The Observant Astronomer

The passing scene as observed by an observant Jew, who daylights as an astronomer.

Monday, September 11, 2006

Aluminum & Mercury

Over at Treppenwitz they're being kept at night by thoughts ofaluminium and mercury. These two elements keep astronomers up at night too.

It's much easier to make a large aperture telescope with a mirror than with a lens. You only have to get the curvature right on one surface, it doesn't have to be perfectly clear, and it is a lot easier to support a big lump of material from behind, than by holding on to a thin edge. First, mirrors were made of solid metal. Speculum, a copper-tin alloy, being the favourite. Speculum corrodes and needs to be repolished every few months, so a big telescope would have two such mirrors, each to be used while the other was being cleaned up. In the 19th century, after the technology had been developed to coat glass with uniform metal films, silver-coated mirrors were favoured. The main problem with silver is that it tarnishes. Silver oxide is black, as evidenced by your silver kiddush cups and candlesticks. So periodically, before the point was reached where the reflecting mirror wasn't, the entire mirror had to be pulled out of the telescope, the silver removed and a new coating deposited.

Aluminum cured this problem, and also is more reflective to boot. The technology to make aluminum mirrors arrived in the early 1930's and almost all reflecting telescopes now use this. Aluminum oxide, in large lumps, looks like this or this with a few impurities. Clear and hard, a perfect protective coating.

Mercury, as David notes, does this in, but there are mercury telescopes. Mercury is a shiny reflective metal; too bad it's a liquid. The trick is to put a thin layer in a rotating tub and make a Liquid Mirror Telescope. The largest is 6m across, and serious telescope makers are thinking about a 100m behemoth to be deployed on the Moon. The advantage is that a liquid mirror telescope is much cheaper to construct than a conventional telescope. The disadvantage is that it can only look up. But for the purposes of a telescopic survey, that's just fine.


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