Originally posted by interestedalways

I would love to know more about a liquid mirror. Sounds delighfully magical.

Ermanno F. Borra, Professor of Physics at Laval University in Quebec, has been making spinning liquid mercury mirrors for telescopes for decades. In fact, an early version of one of Barra's Babes was used to look at the Moon way back in 1908, but the primitive technology back then made it impossible to have a very smooth surface to the mirror. Even the footsteps of a horse 50 metres away put ripples on the face of the spinning liquid mercury!

But Professor Borra had the advantage of modern technology. The bearing he used to hold the bowl of mercury was cushioned with compressed air, instead of oil - so vibrations from the ground did not enter the mercury. He had the bowl spinning with a smoothly-running electric motor, via a floppy rubber belt. He then ran into a problem with the wind currents generated by the spinning mercury - they rippled the mercury. But he found that a thin film of a plastic called Mylar smoothed out the mercury, and did not distort the light. Finally, when he checked the surface of his spinning mercury mirror, he was astonished and delighted to find that it had a superb optical surface. It was smooth to about 20 billionths of a metre. That means that if you expanded the mirror to the size of the Earth, the biggest bump would be only 1 mm high!

However there's a fundamental problem with a liquid-mirror telescope - it always has to point vertically straight upwards, or else the mercury spills out over the edge. But modern technology came to the rescue - he takes a series of quick electronic snapshots of the sky as his telescope was slowly rotated by the Earth, and then electronically combines them.

The biggest liquid mirror so far is 3 metres across. It's in a telescope to search for tiny orbiting particles of space junk, down to one and a half centimetres across.