Liquid-mirror telescope
A rotating liquid mirror forms a parabolic reflector for low-cost telescopes.
A liquid-mirror telescope is a reflecting telescope whose primary mirror is made from a reflective liquid, typically mercury, rotated at constant speed around a vertical axis. The rotation causes the liquid's surface to assume a paraboloidal shape, which can serve as the telescope's primary mirror. Liquid mirrors are notable as a low-cost alternative to conventional large telescopes, as they avoid the expensive casting, grinding, and polishing required for solid glass mirrors.
- Common liquid
- mercury
- Other liquids
- low-melting point alloys of gallium
- Largest ever built
- 6 metres (20 ft) diameter (Large Zenith Telescope, Canada)
- Rotation rate of largest
- about 8.5 revolutions per minute
- Cost of largest
- $1 million
- Decommissioned
- 2016
- Planned larger project
- 8-metre (26 ft) ALPACA; LAMA with 66 individual 6.15-metre (20.2 ft) telescopes
Lore & Background
Isaac Newton noted that the free surface of a rotating liquid forms a circular paraboloid and could be used as a telescope, but he could not build one because he had no way to stabilize the rotation speed. The concept was further developed by Ernesto Capocci of the Naples Observatory in 1850, but it was not until 1872 that Henry Skey of Dunedin, New Zealand, constructed the first working laboratory liquid-mirror telescope.
A liquid-metal mirror can only be used in zenith telescopes, i.e., that look straight up, so it is not suitable for investigations where the telescope must remain pointing at the same location of inertial space. A possible exception may exist for a liquid-mirror space telescope, where the effect of Earth's gravity is replaced by artificial gravity, perhaps by propelling it gently forward with rockets. Only a telescope located at the North Pole or South Pole would offer a relatively static view of the sky, although the freezing point of mercury and the remoteness of the location would need to be considered.
The mercury mirror of the Large Zenith Telescope in Canada was the largest liquid-metal mirror ever built, with a diameter of 6 metres. It rotated at about 8.5 revolutions per minute and was decommissioned in 2016. This mirror was a test, built for $1 million, but it was not suitable for astronomy because of the test site's weather. As of 2006, plans were being made to build a larger 8-metre liquid-mirror telescope ALPACA for astronomical use, and a larger project called LAMA with 66 individual 6.15-metre telescopes.
Reader's Guide
Liquid-mirror telescopes represent a significant low-cost alternative to conventional large telescopes, as the rotating liquid-metal mirror is much less expensive to manufacture than a solid glass mirror that must be cast, ground, and polished. The key limitation is that such telescopes can only observe straight up (zenith telescopes), making them unsuitable for tracking a fixed point in inertial space unless placed at the poles or in space with artificial gravity. The largest example, the 6-metre Large Zenith Telescope in Canada, was built for $1 million but was decommissioned in 2016 due to weather unsuitability. Despite this, larger projects were planned as of 2006, including an 8-metre ALPACA telescope and the LAMA project, which would combine 66 individual 6.15-metre telescopes to achieve the collecting power of a 55-metre telescope and resolving power of a 70-metre scope. The physics of the rotating liquid ensures that the surface shape is a paraboloid regardless of the container's shape, and the focal length depends only on rotation speed and gravity, not on the liquid's density.
Did You Know?
- Isaac Newton noted the principle but could not build a working telescope because he had no way to stabilize the rotation speed.
- The first working laboratory liquid-mirror telescope was constructed in 1872 by Henry Skey of Dunedin, New Zealand.
- The largest liquid-metal mirror ever built was the 6-metre mercury mirror of the Large Zenith Telescope in Canada, decommissioned in 2016.
- A liquid-mirror space telescope might use artificial gravity from gentle rocket propulsion to replace Earth's gravity.
Frequently Asked Questions
What is a liquid-mirror telescope?
It is a reflecting telescope that replaces the traditional solid glass primary mirror with a shallow pool of reflective liquid, most commonly mercury. The liquid is spun at a constant rate around a vertical axis, and centrifugal force shapes its surface into a natural paraboloid that can focus incoming starlight.
How does a liquid-mirror telescope actually work?
A motor rotates a dish of mercury steadily, and the liquid climbs the walls until its free surface settles into a precise paraboloid. That curved surface then reflects and concentrates light in exactly the same way a ground-and-polished glass mirror would, so the rest of the optical train operates normally.
What is the largest liquid-mirror telescope ever built?
The Large Zenith Telescope in Canada held the record at 6 metres (roughly 20 feet) in diameter, spinning at about 8.5 revolutions per minute. It cost approximately one million dollars to construct and was decommissioned in 2016.
Why build a liquid-mirror telescope instead of a conventional one?
The key advantage is cost: because the primary mirror is simply a pool of liquid, you eliminate the enormously expensive processes of casting, grinding, and polishing a massive solid glass blank. That makes a large-aperture reflector achievable on a fraction of the budget of a traditional telescope.
Can a liquid-mirror telescope be pointed at any part of the sky?
No. The paraboloid only exists while the liquid is rotating around a strictly vertical axis, so the mirror can collect light solely from directly overhead at the zenith. That fixed pointing is the main operational trade-off compared to a solid mirror that can be tilted to track objects across the sky.
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