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.
What these numbers mean
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.
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.
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.
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
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
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.
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.
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.
Watch
- YouTubeReal EngineeringThe Insane Engineering of James Webb TelescopeWhy the mirror folds, why it must stay under 50 K, and what the sunshield actually does.
- YouTubeSmarterEveryDayHow Does The James Webb Space Telescope Work? - Smarter Every Day 262A hands-on walkthrough with the engineers who built it, filmed before launch.
Where these numbers come from
- NASA — Webb fact sheet: 6.5 m mirror, 131.4 m focal length, ~6200 kg, 0.6–28.5 µm, 18 gold-coated beryllium segments, 5-layer 21.197 × 14.162 m sunshield, under 50 K, Ariane 5 ECA, L2 at 1.5 million km
- NASA NTRS — The Design, Verification and Performance of JWST: three-mirror anastigmat (Korsch 1972), effective focal length 131.4 m at f/20, primary is f/1.2
- GAO-21-406 — JWST life-cycle cost estimate of $9.7 billion