Gran Telescopio Canarias

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

Largest single apertureLa PalmaOperational

01 — Interpretation

What these numbers mean

Aperture10.4 m

Fifty-one times the diameter of an 8-inch Dobsonian, and about 2,600 times its light-gathering area. The word doing the work here is single: the ELT will be far larger, but it is not finished, and the VLT's four mirrors are four mirrors. Today no other working telescope brings this much glass to one focus.

02 — Specifications

The instrument

Aperture: 10400
Optical design: Segmented Ritchey-Chrétien (36 hexagonal segments)
Mount: Altazimuth

03 — Mission

The observatory

First light: July 2007
Status: Operational
Operator: Instituto de Astrofísica de Canarias (Spain), with Mexico and the University of Florida
Site: Roque de los Muchachos Observatory, La Palma, Canary Islands
Altitude: 2267
Instruments: OSIRIS, EMIR, CanariCam, HiPERCAM and others

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.

Gran Telescopio Canarias10400 mm

Sky-Watcher Flextube 350P SynScan GoTo Collapsible Dobsonian355 mm · 29× smaller

Sky-Watcher Flextube 300P SynScan GoTo Collapsible Dobsonian305 mm · 34× smaller

Celestron StarSense Explorer 12" Dobsonian305 mm · 34× smaller

05 — Context

What "largest single aperture" is actually claiming

Superlatives about telescopes need reading carefully, because there are several ways to be the biggest. The VLT has four 8.2-metre mirrors and can combine them; the Large Binocular Telescope has two 8.4-metre mirrors on one mount. Neither brings all that glass to a single focus in the ordinary way.

The GTC does. Its 10.4-metre primary is 36 hexagonal segments held to work as one continuous surface — the Keck approach, slightly larger — and everything it collects arrives at one place. That is the sense in which it is currently the largest optical telescope in the world.

The qualifier matters because it will expire. When the ELT sees light, this record ends by a wide margin. Until then, this is the biggest single eye anyone has built.

06 — Context

Why a Spanish island in the Atlantic

The Roque de los Muchachos sits at 2,267 metres on La Palma, and the reason a great many telescopes are gathered there is a quirk of local weather rather than politics.

A persistent temperature inversion traps a sea of cloud below the summit for much of the year. Observatories above that layer sit in dry, steady air, with the cloud acting as a lid on the humidity and the light of the towns beneath it. The islands' skies are also protected by law — Spain's 1988 Sky Law restricts lighting, air traffic and radio interference around the site.

It is worth noticing how much of a modern telescope's performance is decided before any glass is cast. Site selection is an optical component.

Sources

Where these numbers come from

Elsewhere

More great telescopes

Vera C. Rubin Observatory8360 mm

An 8.4-metre survey telescope that photographs the entire southern sky every few nights, for ten years, to find what changed.

W. M. Keck Observatory10000 mm

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.

Very Large Telescope8200 mm

Four 8.2-metre telescopes on a Chilean ridge that can observe separately, or combine their light into one instrument no single mirror could be.

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.

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 →