James Webb Space Telescope

A 6.5-metre segmented infrared mirror that had to be folded to fit inside a rocket, unfolded a million miles from Earth, and kept below 50 kelvin to see anything at all.

Near and mid-infraredSun–Earth L2Operational

Mission cost $9.7 billion (lifecycle cost)

01 — Interpretation

What these numbers mean

Aperture6500 mm

Thirty-two times the diameter of a common 8-inch Dobsonian, which means roughly a thousand times its light-gathering area. Aperture is the one telescope number that never stops mattering, at any scale — the same rule that makes a 10-inch beat an 8-inch in your garden is the rule that makes Webb see galaxies Hubble cannot.

Focal ratiof/20

The finished telescope delivers an f/20 beam from a 131.4-metre effective focal length. The primary mirror on its own is f/1.2 — faster than almost any amateur mirror ever sold. Two more mirrors slow that beam down before it reaches the instruments, which is the whole point of a three-mirror design: you get the compact structure of a fast primary and the well-behaved, wide, aberration-free field of a slow one.

Operating temperatureUnder 50 K

An infrared telescope that is warm sees itself. At the wavelengths Webb works in, room-temperature metal glows brightly enough to drown out the sky, so the mirrors and instruments have to be colder than the signal they are looking for. This is why the sunshield exists, and why the observatory cannot simply be parked in low Earth orbit next to Hubble.

02 — Specifications

The instrument

Aperture: 6500
Focal length: 131400
Focal ratio: f/20
Optical design: Three-mirror anastigmat (all-reflective, Korsch form)
Mount / orbit: None (Sun–Earth L2 halo orbit)
Mass: 6200

03 — Mission

The mission

Launched: 2021-12-25
Status: Operational
Operator: NASA / ESA / CSA
Orbit: Sun–Earth L2 halo, about 1.5 million km from Earth
Wavelength range: 0.6–28.5 µm (near / mid-infrared)
Instruments: NIRCam, NIRSpec, MIRI, NIRISS/FGS

04 — Scale

Against your telescope

Aperture is aperture. The same measurement that describes this instrument describes the telescope in your garage — which is the only honest way to feel the difference.

James Webb Space Telescope6500 mm

Sky-Watcher Flextube 350P SynScan GoTo Collapsible Dobsonian355 mm · 18× smaller

Sky-Watcher Flextube 300P SynScan GoTo Collapsible Dobsonian305 mm · 21× smaller

Celestron StarSense Explorer 12" Dobsonian305 mm · 21× smaller

05 — Context

Why it is gold, and why it is folded

Webb is an infrared telescope. That single fact explains most of its odd appearance. Gold is an excellent reflector in the infrared, so the eighteen beryllium mirror segments are gold-coated — a functional choice about reflectivity, not a decorative one. Beryllium is used underneath because it holds its shape as it cools to cryogenic temperatures, which ordinary glass does not.

The segmentation is a launch problem, not an optical preference. A single rigid 6.5-metre mirror will not fit inside any existing rocket fairing, and Webb flew on an Ariane 5. So the primary was built as eighteen hexagons on a structure that folds, with the outer rows hinged back against the sides for launch and swung into place afterwards. Each segment then has to be positioned to a small fraction of a wavelength of light relative to its neighbours, using actuators behind the mirror, before the array behaves as one surface. There is no optical bench, no daylight, and no second attempt.

Optically the telescope is a three-mirror anastigmat: an elliptical primary, a hyperbolic secondary, an elliptical tertiary, plus a fine steering mirror. That fourth element is not part of the imaging prescription — it exists to hold the pointing steady. The three-mirror form is what allows a fast 6.5-metre primary to feed a corrected, flat, wide field to four instruments at once, which a classical two-mirror Cassegrain of this size could not do.

The result is an effective focal length of 131.4 metres inside a structure about the size of a tennis court. That is the compression a folded, multi-mirror design buys you.

06 — Context

Why it sits a million miles away

Webb orbits the second Sun–Earth Lagrange point, roughly 1.5 million kilometres from Earth. L2 is not a place with anything at it. It is a point where the combined gravity of the Sun and Earth lets an object keep pace with Earth's year-long orbit while staying lined up on the far side from the Sun.

That alignment is the reason to go. From L2 the Sun, Earth and Moon are all in the same direction, so a single shield can block all three at once. Webb's sunshield is five layers, 21.2 by 14.2 metres, and it maintains a temperature split of hundreds of degrees across itself: the spacecraft bus runs near ambient on the hot side, while the telescope and instruments sit below 50 kelvin on the cold side. In low Earth orbit, where Hubble is, the Earth itself fills a large part of the sky and radiates heat — an impossible place to keep an infrared telescope cold passively.

The price of that location is finality. L2 is about four times further away than the Moon, and no crewed vehicle has been there. Nothing about Webb was designed to be repaired, replaced or upgraded after launch. The instruments it carries — NIRCam, NIRSpec, MIRI and NIRISS/FGS — are the instruments it will carry for its entire life.

This is the exact inverse of the bet NASA made with Hubble, and it is worth noticing that both bets paid off. Hubble was reachable, so a catastrophic mirror error became a repair job. Webb was not reachable, so every one of the several hundred deployment steps had to work the first time. They did.

Sources