five layers, the pallets and booms, and the spacecraft bus move apart, each its own way. Rest the pointer on one and it is lifted out on its own and named.