Boston
ASTRONOMERS will be crossing their fingers as a team at the University of
Arizona in Tucson has a second go at making the world鈥檚 largest one-piece
telescope mirror.
In January, Roger Angel and his colleagues cast an 8.4-metre mirror for the
Large Binocular Telescope being built on Mount Graham in Arizona. But some glass
leaked out. And when Angel鈥檚 team opened the mould last week, they found that a
tenth of the mirror鈥檚 surface was too thin.
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Huge telescope mirrors are usually made from several segments. But Angel has
pioneered a 鈥渟pin-casting鈥� process to make one-piece mirrors in which the
reflecting top surface is supported by a lightweight honeycomb structure.
Rotating the furnace 6.8 times a minute shapes the top surface of the molten
glass into a parabola, which can be polished to the required curvature by
removing about 8 millimetres of glass. Over the past few years, Angel has cast
mirrors up to 6.5 metres in diameter.
The latest mirror, nearly two metres larger, posed a greater challenge, and
the Tucson team suspected that things had gone awry in January. 鈥淚 could tell
some glass was on the floor,鈥� says project scientist John Hill. The researchers
had placed two tonnes of extra glass into the mould in anticipation of such
leaks鈥攂ut this was not enough. Ten per cent of the mirror鈥檚 top surface is
now less than 20 millimetres thick, instead of the planned 36 millimetres.
To fix the problem, the researchers plan to 鈥渢op off鈥� the mirror by adding an
extra layer of glass. They will lay several hundred 5-kilogram chunks of new
glass on the mirror and reheat its top surface.
Thick as honey, the fresh molten glass should spread, filling in up to 25
millimetres on the outer rim where the top surface is too thin, but only a
millimetre or two elsewhere.
Hill says that the extra work should not delay construction of the binocular
telescope. Current plans call for the first mirror to see 鈥渇irst light鈥� in 2002.
The second mirror should follow a year or two later.