Arecibo Observatory

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.

RadioPuerto RicoCollapsed 2020

01 — Interpretation

What these numbers mean

Aperture305 m — but not the kind on the rest of this site

Fifteen hundred times the diameter of an 8-inch Dobsonian, which sounds absurd until you notice it collected radio waves rather than light. Radio wavelengths are centimetres where visible light is a fraction of a micrometre, so a radio telescope needs an enormous dish to achieve resolution an amateur refractor manages at 60 mm. The number is real; comparing it to an optical aperture is not.

02 — Specifications

The instrument

Aperture: 305000
Optical design: Fixed spherical reflector with a steerable suspended feed platform
Mount: None — the dish was fixed; the receiver moved

03 — Mission

The observatory

First light: 1963
Status: Collapsed 1 December 2020
Operator: US National Science Foundation
Site: Arecibo, Puerto Rico
Wavelength range: Radio
Instruments: Radio receivers and a planetary radar transmitter

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.

Arecibo Observatory305000 mm

Sky-Watcher Flextube 350P SynScan GoTo Collapsible Dobsonian355 mm · 859× smaller

Sky-Watcher Flextube 300P SynScan GoTo Collapsible Dobsonian305 mm · 1000× smaller

Celestron StarSense Explorer 12" Dobsonian305 mm · 1000× smaller

05 — Context

The telescope that could not point

Almost every telescope moves. Arecibo did not. Its reflector was a fixed 305-metre spherical bowl fitted into a natural sinkhole in the Puerto Rican karst, and it stayed exactly where it was for the whole of its life.

What moved was the receiver. A 900-tonne platform hung on cables from three concrete towers above the dish, and steering it across that suspended arc pointed the telescope — within about twenty degrees of overhead. Everything else, Arecibo simply waited for the Earth to bring past.

The spherical shape is the reason this works at all. A parabola has one focus and must be aimed; a sphere is the same shape seen from every direction, so a feed moved above it can select a different patch of sky without the bowl moving. The cost is spherical aberration, corrected by the shaped feed structure.

It also transmitted. Arecibo carried a planetary radar powerful enough to bounce signals off asteroids and read the echo — one of very few instruments that did not merely listen.

06 — Context

1 December 2020

In August 2020 a cable pulled out of its socket and tore a gash in the dish. In November a main cable snapped. On 1 December, at about 7:55 in the morning, the remaining cables gave way and the 900-tonne platform fell into the bowl below.

The investigation found the cause to be zinc creep in the cable sockets — the zinc that anchored the cables had slowly flowed under long-term load, letting the wires pull free. The National Academies' report noted that this kind of failure had never been reported previously in over a century of widespread zinc spelter socket use. The structure was not neglected and did not fail in a way anyone had been watching for. It failed in a way nobody had seen before.

Arecibo is on this page because a collection of great telescopes that includes only the working ones is a collection that has quietly edited out the risk. Instruments of this scale are singular objects, built once, and when they end they are not replaced. Fifty-seven years of observations exist because it was built; nothing has taken its place.

Sources

Where these numbers come from

Elsewhere

More great telescopes

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Very Large Telescope8200 mm

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Gran Telescopio Canarias10400 mm

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Hale Telescope5100 mm

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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 →