Positivism to photonics
What would a bunch of astronomy tourists be doing in Paris in June? Pretty much the same as anyone else, for the most part—seeing the sights and soaking up the atmosphere. As they were Stargazer II tourists, of course, there had to be a bit of study thrown in, but our genial host, Dominique Proust, made light work of that. A memorable Saturday-morning visit to the Paris Observatory, in leafy boulevard Arago, in the 14th arrondissement, gave us an insider’s view of the evolution of Parisian astronomy since the observatory’s foundation by Louis XIV in 1667. The magnificent building, the first national scientific institution of its kind, was truly the epitome of the Enlightenment.
Then, when we took the half-hour coach drive to Meudon later in the day, we were able to visit the observatory’s biggest telescope—and the largest lens telescope in Europe. Sadly, the 83-centimetre-diameter Grande Lunette (Great Telescope) of 1889 was not accessible to visitors, due to weather damage sustained during a bad storm in December 1999. Though restoration had started, the telescope was a pitiful sight in its protective plastic sheeting when we glimpsed it through an access doorway. But its situation overlooking Paris is magnificent, and the telescope’s building is itself a remarkable piece of recycling. It is built on the foundations of the eighteenth-century Château Neuf du Domaine de Meudon, which burned down in 1871.
Paris is also notable as the home of Louis-Jacques-Mandé Daguerre, pioneer of photography in the 1830s. Though it needed several decades of further development, photography revolutionised astronomy in the 1880s with its ability to detect and permanently store the images of celestial objects seen faintly through the telescope. My co-host on the trip was the great modernday astrophotographer David Malin, and we relished his unique insight in a public lecture at the observatory.
So, our visit to Paris took in all of that and more, including one of those extraordinary coincidences that happen from time to time, when we ran into a good friend of mine from Oxford—a former colleague—on the second level of the Eiffel Tower. And the tour was capped off on its final evening with dinner on the first level, with another distinguished special guest, Ian Corbett, general secretary of the IAU. (Remember the IAU? Yes, the organisation that famously demoted Pluto is based in Paris.)
CELESTIAL SIGNATURES
But now let me tell you about someone we didn’t celebrate in Paris. Not intentionally, in fact—I would certainly have mentioned him if I’d remembered to. Auguste Comte was a great French philosopher who lived in the city on and off during the first half of the nineteenth century. He is famous as the founder of the doctrine known as ‘positivism’, in which reason and scientific testing are proposed as the only sensible routes towards understanding both physical and human processes. Actually, positivism encompasses much more than that, but its basic principles are what drive all scientific enquiry. In that regard, despite his occasionally eccentric views, Comte is regarded as one of the defining figures of the philosophy of science in the nineteenth century. One of science’s heroes, in fact.
There is one aspect of his work, however, that invariably brings a little smirk of satisfaction to the faces of astronomers. In one of the volumes of his Course in Positivist Philosophy, published in 1835, he makes some rash statements about the stars. For example, he insists that
we shall never be able by any means to study their chemical composition . . . We shall not at all be able to determine their chemical composition or even their density . . . I regard any notion concerning the true mean temperature of the various stars as forever denied to us.
And so on. You have to have some sympathy for him, I suppose. Comte imagined that in order to work out what the stars were made of or how hot and dense they were you’d have to have physical samples from them. He believed there was no way that simply looking through telescopes at stars could ever reveal their true nature. But he was wrong.
Two years after Comte’s death in 1857, the German physicist Gustav Kirchhoff made a profound discovery. He demonstrated that every chemical element has a characteristic signature in the light it emits when burned in a flame or excited with an electric current. The hidden signature is unlocked when the light is viewed through a prism, revealing a ‘barcode’ of coloured lines in the rainbow spectrum that uniquely identifies the particular element.
After a further two years, in 1861, Kirchhoff succeeded in applying that knowledge on a grand scale. Working with his colleague Robert Bunsen (of burner fame) at the University of Heidelberg, he identified the chemical elements present in the Sun’s atmosphere by means of corresponding lines in the rainbow spectrum of sunlight. Dark lines imprinted on the Sun’s spectrum are simply the bright lines of glowing chemical elements reversed in intensity, almost like a photographic negative. This astonishing feat was proof that you don’t need to have a physical sample of something to know unequivocally what it’s made of. You don’t even have to be anywhere near it. It was a truly remarkable discovery.
The next step was even more remarkable, however. Over in Victorian London, a 37-year-old astronomer of independent means was inspired by the work of Kirchhoff and Bunsen to wonder if the same trick could be applied to the light of other celestial objects. William Huggins had a sizeable telescope by the standards of the day—a refracting telescope 20 centimetres in diameter—and he had time on his hands. With the help of his friend William Miller, a professor of chemistry at King’s College, Huggins fitted this instrument with a spectroscope—a device for examining the rainbow spectrum of light—and together they began exploring the heavens in a new way. While the Moon and planets showed essentially the same spectroscopic barcode as the Sun (not surprisingly, since they all shine with reflected sunlight), the stars took the scientists’ breath away. The stars, they found, also had spectra that showed recognisable patterns. By 1864, Huggins and Miller had carefully studied 50 stars, unmistakeably identifying the barcodes they had found in their spectra as those of chemical elements found on Earth. As Huggins later wrote,
one important object of this original spectroscopic investigation of the light of the stars and other celestial bodies, namely to discover whether the same chemical elements of those of our earth are present throughout the Universe, was most satisfactorily settled in the affirmative; a common chemistry, it was shown, exists throughout the universe.
That was an extraordinary discovery not just for scientists but for philosophers, too, blowing right out of the water Auguste Comte’s ideas of things we can never know. At the same time, it gave birth to the new science of astrophysics—the physics of the stars—in which astronomers attempted to understand the processes that caused spectroscopic differences between one star and another. Today, we know that most of those differences come about because of variations in the size and temperature of the stars, but this was completely unknown territory in the 1860s.
Another property of a star is imprinted on its spectroscopic barcode: its radial velocity, or its speed towards or away from the observer. It comes about for the same reason that the siren of an emergency vehicle seems to change pitch as it goes past, a phenomenon called the Doppler effect. Light behaves rather like sound in that its ‘pitch’ becomes different when its source is moving. If for ‘pitch’ you read ‘colour’, you can perhaps understand how the position of the tell-tale barcode in the spectrum of a star changes slightly with its velocity, giving astronomers an unexpected celestial speedometer. Huggins made the first attempts at such delicate measurements in 1868, but it was not until twenty years later that the first reliable velocities were measured photographically, by Hermann Karl Vogel at the Potsdam Observatory.
ABSOLUTELY NEBULOUS
In his wide-ranging explorations of the spectroscopic sky, William Huggins solved one other problem that had long dogged astronomers. It concerned the true nature of nebulae—the mysterious fuzzy patches in the sky that were neither stars nor planets. As we saw in the preceding chapter, William Herschel had long wondered about these, and many astronomers believed that they were gigantic aggregations of stars that were just too far away to be separated into individual points of light. Indeed, that’s what many of them were eventually discovered to be. But not all.
In 1864, Huggins turned his spectroscope onto one of these nebulae and was astonished to see the unmistakeable barcode of a glowing gas: a series of narrow, coloured lines spaced out from one another and quite different from the continuous rainbow ribbon of light crossed by dark lines that is characteristic of a star like the Sun. As he later wrote, ‘the riddle of the nebulae was solved. The answer, which had come to us in the light itself, read: Not an aggregation of stars, but a luminous gas.’ This triumph of scientific endeavour put astrophysics—and Huggins—firmly on the map. He must have imagined there was nothing his marvellous technique couldn’t do. Indeed, his career flourished, and eventually, in 1897, he was awarded a knighthood.
There was just one niggling problem that took a little of the shine off Huggins’ discovery. Embarrassingly, most of the bright, coloured lines, or emission lines, in the spectra of nebulae could not be identified: they didn’t seem to correspond with any of the chemical elements found on Earth. This was in stark contrast to the dark lines that Huggins and Miller had found in the spectra of stars. True, there were lines in the blue and violet part of the spectra of nebulae that seemed to coincide with lines known to be emitted by glowing hydrogen, but others didn’t fit any known pattern. In particular, the brightest and most prominent lines, which were in the green part of the spectra, completely defied identification. This mystifying lack of correspondence was not something that could be put down to a velocity shift caused by the Doppler effect, for example; the barcodes of the nebulae simply had no terrestrial counterparts.
It was several years before further progress was made. But what happened next was perfectly logical and followed a similar conundrum that had emerged in August 1868 during a total eclipse of the Sun. On that occasion, a number of well-known scientists, including a French astronomer, Georges-Antoine-Pons Rayet, had made spectroscopic observations of solar prominences—huge clouds of glowing gas billowing from the Sun’s surface. Rayet had found no fewer than nine bright emission lines, among which was one he took to be something called ‘sodium D’, the well-known orange-yellow emission line that gives today’s sodium street lamps their characteristic colour. But further investigation by a London-based scientist, Norman (later Sir Norman) Lockyer, and his colleague, Edward (later Sir Edward) Frankland, soon revealed that this was in fact a different spectrum line—and one that did not correspond to any known substance. They therefore assumed that it originated in a chemical element unknown on Earth, which they eagerly christened ‘helium’, a beautiful name derived from the Sun’s personification in Greek mythology, Helios. This rash act of faith in the power of spectroscopy was vindicated in 1895, when helium was extracted from the mineral cleveite by an enterprising Scottish chemist called William (later, of course, Sir William) Ramsay, who boiled it in weak sulphuric acid. It is easy to imagine the glee with which Lockyer and Frankland must have greeted the news of this feat of latter-day alchemy. For the first time, an element had been identified in a celestial object before being discovered on Earth.
Astronomers who were worried about the failure to identify the spectrum lines in nebulae took heart from the work of Lockyer and Frankland. Following their lead, they declared that the mysterious green lines in their nebulae spectra must come from an undiscovered element, which they called ‘nebulium’. (Another nice name, you have to admit, albeit a little less catchy than ‘helium’.) In the gung-ho climate of the day, it was the only sensible thing to do and was heralded as a further triumph of astrophysics. Problem solved.
However, as time went by, more and more experiments failed to reveal any trace of these nebulium lines in laboratory spectra, and gloom gradually settled once more over the astronomers’ camp. Moreover, a new tool for chemical investigation came into vogue, in the shape of the periodic table of the elements, and this didn’t seem to have any gaps among the lighter elements where you might expect nebulium to appear. Could nebulium perhaps be something that was extraordinarily rare in the Universe? Absolutely not—the stuff was everywhere. The brilliance of the green lines testified to its abundance in the gaseous nebulae. As the twentieth century dawned, flourished and was all but obliterated in the carnage of the First World War, the mystery of nebulium deepened into a constant irritation in the minds of the world’s physicists.
It was not until 1927 that the first glimmer of light began to shine on the problem—if you’ll pardon the pun. In a book that was still a standard text when I was a lad (albeit, by then, in a much later edition), a gifted US astronomer threw out an illuminating suggestion. Henry Norris Russell was one of the leading lights of the Princeton University Observatory when he collaborated with his colleagues Raymond Smith Dugan and John Quincy Stewart to write a textbook entitled simply Astronomy. (A good title—nothing beats telling it like it is.)
In the book, Russell speculated sagely on the origin of the nebulium lines: ‘It is now practically certain that they must be due not to atoms of unknown kinds but to atoms of known kinds shining under unfamiliar conditions.’ He went further, suggesting that those unfamiliar conditions might be what you would find in a gas of very low density—almost akin to a vacuum. Russell knew that the coloured emission lines characteristic of an element had their origin in atoms of that particular element changing their energy levels, from more to less excited states. The excitement might come from electrical currents in the laboratory or radiation from stars in space. He then postulated that the reason for the emission of the unidentified lines could be that it took ‘a relatively long time (as atomic events go) for an atom to get into the right state to emit them’—a state that, under normal laboratory conditions, would be rudely interrupted by a collision with a neighbouring atom. In the incredibly low pressure prevailing in a gaseous nebula, atomic collisions would be rare, and who knew what might happen to the states of the relatively undisturbed atoms?
The person who answered that question in characteristically brilliant and lucid fashion was another American, Ira Sprague Bowen. Working at the California Institute of Technology, Bowen had been calculating the possible energy states that could be taken on by various atoms and the colours of light that would be emitted when the atoms jumped from one energy state to another. Such transitions are, in fact, the origin of all coloured emission lines observed in the spectra of glowing gases, whether in a flame in the laboratory or in a nebula in the depths of space. Physicists had learned that the energy jumps were governed by certain selection rules and that some were permitted while others were, well, forbidden. That rather stern description was, in fact, a bit misleading, as the ‘forbidden’ energy jumps are not really forbidden. More accurately, they are highly improbable under laboratory conditions, because atoms are packed relatively closely together and bump each other into new energy states long before the forbidden radiation can be emitted. That is very different from the almost-perfect vacuum of space—even in the centre of a glowing nebula. Bowen had been pondering these various transitions while also reading Russell, Dugan and Stewart’s Astronomy. Suddenly, the penny dropped. As he recounted in 1968,
one night, I went down to work and came home about nine o’clock . . . and started to undress. As I got about half undressed, I got to thinking about what happens if atoms get into one of those states. Are they stuck there forever? Then it occurred to me, having read this [Russell’s comments on nebulium], maybe they can jump if undisturbed in a nebula, but we can’t see them here [in the laboratory] because of collisions . . . Well, I quickly put a reverse on my dressing, and went down to the lab again. Since I had these levels it was very easy to take these differences and check them up . . . It was a matter of minutes to establish it . . . I worked until midnight and I knew I had the answer when I went home that night.
The answer was that the so-called ‘nebulium lines’ were caused by a forbidden transition between different energy states of oxygen atoms. They were ‘forbidden lines’. Bowen’s familiarity with the energy states meant that he could quickly narrow down the possibilities and then calculate accurately the colours of light that would be emitted by the forbidden transitions. They agreed exactly with where the nebulium lines appeared. The puzzle that had dogged astrophysicists for more than 60 years was solved—and the answer lay in those shady-sounding forbidden lines: not stanzas of off-colour poetry, but ordinary atoms behaving unusually under extraordinary conditions.
In the flurry of inspired research that followed, Bowen was also able to match other previously unidentified nebula lines with terrestrial elements and, in the process, solve most of the outstanding problems of nebular astronomy. Nebulium itself was consigned to the history books—it simply did not exist. Bowen quickly dashed off a note for the Publications of the Astronomical Society of the Pacific and then, in 1928, presented his results in a seminal paper in the Astrophysical Journal. It makes pretty exciting reading, even today.
Ira Bowen was in his late twenties when he unravelled the mystery of nebulium, and he went on to a most distinguished career. For eighteen years (1946–64), he was director of the Mount Wilson and Palomar Observatories in California, overseeing the completion of both the 5-metre Hale Telescope—for 28 years the largest in the world—and its smaller sibling on Palomar Mountain, the 1.2-metre Palomar Schmidt Telescope, which is now called the Oschin Schmidt Telescope. He died, in 1973, at the age of 75.
The saga of nebulium is one of the great detective stories of astronomy. The stuff even found its way into the popular culture of the day as an epithet for mysterious things—although there can’t be too many people out there these days who go around thinking the Universe is full of nebulium. On the other hand, there do seem to be plenty of folk worrying about alien invasions, Moonlanding conspiracies and faces on Mars. Science has ever to be on its guard to keep the facts high in the public consciousness.
One of the astronomer’s most effective weapons in pushing back the frontiers of ignorance is the same one that William Huggins wielded to such effect in the nineteenth century—the spectroscope, and the photographic version he later developed, the spectrograph. Things have moved on a bit, though. Today’s spectrographs have a level of sophistication that stretches technology to its very limits. They are so frugal with the faint whispers of light from distant objects that almost none is wasted. They no longer use Huggins-style photography to record the spectra but have special TV cameras super-refrigerated with liquid nitrogen that are almost as efficient as the laws of physics will allow. And, as we shall see, some spectrographs have been designed in such a way that they are capable of observing not just one target at a time but hundreds. In that way, the efficiency of data collection has been improved beyond all recognition in recent years. For its part, astrophysics is still intoxicated with the wealth of information that comes from spectroscopy (as the technique is still known, despite the advent of modern spectrographs). Even the most basic observations demonstrate that the Universe is expanding and that unseen planets orbit other stars. And spectroscopy allows us to probe the limits of our understanding—for example, in the dark matter and dark energy of the Universe, two more examples of mysterious stuff from the Cosmos that have infiltrated popular culture. We will hear more about them, too, in due course.
So there you have it. As a tool for unravelling the innermost secrets of nebulae or as a harvester of hard facts from the sky, you can’t beat a good spectrograph backed up by a sound theory. Spectrographs are—in the world of astronomy, at least—absolutely fabulous.
CATHEDRAL OF REASON
Of all the places in the world where I have shown folk around, there’s one that easily outstrips the others in the number of tours I’ve led. You probably won’t be surprised to hear that it’s the place where I work. In many respects, it’s the southern-hemisphere equivalent of the facility I just mentioned on Palomar Mountain—the one where Ira Bowen played a starring role. Please don’t be deluded into thinking there’s any comparison between him and me, though. If it had been up to me, we’d still be on the lookout for nebulium.
But one thing I can do is take another tour around this facility now, so you can come along too. The journey takes us a few hundred kilometres inland from Australia’s eastern seaboard—yes, via the vineyards, if you like—to where one of the continent’s most remarkable natural regions lies. It rises abruptly out of the plains of north-western New South Wales like an ancient sentinel watching over a timeless landscape. Thirteen million years ago, this was a vigorously active shield volcano. Vast quantities of ash and lava were spewed from beneath the sandstone beds of the plains to produce a broad, 1000-metre-high cone that dominated the landscape for 100 kilometres around. Gradually, though, the hot spot in the Earth’s mantle (the soft rock underlying the crust) that had given it life was left behind by the steady northward drift of the Indo-Australian continental plate. As the volcano slumbered towards extinction, more moderate forces came into play. Slowly but insistently, sunshine, rain, wind and frost began laying bare its inner workings.
Today, the effect of that dissection is clear. The volcano’s foundations are exposed in a maze of domes, ridges and towering spires, while between them are deep valleys clothed in eucalypts and acacias, and populated by kangaroos, koalas and emus. Brightly coloured parrots flit through the trees, while eagles soar on thermals over the mountain slopes. From the furnaces of hell has come a tranquil and beautiful place.
In 1818 the first European explorers of inland New South Wales set eyes on this ‘most stupendous range of mountains, lifting their blue heads above the horizon’. The words of their discoverer, John Oxley, were spiced with awe, and no doubt it was a similar awe that led him to honour an officer in His Majesty’s Treasury by naming them Arbuthnot’s Range. Fortunately, this ridiculously inappropriate name didn’t last. The range had been home to the Gamilaraay people for thousands of years, and their Aboriginal name soon reasserted itself. It is startlingly apt: Warrumbungle simply means ‘crooked mountains’.
In the midst of all this natural grandeur, on the summit of one of the high ridges, is a unique place where primeval wilderness and modern technology collide head-on. The mountain top is called Siding Spring—though there’s not much evidence of the spring until summer storms bring waterfalls cascading into life. It’s home not just to kangaroos and emus but also to a dozen or so futuristic white buildings in a weird and wonderful array of shapes. One of them looks for all the world like an oversized Portaloo, while another resembles a gigantic silver Rubik’s Cube, but they all have an air of clinical functionality that seems slightly incongruous in the rugged landscape.
They are here to escape the pollution of cities—the dust, the smog and the blinding light of a sky illuminated by a million street lamps. And when darkness falls a subtle transformation takes place. Great doors slide open, and the telescopes inside peer silently into the velvet depths of the sky. Their task is simple—to satisfy a human race curious about its place in the grand scheme of things. Siding Spring Observatory is a place of exploration, not of the wilderness around it but of the greater wilderness above.
The largest of the buildings is a truly gigantic structure, much bigger than the others. It can easily be seen 60 or 70 kilometres from the mountains, particularly if the sun picks out its dazzling white shape from the blue eucalypt haze. A perfect cylinder, 37 metres in diameter and 26 high, surmounted by a dome of slightly more than half a sphere. Within the cylinder are eight floors of offices, labs and workshops. On top, enclosed by the immense void of the dome, is Australia’s largest optical instrument, the Anglo-Australian Telescope (AAT). The structure wouldn’t look out of place in the CBD of a major city—apart from the complete absence of windows and that shiny, bald dome. You may not be surprised to hear that I’ve been working in this building for so many years that I’ve started to look like it . . .
Though not a giant by modern standards, the AAT is still one of the most powerful of its kind in the world. It’s used by astronomers from all over the planet to look into the vastness of the Universe and study events in the remote past. As is well known, in this business, distance and time are equivalent by virtue of the finite speed of light. Though that is more than 1 billion kilometres per hour, it’s not fast enough to allow the instantaneous transfer of information over distances much greater than the size of the Earth. Moonlight, for example, takes 1.3 seconds to arrive, while sunlight has to bowl along for 8 minutes to get here. But starlight takes years, and that’s only scratching the surface of the Cosmos. The deeper into space we look, the further back in time we are seeing. The most distant objects observed with the AAT are seen as they were billions of years ago. By comparison, the age of the mountain top is a mere heartbeat.
A place so generously endowed with natural beauty as the Warrumbungle Mountains attracts a steady stream of holiday-makers. They come to experience the flora and fauna, the spectacular scenery, the peace and quiet. If they camp in the Warrumbungle National Park, they might also experience—sometimes for the first time—a truly dark night sky. No visitor to the area can fail to notice the domes on the mountain top, particularly the insistent presence of the AAT. To a few it’s an eyesore, but to most it’s a monument to scientific endeavour—a great cathedral of the Age of Reason. Nearly all of them, however, make the drive to Siding Spring to have a look.
The first thing they learn, as they wind their way up the steep access road, is that the AAT dome is the focus of the World’s Largest Virtual Solar System Drive, which I mentioned in Chapter 2. On the mountain road, visitors pass scale models of Earth, Venus and Mercury, surprising in their smallness compared with the bulk of the dome representing the Sun. Once they enter the building, they find themselves in a lift taking them to the observing floor of the AAT. It’s one of only three lifts in the whole district. The other two are a few metres away in the same building. The visitors are finally ushered into a long room, one wall of which is windowed from end to end and faces into the dome.
‘Jeez! That is bloody big!’ The reaction is always the same. Utter astonishment. Even though the AAT no longer ranks among the largest optical (visible-light) telescopes in the world, as it did when it was built, it is still a most imposing instrument. But it’s not just the telescope that elicits surprise from onlookers. Because it was built at a time when science funding was more generous than it is today, the AAT still boasts one of the biggest domes of any telescope in the world, and from the inside it looks huge.
Astronomers from the Mount Stromlo Observatory of the Australian National University, in Canberra, first looked at the Warrumbungle Mountains as a possible location for a major observatory. Their quest was for a new site free from the growing light pollution of their home base. In 1964, the first telescope was built on Siding Spring Mountain—a 1-metre reflecting telescope made famous by a distinguished husband-and-wife team, Bart and Priscilla Bok, who used it to carry out pioneering studies of the Milky Way. It was followed by two smaller Australian National University telescopes and, perhaps more importantly, by infrastructure such as observer accommodation, power, water and a paved road to the country town of Coonabarabran, 30 kilometres away. Like the mountains themselves, Coonabarabran has a Gamilaraay name—and it is no less apt. It means ‘inquisitive person’.
When the British and Australian governments looked jointly at possible sites for a new 4-metre-class telescope in the south during the late 1960s, the existing infrastructure at Siding Spring was certainly a consideration. But of prime importance was the quality of the atmosphere there, and tests revealed that it was, indeed, a suitable location for a new, large telescope. With spectroscopic conditions—clear apart from occasional thin cloud—prevailing for 65 per cent of nights and completely clear skies for up to 50 per cent, together with reasonable conditions of atmospheric turbulence (which dictates image sharpness), Siding Spring was considered to be the best place in Australia for an optical observatory. More recent site testing elsewhere in Australia has demonstrated that this is indeed true; but on a world scale the continent lacks the geographical features necessary to produce the consistent high transparency and exquisite imaging of the world’s best high-altitude observatories. Thus, the AAT is likely to remain the largest telescope on Australian soil, while Australian astronomers and their British counterparts invest in international collaborations that locate their 8-metre-class (and larger) facilities on top of Mauna Kea in Hawaii, or in the high, arid Atacama Desert of northern Chile.
The planning and construction of the AAT as a joint British–Australian project took place throughout the late 1960s and early 1970s. The dished 16-tonne primary mirror was cast in the United States from a highly stable glass-ceramic material called Cervit by Owens-Illinois Inc., in April 1969, and delivered later that year to Newcastle-upon-Tyne in the United Kingdom. There, the firm Sir Howard Grubb, Parsons & Co. Ltd began the long process of polishing the glass blank to turn it into a finished mirror under the direction of the late David Brown, one of the country’s most gifted optical scientists—and my first boss. In March 1973 it was declared ready for final testing, and its superb optical quality was revealed. If you imagine the 3.9-metre-diameter mirror expanded to be the size of New South Wales, then the biggest departure from a perfect surface you could find on it would be about the height of a pencil when laid on its side. Not bad.
Eventually, having satisfied all the technical requirements, the mirror was transported to Australia, and given a hero’s welcome in Coonabarabran on 5 December 1973. The low-loader on which it had been carried from the New South Wales port of (rather appropriately) Newcastle was made to perform not one but two laps of honour around the small country town. No one was under any illusions about the changes this precious lump of glass would bring to the community. It was also becoming obvious that, because the AAT would be operated completely under computer control (the first large telescope to do so), it was going to be a very fine instrument indeed, with a pointing accuracy much better than any comparable facility. The telescope finally entered service in June 1975, eight months after its formal opening by Prince Charles.
The AAT was originally designed with the idea that photographic plates would be the main detectors, both for taking direct pictures of the sky and for recording the rainbow spectra of celestial objects. For the picture taking (which is technically known as ‘large-format imaging’), that was indeed what transpired, and one of the unexpected contributors to the telescope’s early reputation was the profusion of remarkable astronomical images made by its photographic specialist. This was none other than David Malin, who was with us in Paris a few pages ago. When, in the late 1970s, Malin pioneered a new technique for recording celestial objects in true colour, the impact was dramatic, rocketing both the telescope and Malin himself to world fame. For spectroscopy, however, you don’t need the large area offered by photographic plates, and various experimental electronic cameras found favour because of their far greater sensitivity to faint light. By the late 1980s, these had evolved into the solid-state charge-coupled devices universally used today in both professional and amateur astronomy.
Two other circumstances conspired to increase the AAT’s potency as a discovery machine. The first was that the southern sky was essentially unexplored by large telescopes. Even such obvious targets as the centre of the Milky Way Galaxy, our nearest neighbour galaxies (the two Magellanic Clouds, visible throughout Australia) and the closest large star clusters to the Sun had been observed only at low elevations by northern-hemisphere instruments. The second was that the challenge presented by this virgin territory had been met by the British when they had decided, in 1970, to go ahead with the construction of a wide-angle photographic survey instrument, the 1.2-metre United Kingdom Schmidt Telescope (UKST). That instrument, located a few hundred metres from the AAT at Siding Spring, was formally opened on 17 August 1973 and entered service two weeks later. Its initial task was to photograph the whole of the southern sky not covered by its near twin on Palomar Mountain during the 1950s, a job that eventually took the better part of a decade.
Many reputations were made during that frenzied period when astronomers at the UKST collaborated with their counterparts at the AAT to scout out the most interesting southern objects and follow them up with the larger telescope. It was, in many respects, a southern-hemisphere re-enactment of the success of Ira Bowen’s telescopes on Mount Palomar. Eventually, in June 1988, the symbiotic relationship was formalised when the UKST became part of AAT’s parent institution, the Anglo-Australian Observatory (AAO), instead of a distant outstation of the Royal Observatory, Edinburgh.
From the beginning, astronomers and engineers at the AAO showed themselves to be adept at building novel instruments for use with the telescope. For example, observations at infrared (redder-than-red) wavelengths became possible in 1979 with the introduction of the infrared photometer-spectrometer, unflatteringly known by the acronym IRPS. At a stroke, the AAO had both opened a new window on the Universe and set a trend for daft acronyms that continues to this day. But, with IRPS, the AAT could now see through dust clouds and study the earliest stages of the formation of stars in their dusty cocoons. Today’s flagship instrument for infrared observation is IRIS2, a hybrid imager-spectrograph that was completed in 2003 and allows front-line astronomical research to continue through the Full Moon period, when the sky is too bright to observe faint objects in visible light. Investigations of subjects as diverse as the characteristics of Venus’ upper atmosphere, weather on brown dwarf stars and filaments of galaxies in clusters—as well as survey-type work—are IRIS2’s stock-in-trade.
Another new technique, while not invented at the AAO, was certainly perfected there—and has since swept through astronomy as nothing less than a revolution. It came about because of the development, in the early 1970s, of optical fibres for telecommunications—strands of glassy material a fraction of a millimetre in diameter with the extraordinary property that if you beam light in at one end it comes out almost undiminished at the other. A few years later, a handful of excited astronomers realised that these flexible, highly efficient light guides could have a remarkable application in astronomy. The idea was that you could use them to play God—in a manner of speaking—by rearranging the random distribution of stars and galaxies in the field of view of your telescope into a much neater, tidier format. To be more precise, you could allocate one fibre to each target object and then use the flexibility of the fibres to bring them all into a straight line at the other end, with each fibre emitting the light of its own target. The straight-line configuration was exactly what was needed for feeding light into a spectrograph. Thus, you could take a bundle of fibres, line up one on each target and then obtain the rainbow spectra of all your targets simultaneously. Before that, the only way to get the spectra of many objects had been to do them one at a time. The potential benefits in the rate at which data could be gathered were truly enormous, and this promised to transform our understanding of the Universe. It was the dawn of a new era in astronomy—the age of multi-object spectroscopy.
In the early 1980s a nucleus of that excited bunch of astronomers was at the AAO, with a couple more at the UKST (still, at that time, a Scottish outstation). One of those was a much-missed former astronomer-in-charge at the Schmidt Telescope, the late John Dawe, and the other was me. We kicked off fibre development there, and I took it forwards as my research project.
Working closely with the AAT’s fibre optics specialist Peter Gray, I spent the 1980s developing a succession of fibre-optic instruments for the UKST. While Gray’s own creations were well engineered and highly productive systems (his ‘fibre-optically coupled aperture plate’ revolutionised spectroscopy on the AAT), mine were string and sealing wax experiments with, I’m afraid, steadily more ridiculous names. The prototype was the ‘fibre-linked array image reformatter’—FLAIR—although David Malin tended to refer to it (rather unkindly, I thought) as Watson’s Folly. It did show promise, however, so in 1988 we built an improved model called the ‘panoramic area coverage with higher efficiency’ version, which became known as PANACHE. What else? When further improvements were suggested, I thought FINESSE might be rather an appropriate name, but another colleague put me right by suggesting that it would have to mean ‘fails to interest nearly everyone save spectrograph engineers’. In the end, duly chastened, we just called it FLAIR II.
The drawback of the early fibre optics systems on both the AAT and the UKST was that they relied on each fibre being manually positioned in exact alignment with its target object. Not only was that a time-consuming process, but, in the case of Gray’s system on the AAT, it had financial consequences, with the need for expensive brass plates accurately drilled with holes into which the fibres were plugged (or ‘stuffed’, to use the technical term) before the astronomer could observe each set of targets. Some sort of robotic positioning of the fibres was clearly necessary, and, during the 1980s, in collaboration with the University of Durham, in the United Kingdom, engineers at the AAT began experimenting with intelligent machines called ‘pick-place robots’ that would pick up, move and put down tiny magnets on a large steel plate. With a fibre attached to each magnet and a rightangled prism stuck on the end of each fibre, the stage was set for a transformation in the way multi-fibre spectroscopy was carried out.
That early work culminated in what are still today’s workhorse fibre-positioning robots on the AAO’s two telescopes. The 2-degree field, or 2dF, system (named after the diameter of the area of sky it can see) was commissioned on the AAT during the mid-1990s, while the 6dF (yes, you’ve guessed it—the 6-degree field) system on the UKST first saw light in 2001. Although the constructional details of these machines could hardly be more different, they both do essentially the same job. First, they take the positions of their target objects from catalogues stored in their electronic memories. Then, they place a sequence of 0.1-millimetre-diameter optical fibres, equipped with magnets and right-angled prisms, onto a metal plate in the correct position for each target (to an accuracy of 0.01 millimetres), taking no more than a few seconds per fibre—and they do it entirely automatically. Since the fibres themselves are rather delicate, another important requirement for the robots is that they try to remember not to break them.
Watching these two machines diligently going about their business is always impressive for visitors, rivalling the size of the telescope and dome in its wow factor. But some of their inherent reliability comes from the work of one particular scientist formerly at the AAO, a ‘robot whisperer’ of great skill called Ian Lewis. We have met him already in this chapter, completely by chance—halfway up the Eiffel Tower.
MOST PRODUCTIVE IN THE WORLD
When the AAT celebrated its 25th birthday, in 1999, it was with the recognition that the new millennium would bring challenges to a telescope that was starting to look small by world standards. More than a dozen groundbased telescopes with mirrors bigger than 6.5 metres in diameter were under construction or planned, and the Hubble Space Telescope had been producing breathtaking razor-sharp colour images of the Universe for almost a decade. Moreover, even then, astronomers had their sights on a new generation of what are called ‘extremely large telescopes’, with mirrors bigger than 20 metres in diameter.
The AAO had a proven record in building innovative instrumentation and already had an external projects group to make its expertise available on a commercial basis in collaboration with other Australian or British institutions. Recognising this expertise, and grabbing the scientific niches in which a 4-metre-class telescope on a less-than-perfect observing site could flourish, the observatory’s management charted a future for the AAT that would keep it more than competitive in an era of larger telescopes. Specialisation in a small number of world-class auxiliary instruments rather than a large collection of hard-to-maintain bits of equipment was a key ingredient, together with an emphasis on survey astronomy—the gathering of census-style data on large populations of celestial objects.
The flagship instrument then newly commissioned—2dF—was unique on a 4-metre telescope. Its first task was a survey of the three-dimensional positions of galaxies to provide a detailed cross-section of the local Universe. By measuring the amount by which a galaxy’s light is shifted to the red end of the spectrum, or its redshift, the galaxy’s distance can be determined by virtue of the redshift–distance relationship discovered by Edwin Powell Hubble in 1929 (about which we’ll learn more in Chapter 10). Technically, this comes about not because of the Doppler effect we met earlier but because of the expansion of the space through which the light has travelled since it was emitted. As space expands, light waves are stretched, and the light gets redder. The project was therefore known as the 2dF Galaxy Redshift Survey. It was completed in 2002 and catalogued 220 000 galaxies, resulting in a deluge of scientific papers.
In contrast with 2dF’s ability to look deep into space in narrow pencil beams, the strength of 6dF on the UKST lies in its ability to perform surveys over the entire southern sky because of the telescope’s wide angle of view. Thus, its first major task was a survey of 136 000 galaxies, in the 6dF Galaxy Survey, completed in 2005. That, too, has produced some spectacular advances in our understanding of the local Universe.
Today, 2dF has metamorphosed into AAOmega. (I’m afraid the name is another in-joke among optical engineers, the A-Omega product being a measure of efficiency in an instrument.) Like 2dF, AAOmega utilises 400 optical fibres allocated one to each object but feeds them to a new highly efficient and stable spectrograph. Since its completion, in 2006, AAOmega has been used for surveys of distant galaxies and also for surveys of stars in our own Milky Way Galaxy. AAOmega will remain the world’s most powerful spectroscopic survey instrument for a few years to come, but, as we will see in the next chapter, a further metamorphosis is on its way.
Another string to the AAT’s bow is the University College London Echelle Spectrograph (UCLES). Like IRIS2, it is a bright-time instrument—it can be used when the Moon is lighting up the sky. AAO engineers are always at pains to point out that, despite some early teething troubles, UCLES is not pronounced ‘useless’ but rhymes with ‘chuckles’. UCLES is a survey instrument, too, but it’s used to obtain extremely detailed spectra of stars one at a time. It is perhaps most famous for its contribution to our knowledge of planets in orbit around other stars. The Anglo-Australian Planet Search program has discovered around 8 per cent of all known extrasolar planets (currently about 800) by means of the Doppler wobble technique, which looks for stars being pulled to one side or the other by the gravitational attraction of their planets. UCLES has also been used for pioneering work in asteroseismology, in which minute oscillations in the surfaces of stars reveal details of their structure and age.
As well as its nightly exploration of the Universe, the AAT is frequently used for developmental work in instrument science, with recent highlights centring on the relatively new field of astrophotonics. The idea here is that once the light from your star or galaxy is inside an optical fibre, there are other clever things you can do with it rather than simply letting it out again at the other end. This manipulation of photons—particles of light—is similar to the way electrons are manipulated in electronic circuits, hence the name ‘photonics’.
In collaboration with other Australian institutions and the Leibniz Institute for Astrophysics, the AAO has tested some truly exotic and ground-breaking photonic devices like Starbugs (miniaturised robots), fibre Bragg gratings, photonic spectrographs and laser-comb calibration cells. Don’t worry if you have no idea what these things are. I can only just get my own head around them—and, apparently, I invented one of them. These devices, or their successors, are expected once again to radically change the way in which astronomical instruments are built and to add wholly new capabilities.
All the staff of the AAO, whether they be astronomers, instrument scientists, engineers, technicians or administrators, contribute to its functioning, and it is largely due to them that the institution has maintained a high record of productivity over the years. Repeatedly, in studies of the effectiveness of astronomical facilities worldwide, the AAT has come out at or near the top, and a recent analysis of this kind has demonstrated that the strategies adopted have paid off in keeping the telescope at the cutting edge of astronomy. Published in 2008, the analysis shows that the AAT is the first-ranked 4-metre telescope in the world, in both productivity and impact, achieving 2.3 times as many citations as its nearest competitor. Moreover, among optical telescopes of any size, on the ground or in space, the AAT is ranked just fifth in productivity and impact.
This extraordinary achievement is one of the highlights in a process that has recently taken the AAO into a totally new era. In 2002, the United Kingdom became a partner in the European Southern Observatory, whose telescopes are in northern Chile, and its science agency signalled its wish to withdraw from the AAT agreement. So, on 1 July 2010, the AAO underwent the biggest change in its long history, when it metamorphosed from the Anglo-Australian Observatory into the Australian Astronomical Observatory. The AAO is now a division of the Department of Industry, Innovation, Science, Research and Tertiary Education of the Australian Commonwealth government—which was delighted to find there was no need to fund the design of a new logo.
And there was another spin-off. While the AAT itself was obliged to retain its original name, the anomaly in the observatory’s name that had irritated generations of Scottish, Welsh and Northern Irish astronomers was, at last, laid to rest. The misnomer that had for so long been a thorn in the side of those who sit in Pedants’ Corner (and, let’s face it, that means most astronomers) finally disappeared. After 36 years of high-profile misrepresentation, the Anglo in Anglo-Australian Observatory was consigned to history. So, rather than being Anglo-Australian, the observatory’s astronomers have now become ‘all-Australian’.
THE ALL-AUSTRALIAN ASTRONOMER
Despite the advent of cloud cameras and web-accessible meteorology data such as weather radar and satellite imaging, all astronomers like to get a first-hand impression of the condition of the sky. Whether or not there is cloud, which way it is moving, the smell of rain on the wind, perhaps—these are all signs that are difficult to read from a windowless control room, no matter how well equipped it is. Astronomers at the AAT are no exception, and the building’s high exterior walkway is a place that affords not just routine weather checks but some of the most magnificent views in the world.
Let’s end our tour of Australia’s premier optical astronomy facility by stepping outside for a final night-time observation. From here, the vault of the Warrumbungles’ pollution-free sky is visible in all its breathtaking glory. Even when there is no moonlight, the stars of the southern hemisphere shine brightly enough to allow the handrail and open-mesh flooring of the walkway to be plainly seen without a torch. Throughout much of the year, the Milky Way arcs from horizon to horizon in a radiant band, and it’s possible to visualise the flattened disc of our Galaxy encircling the sky completely, if we could but see it through the Earth’s dark form. Science tells us that this glowing swathe outlines the Solar System’s home in the Universe. But from the walkway of the AAT it looks for all the world like the Dreamtime river of Aboriginal legend, and it’s easy to feel an affinity with the first humans who observed the sky from this place.
To some Aboriginal people, the Milky Way’s dark dust clouds are the head, neck and body of an emu. To others, its two glowing companions represent an old man and an old woman sitting by a camp fire, which is the star we call Achernar. Nothing the unaided eye can see gives the remotest hint that these two Magellanic Clouds are whole galaxies of stars and that the light from the nearest of them has been on its way for 170 000 years. Aboriginal legend also has much to say about the stars themselves, as they dutifully mirror the rotation of the Earth in their nightly excursion around the sky. Stories of hunters, beautiful young women, sacred creatures and munificent spirits seem to complement science’s view that they are just other suns, fellow travellers in the Galaxy with our own.
For more than 10 000 years, thinking people have watched the Universe from Siding Spring Mountain and have been inspired by it. Today’s all-Australian astronomer has much to be proud of, with an ancestry like that.