W. M. Keck Observatory

Thirty-six hexagonal mirror segments, each adjusted to a few nanometres by sensors and actuators, working as one ten-metre mirror on the summit of Maunakea. Keck showed that a mirror could be made of pieces, and that a telescope could correct the air above it.

Segments and control

Jerry Nelson of the University of California proposed in the late 1970s that a very large mirror could be assembled from many small hexagonal segments held in shape by computer-controlled actuators, instead of cast in one piece. In 1985 Howard B. Keck gave $70 million through the W. M. Keck Foundation to start Keck I. The telescope saw first light in 1990 with nine of its 36 segments installed, and began science operations in May 1993.

Each segment is 1.8 metres across, and every segment is held by sensors at its edges that measure its position relative to its neighbours twice a second; actuators correct its tilt and height to within a few nanometres. The mirror's shape is therefore maintained, not fixed.

Twins and adaptive optics

Keck II saw first light in 1996, when NASA joined the partnership with the California Association for Research in Astronomy. The twin telescopes can operate together as an interferometer. Both were fitted with adaptive optics, in which a deformable mirror, shaped hundreds of times a second, corrects the blurring of the atmosphere, using the light of a star or of an artificial star made by shining a laser into the upper air.

With adaptive optics, Andrea Ghez and her team tracked the orbits of stars around the black hole at the centre of the Milky Way, work for which Ghez shared the 2020 Nobel Prize in Physics.

On a sacred mountain

Maunakea, at 4,145 metres, has some of the best astronomical conditions on Earth and is also sacred to many Native Hawaiian people. The telescopes sit on leased land that is managed, since 2022, by an authority with Native Hawaiian representation; the relationship between science and the mountain's cultural significance continues to be debated and negotiated. The observatory runs a programme with local communities, and is working on instruments such as the Keck Planet Finder, which measures the motions of stars to search for planets.

Twins that became a model

The success of Keck's segmented mirror led directly to the next generation: the Thirty Meter Telescope, the Giant Magellan Telescope and the European Extremely Large Telescope all rely on segments or on very large mirrors controlled by actuators. Keck's engineers solved the problem of keeping segments aligned by measuring edge heights with capacitive sensors and correcting them continuously, and the design was adopted by others, including the Gran Telescopio Canarias. The telescope's technology outlived its patent protection in practice because it was published. A good technical idea in an observatory is usually shared quickly, because the community's progress depends on it.

Instruments and technologies

  • Segmented primary mirror Thirty-six hexagonal segments of Zerodur, 1.8 metres wide and 7.5 centimetres thick, forming a 10-metre mirror, each held by three actuators and edge sensors.
  • Adaptive optics A deformable mirror and a laser guide star correct atmospheric turbulence in real time.
  • Spectrographs Instruments such as HIRES split starlight into fine spectra for composition and motion.

The people

  • Jerry Nelson Astronomer and engineer who conceived the segmented mirror.
  • Andrea Ghez Astronomer who used Keck to follow stars around the Galactic Centre; Nobel Prize 2020.

Timeline

  • 1985 Howard B. Keck gives $70 million through the W. M. Keck Foundation to start Keck I.
  • 1990 Keck I first light with nine of its 36 segments.
  • 1993 Keck I begins science operations in May.
  • 1996 Keck II first light; NASA joins the partnership.
  • 2020 Andrea Ghez shares the Nobel Prize in Physics for Galactic Centre work using Keck.

Key lessons for observation today

  • Control can replace rigidity A mirror of pieces, held by sensors and actuators, outperforms a rigid block. Closed-loop control is a general design principle.
  • Correct what you cannot avoid Adaptive optics cancels the air's blur. Measure the disturbance, then correct it, as in modern timing and calibration.
  • Share the site with its people A site's cultural meaning is part of its governance. Observatories need agreements with the communities who live with them.

Sources

  • Wikipedia, W. M. Keck Observatory
  • NASA Astrobiology, Keck
  • Wikipedia, Jerry Nelson (astronomer)
  • ESO, Nobel Prize in Physics 2020 press release