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Vacuum plant at the AAT

Tags: AAT, Peter Gillingham

It is at first surprising to find a huge vacuum plant inside a telescope building, however, it is an essential feature.

Early telescopes used lenses to gather the light from the stars, and the bigger the lens the more light it gathers. But there's a limit to the size of lens that can be made without it sagging under its own weight — about a metre. That problem was overcome by using concave mirrors, which can be supported from the back. However, the surface (not the back) of the mirror has to be reflective, and the modern way of doing this is evaporate a thin layer of aluminium onto the surface in a vacuum chamber. However, mirrors get dusty and dirty, so they removed from the telescope and recoated. This is done typically once a year, in the summer months (shorter nights) and at full moon (less valuable nights), and takes about three days.

The AAT's vacuum chamber is over 4 metres in diameter, but is not shown here. This is an enormous diffusion pump, receiving attention from Peter Gillingham before an aluminising session.

Credit: David Malin

© Australian Astronomical Observatory