World's Largest Liquid Mirror: Large Zenith Telescope

Interesting Engineering

zenith[Image Source: Large Zenith Telescope]

The largest liquid mirror telescope on earth is the Large Zenith Telescope. It's also the third largest optical telescope in North America. Instead of a glass mirror, the mirror in the Zenith Telescope is made from liquid mercury and it rotates at a rate of about 8.5 revolutions per minute. Creating telescopes with mercury is much cheaper than using glass mirror lenses that have to be ground down and shaped to form the correct curvature. Liquid mirrors cost about 1% of a conventional telescope mirror. However, because the mirror is made from a liquid, the telescope can only be pointed straight upwards, hence the name zenith, which literally means "path above the head". If it was tilted in any direction, the mercury would pour out the side. Here's one of photos produced by the Zenith Telescope:

zenith2[Image Source: The Large Zenith Telescope]

Isaac Newton knew about the possibility of creating a liquid mirror telescope long before one was actually constructed. He realized the surface of a rotating liquid creates a circular paraboloid, but he could not actually build one because he had no way to control the rotation speed. The concept of liquid mirrors was further developed by Ernesto Capocci in 1850, but it wasn't until 1872 that Henry Skey in New Zealand constructed the first working liquid mirror telescope.

The Zenith Telescope is located at the Liquid Mirror Observatory in the UBC Malcolm Knapp Research Forest in British Columbia, Canada. Its main objectives are to measure "spectral energy distributions and redshifts of over 100,000 galaxies and quasars and to detect distant supernovae." The telescope was designed to collect valuable information about the evolution of galaxies and the large-scale structure of the universe. The telescope was finished in 2003 and funded by the Natural Sciences and Engineering Research Council of Canada and UBC.

a2112_ubcliquidmirror2_g[Image Source: Astro Canada]

Here's how the Zenith Telescope works:

"The primary mirror consists of a rotating steel truss and 30 adjustable pads that support a 6cm thick dish. The dish is fabricated from seven hexagonal segments, plus six smaller pieces, glued together to form a shell 6.1 meters in diameter. These are made from high-density PVC foam covered with fiberglass. The foam cores were formed to a concave shape, with 18-meter radius, by heating to 100C in a large oven. A wall at the rim of the mirror prevents mercury from spilling. The diameter of the reflecting area is 6.00 meters." -UBC Liquid Mirror Observatory

spincast05[Image Source: The Large Zenith Telescope]

When rotating, the thin layer of liquid mercury is kept in the shape of a parabola. That’s the shape needed for the liquid mirror to be able to collect and focus incoming light. Because of the low cost of liquid mirror telescopes, many astronomers are looking to put them on the moon's surface.

Article written by Leah Stephens. She is a writer, artist, and experimenter. She ghostwrites for CEOs and recently self-published her first book, Un-Crap Your Life. Fun fact: she’s been cutting her own hair since she was a teenager and calculates she’s saved over $3,500 in haircutting expenses so far. You can follow her on Twitter or Medium.

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