The James Clerk Maxwell Telescope

by Douglas Pierce-Price, JAC

The James Clerk Maxwell Telescope (JCMT) is one of two telescopes operated by the Joint Astronomy Centre, the other being the United Kingdom Infrared Telescope (UKIRT).

With its 15-meter (50-foot) diameter dish the JCMT is the largest telescope of its kind in the world. It detects light with "submillimeter" wavelengths, between infrared light and radio waves on the wavelength scale. This light comes not from stars but from the coldest material in the universe, such as the clouds of gas and dust found between the stars. This "interstellar medium" is the stuff from which new stars are born, and into which stars disperse or explode as they die.

Water vapor in the Earth's atmosphere gets in the way of these faint cosmic signals, which is why the high, dry site of Mauna Kea is one of the best places on the planet for submillimeter astronomy.

The JCMT's dish, as wide as a basketball court, collects the submillimeter light and feeds it to a set of sensitive detectors. One of these is 'SCUBA', the world's most powerful submillimeter wave camera.

SCUBA takes pictures showing the faint heat glow of interstellar dust grains. These fine particles, like soot or sand, are at temperatures below -400F. SCUBA itself is kept even colder than this, to improve its sensitivity. In its jacket of liquid helium it gets down to less than a tenth of a degree above absolute zero, around -459.5F!

The JCMT also has 'heterodyne receivers' which detect light from gas molecules in space. These molecules emit characteristic submillimeter radiation patterns when they rotate. The patterns are 'fingerprints' that tell us about the temperature, density, or motion of the gas.

The nearest massive star formation to Earth is happening 1500 light years away in the constellation of Orion, the Hunter. SCUBA's maps of Orion reveal bright knots where stars are being born, strung along fainter filaments of interstellar dust.

A similar map of the center of our Galaxy shows a complex region of shells, filaments, and clouds like cotton candy in the heart of the Milky Way. This exotic region lies 27000 light years from Earth, and our view of it in visible light is obscured by the intervening dust.

Far beyond our Milky Way, SCUBA sees galaxies more than ten billion light years away. These galaxies are enshrouded in dust at the edge of the universe, and tell us about star birth more than 90% of the way back to the start of time.

Closer to home, pictures of the dust around nearby stars provide tell-tale evidence for new planets around suns other than our own.

The gases and dust grains seen by the JCMT are not just important for the life cycle of stars. They are also the origin of the chemical elements in the planets, and in our own bodies. We are all made of stardust.

Future instruments such as SCUBA-2 (the next generation SCUBA) and HARP-B (an array of heterodyne receivers) will keep the JCMT at the forefront of submillimeter astronomy, complementing the new Submillimeter Array on Mauna Kea and forthcoming telescopes such as the Atacama Large Millimeter Array in Chile.

The JCMT is internationally funded, by the UK, Canada, and the Netherlands. It was opened in April 1987.

You can find some more technical information in a previous article about the JCMT.