The SCUBA Revolution

by Prof. Ian Robson, Director of JCMT and JAC

Astronomers use two key tools in their investigations of the Cosmos: the telescope and a detector. Everyone will be familiar with the first; these are the most obvious signs of astronomical activity anywhere in the world. The clustering of telescope domes on Mauna Kea represents the world's largest investment on a ground-based site.

The first telescope was used by the Italian scientist Galileo around the year 1610. Although this was a very small and rudimentary telescope, he observed that the planet Jupiter had four 'moons' and that these moons went around Jupiter in a regular fashion. Until that time the prevailing view of mankind was that the Earth was the centre of the Universe and that everything, including the Sun and Moon, went around us. The observations of Galileo showed this was incorrect and heralded a major revolution in our understanding of the Cosmos. We now know that while the Moon goes around the Earth, we and the Moon and all other bodies in the Solar System orbit the Sun, and the Sun is just one of a hundred thousand million stars in our Galaxy, which is but one in billions of billions in the Universe.

So what about the detector? Galileo used a primitive astronomical detector, the same detector that we all possess: the eyeball! Viewing Jupiter through the telescopes at the Visitor Centre at Hale Pohaku you will easily be able to see what Galileo saw, the four 'Galilean' moons of Jupiter.

The eyeball works with visible light, but that is just part of a wide range of radiation called the 'electromagnetic spectrum'. The eye can see light waves with a wavelength between two and three hundred-thousandths of an inch. Astronomers, however, look at the Universe over all the electromagnetic spectrum, from gamma and X-rays through visible light to the longest wavelengths of the radio, and so require different detectors.

While the eyeball serves us humans well, it can be greatly improved for astronomical use. The advance of technology allowed the eyeball to be replaced by the photographic detector in the early years of the last century and from the 1970's by the Charge Coupled Detector (CCD). The CCD is now found in many guises in all sorts of products, including one of the best selling Christmas presents, digital cameras. But to work at other wavelengths requires much advanced technology and this is where SCUBA comes in.

SCUBA is the world's most advanced submillimetre camera and was installed on the James Clerk Maxwell Telescope (JCMT) on Mauna Kea in 1996. Since then it has caused its own revolution in our understanding. So what is SCUBA? SCUBA stands for Submillimetre Common User Bolometer Array, which is a bit of a mouthful. In short, SCUBA is a revolutionary camera working in the submillimetre. Which begs the next question; what is the submillimetre? This is a range of wavelengths between the radio and infrared (remembering that the infrared is those wavelengths immediately beyond the red end of the visible spectrum). The submillimetre region ranges from wavelengths of about one twenty-fifth of an inch to about one hundredth of an inch (by comparison a microwave oven uses wavelengths of around one third of an inch).

Astronomers need to use the whole spectrum of wavelengths because different bodies in the Universe shine brightest at certain wavelengths. Stars like the Sun shine brightly in the visible, while the very hottest bodies in the Universe shine in the X-ray region. The submillimetre is the domain of the very coldest bodies in space. In fact SCUBA mostly looks at dust in space that is at temperatures lower than -400 degrees F. Until the advent of SCUBA submillimetre astronomy languished far, far behind the other wavelength regimes. Why was this? Firstly, the signals from space are extremely faint, secondly, it is difficult to build highly sensitive detectors, and finally, the Earth's atmosphere is a huge barrier to observing. This is why the world's best submillimetre telescopes are located on the high summit of Mauna Kea, above much of the absorbing water vapour that blocks our submillimetre views of the Cosmos.

SCUBA made a huge breakthrough that dramatically opened up submillimetre astronomy and provided a new insight into a wide range of phenomena. Indeed, SCUBA is one of the most successful instruments ever produced for a ground-based telescope. It was so successful for four reasons: it was the first large-scale camera to be used in the submillimetre; it was extremely sensitive; it was located on the world's largest submillimetre telescope (the JCMT) on the world's best submillimetre site (Mauna Kea) and, last but not least, it was provided with excellent and user-friendly software that allowed astronomers to readily convert the electronic output into sensible pictures.

How does SCUBA manage to convert the exceedingly faint whispers of submillimetre radiation from space into a signal that the astronomer can detect? The answer is to use lots of complex technology, one of which is the cooling of the detecting element of the camera to an extremely low temperature, less than one tenth of a degree above absolute zero, or -459.5 degrees F! Maintaining this level of complex instrumentation is far from easy and it is a tribute to all the technical staff who work at the JCMT, most of whom are Hilo raised.

The talk at Hale Pohaku will focus on three main areas of research with a range of pictures of the submillimetre Universe compared to what we see in the visible region with telescopes such as the Hubble Space Telescope. The first of these is star formation. Astronomers still do not fully understand the process of how stars form but we know that the very earliest stages of the formation of a new star are cloaked in so much dust that they are totally invisible except to submillimetre astronomers. Clearly this is important and is similar to studying the earliest days of a developing baby in the womb. Therefore much work is being targeted at investigating those objects that will ultimately develop into new stars and a number of spectacular SCUBA pictures have been taken.

Second is dust disks around nearby stars. These disks are another way of telling us about planetary formation and some truly spectacular pictures have been taken that have appeared in many journals and TV programmes. Finally, there is the study of the evolution of galaxies in the Universe. It turns out that SCUBA looks at galaxies much farther back in time (closer to the Big Bang) than the Hubble Space Telescope. The results were surprising in that these very early galaxies were full of dust. This means that there must have been an intense period of violent star formation and destruction at extremely early times in the Universe because dust can only be formed from chemical elements made inside stars and ejected into space (just as for most of the chemical elements from which we are made).

Just like the other fantastic telescopes on Mauna Kea, astronomers use the best tools they can build to study our Universe, and SCUBA takes its place as one of the most successful and famous ever built. Welcome to the SCUBA revolution.