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.

InterferometerChajnantor 5,000 mOperational

ALMA antennas on the Chajnantor plateau, 5,000 metres up in the Chilean Atacama. ESO

01 — Interpretation

What these numbers mean

ApertureThere isn't one

This entry has no aperture figure, and that omission is the reason it is here. ALMA's resolution comes from the distance between its antennas — up to 16 km — not from the size of any dish. Its light-gathering power is the summed area of 66 antennas; its sharpness is a separate number entirely. Everywhere else on this site those two things travel together in one figure. Here they come apart.

Altitude5,000 m

High enough that staff work on oxygen and the site is approached from a lower base camp. Millimetre-wave astronomy is defeated by atmospheric water vapour, and there is almost nowhere on the planet drier or higher that you can also drive a 100-tonne antenna transporter to.

02 — Specifications

The instrument

Optical design: Interferometric array — 54 antennas of 12 m and 12 of 7 m
Mount: Altazimuth, on transporters

03 — Mission

The observatory

First light: March 2013
Status: Operational
Operator: ESO / NRAO / NAOJ
Site: Chajnantor plateau, Atacama Desert, Chile
Altitude: 5000
Maximum baseline: 16000
Wavelength range: Millimetre and submillimetre
Instruments: 66 antennas, movable across the plateau by custom transporters

05 — Context

Resolution without a mirror

An ordinary telescope's sharpness is set by its aperture: the wider the mirror, the finer the detail. An interferometer breaks that rule by combining signals from separated antennas and measuring how their waves interfere. The detail it resolves is governed by the separation, not by the dishes.

ALMA's 66 antennas can be spread across 16 kilometres of the Chajnantor plateau, and are physically moved between configurations by two custom transporters. Compact arrangements are sensitive to large faint structures; spread-out ones resolve fine detail. The array is reconfigured through the year, so the telescope's characteristics are something operators choose rather than something the hardware fixes.

What it does not gain is light. Sixty-six antennas collect exactly as much as their combined area, which is why an interferometer can produce an extraordinarily sharp image of something that is nonetheless bright enough to detect. Resolution and sensitivity are separate purchases.

06 — Context

Why it breaks this collection on purpose

Every other entry on these pages can be placed on a single line, ordered by how wide its mirror is. That line is a useful thing: it puts a garden telescope and the ELT into one honest comparison, and it makes the scale of the largest instruments legible.

ALMA does not fit on it, and neither does Arecibo. One is an array whose resolution is a distance rather than a diameter; the other collected radio waves where the whole relationship between aperture and detail is different. Placing them on that line would produce a number that is arithmetically fine and physically meaningless.

That is worth ending on. Aperture is the most useful single number in amateur astronomy, and it stays useful all the way up to a 39-metre mirror. But it is a convention that suits one kind of instrument, and the moment you leave visible light or a single focus, it stops describing what the telescope actually does.

Sources

Where these numbers come from

Elsewhere

More great telescopes

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W. M. Keck Observatory10000 mm

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Very Large Telescope8200 mm

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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.

All ground telescopes →