W. M. Keck Observatory

Two ten-metre telescopes whose mirrors are not mirrors at all, but thirty-six hexagons each, held in formation to a fraction of a wavelength.

SegmentedMaunakeaOperational

The twin domes of the W. M. Keck Observatory near the summit of Maunakea, Hawaiʻi. International Gemini Observatory/NOIRLab/NSF/AURA

01 — Interpretation

What these numbers mean

Aperture10 m, in 36 pieces

Forty-nine times the diameter of an 8-inch Dobsonian, and about 2,400 times its light-gathering area. But no single piece of glass in it is larger than 1.8 m — roughly a large amateur mirror. The size is an arrangement, not a casting.

Altitude4,145 m

High enough that the air holds around forty percent less mass above you than at sea level, and most of the water vapour is below. Site altitude is not a luxury for a telescope this size; it is part of the optical design.

02 — Specifications

The instrument

Aperture: 10000
Optical design: Segmented Ritchey-Chrétien (36 hexagonal segments, 1.8 m each)
Mount: Altazimuth
Mass: 245000

03 — Mission

The observatory

First light: 1990-11-24
Status: Operational
Operator: Caltech / University of California / NASA
Site: Maunakea, Hawaiʻi
Altitude: 4145
Instruments: Two 10 m telescopes, Keck I and Keck II, with laser guide star adaptive optics

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.

W. M. Keck Observatory10000 mm

Sky-Watcher Flextube 350P SynScan GoTo Collapsible Dobsonian355 mm · 28× smaller

Sky-Watcher Flextube 300P SynScan GoTo Collapsible Dobsonian305 mm · 33× smaller

Celestron StarSense Explorer 12" Dobsonian305 mm · 33× smaller

05 — Context

Why the mirror is in pieces

There is a hard limit to casting telescope mirrors. Glass sags under its own weight, and the thicker you make it to resist sagging, the heavier it gets and the more it sags. Past about eight metres the problem stops being difficult and starts being structural.

Keck's answer was to stop trying. Each 10-metre primary is 36 hexagonal segments, each 1.8 metres across — a size that was already routine to make. The segments are held in a honeycomb and actively pushed and pulled, twice a second, to keep the whole surface behaving as one continuous curve.

The tolerance is the striking part. The segments must stay aligned to a few nanometres relative to each other. If a segment drifts, the telescope stops being a 10-metre mirror and becomes 36 small ones pointing in slightly different directions.

Every giant telescope built since has used this idea. The ELT's 39-metre mirror is 798 segments. Keck is the reason that sentence is possible.

06 — Context

Beating the air you are sitting under

A ground telescope has a disadvantage no space telescope shares: about a hundred kilometres of moving atmosphere between it and everything it wants to look at. Air cells of different temperature bend light continuously, which is why stars twinkle and why, for most of the twentieth century, a very large ground telescope could not resolve much more detail than a modest one.

Adaptive optics removed that ceiling. A deformable mirror measures the incoming distortion and cancels it by changing shape hundreds of times a second. Where there is no bright star nearby to measure against, a laser is fired into the upper atmosphere to make one.

The result is that a 10-metre telescope on the ground can, in the infrared, resolve finer detail than Hubble. This is the fact that most surprises people: the atmosphere is a solvable problem, and once solved, aperture wins. It is also why the largest telescopes are built on mountains rather than launched.

Sources

Where these numbers come from

Elsewhere

More great telescopes

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Extremely Large Telescope39000 mm

A 39-metre mirror being assembled on a Chilean mountaintop from 798 hexagons — larger than every other optical telescope on Earth combined.

Gran Telescopio Canarias10400 mm

The largest single optical aperture working anywhere on Earth — 10.4 metres of mirror on a volcanic ridge above the Atlantic.

Hale Telescope5100 mm

The 200-inch that held the record for forty-five years, still working on a California mountain, and still the telescope every larger one was measured against.

Arecibo Observatory305000 mm

A 305-metre dish built into a Puerto Rican sinkhole that could not point at anything — and, for fifty-seven years, heard more than anything else on Earth.

Atacama Large Millimeter/submillimeter Array

Sixty-six antennas on a plateau five kilometres above sea level, which together resolve detail as a single dish sixteen kilometres wide would.

All ground telescopes →