Astronomy Principles and Practice Fourth Edition.pdf

Millimetre astronomy 395
Figure 23.14. The 15 m dish of the James Clerk Maxwell Telescope (JCMT) at Mauna Kea, Hawaii, without
the protective membrane which normally fits between the large slit of the observing turret. (By permission of the
Royal Observatory, Edinburgh.)
view. However, the dish aperture is normally protected from the wind and from solar radiation
by a large PTFE (polytetrafluoroethylene) membrane across the dome slit. This plastic sheet has a
transmittance of 98% at mm wavelengths but absorbs 80% of the general solar radiation and allows
observations to be made up to wind speeds of the order of 70 km h −1 . When the weather is calm,
the shield can be removed to allow the most sensitive measurements which are degraded by the small
amount of scattering that the membrane introduces.
One of the chief instruments developed at the Royal Observatory, Edinburgh, and attached
to the JMCT is SCUBA (Submillimetre Common-User Bolometer Array) which allows images to
be obtained. The array of bolometers is cooled to 0·06 K making it the world’s most powerful
submillimetre camera. It has had great success in mapping the centre of our galaxy, the galactic plane