快猫短视频

YOU can tell that there鈥檚 something unusual about this telescope just from its name. Its competitors describe themselves as 鈥渧ery large鈥�, 鈥渆xtremely large鈥�, 鈥渙verwhelmingly large鈥� and 鈥済iant鈥�. This one, however, is unexpectedly modest about its dimensions: it鈥檚 just 鈥渓arge鈥�.

And it is not only the unassuming name that sets the Large Aperture Mirror Array (LAMA) apart. Most astronomical telescopes have a solid, polished mirror at their heart. But LAMA is a liquid mirror telescope that will gather light from the stars using 2000 litres of mercury. The metal will swill around in 18 shallow rotating dishes, each one 12 metres across. The rotation turns the flat surface of the mercury into a highly reflective parabolic mirror, which is exactly what an optical telescope needs.

Spinning vast platters of poisonous liquid metal might not sound like a sensible idea, but a growing group of researchers believe this could be the future of astronomy. And that鈥檚 a little odd because the idea comes from the distant past. Liquid mirrors were one of Isaac Newton鈥檚 inventions. He never saw one work because the technology to build them didn鈥檛 come along until the second half of the 19th century. But then the English astronomer Henry Skey constructed a 35-centimetre mirror from a bowl of mercury and showed that it could give clear images. Skey even managed to alter the telescope鈥檚 focal length by changing the rate at which the mercury was spun.

A few decades later, physicist Robert Wood built a 51-centimetre mirror at Johns Hopkins University in Maryland and used it to photograph stars moving overhead. But he eventually abandoned the scheme, and the idea fell into disuse. There was an obvious reason for that, which alert readers may already have thought of. If your observations rely on a spinning bowl full of mercury, there鈥檚 only one place you can look: straight up.

That sounds like a fatal flaw, something that should consign liquid telescopes to the scrap heap. But Ken Lanzetta of Stony Brook University in New York state, one of the astronomers keen to see liquid mirrors make a comeback, insists it鈥檚 not. 鈥淚n cosmology it makes no difference where you look. On average, the Universe is the same in all directions,鈥� he says.

Indeed, Lanzetta believes that a liquid mirror telescope can do everything that is needed for astronomy鈥檚 two big goals: studying cosmology and the formation of galaxies, and the search for extrasolar planets. For both of these we are going to need a new generation of bigger telescopes. Across the world, there are a few telescopes equipped with mirrors 6.5-metres across, and about a dozen with mirrors measuring 8 to 10 metres, but that is no longer enough. In astronomy, size matters: the bigger the mirror, the more you can see. As well as collecting more light, a bigger mirror gives you better resolution. 鈥淭he next step is a much larger telescope of between 20 and 100 metres in diameter,鈥� Lanzetta says.

快猫短视频s, including Lanzetta, are currently working out the details of LAMA and are aiming to build a single 12-metre liquid telescope as a first step to test the technology. Ultimately the light collected by LAMA鈥檚 18 dishes will be pooled so that it works as well as a telescope with a 50-metre mirror. That is far better than anything in existence and, crucially, this performance comes at a fraction of the cost of other telescopes being proposed, such as the European Southern Observatory鈥檚 Overwhelmingly Large Telescope (OWL), and the California Extremely Large Telescope (CELT), being developed by the University of California and Caltech. 鈥淲e estimate the cost savings to be a factor of between 10 and 20,鈥� says Paul Hickson, an astronomer at the University of British Columbia in Vancouver, who is one of the driving forces behind LAMA. CELT, for example, will be 30 metres across and will cost about $700 million. 鈥淟AMA is estimated to cost between $50 and $100 million,鈥� Hickson says.

LAMA might even be cheap enough for astronomers to build several instruments at various sites around the world. With a number available for use, one or two could be dedicated to answering important cosmological questions that have been impossible to address till now. This could, for example, finally tell us about the Universe鈥檚 rate of expansion.

Observations of distant supernovae indicate that its expansion is accelerating, which has enormous consequences for our understanding of how the Universe works, as well as its past and future (快猫短视频, 11 April 1998, p 26). But the supernova observations are not conclusive. Inferences about the expansion of the Universe rely on assumptions about the mechanism behind supernova explosions, their brightness, and the way dust in our line of sight will absorb some of their light. A number of physicists believe the conclusion that the expansion is accelerating is not to be trusted, and some say it could even be slowing down. To resolve this important issue, astronomers need long-term, direct measurements of the way astronomical objects are moving away from us. LAMA could be just the tool to provide them.

When researchers want to look at the sky, they are usually allocated telescope time in chunks of three to five nights. Now imagine you wanted a detailed set of measurements of the spectrum of light emitted by the energetic core of a distant galaxy. The 50 hours or so you would get from five nights鈥� viewing would be nothing like enough for a truly long-term view. LAMA could change that. One proposal is to use the liquid telescope to observe 100 quasars, night by night, for a decade.

As the Earth rotates, the telescope鈥檚 overhead view would trace out a strip in the sky. Over the course of each 24 hours, LAMA would have access to about half that strip, the part that is overhead at night. Over the course of a year it will have access to the whole strip. Factoring in the sky conditions, including the brightness of the Moon, this would give something like 9 hours per quasar per year, or 90 hours per quasar per decade.

Only time will tell

After this length of time, you not only get very high-quality spectra from 100 quasars, you can also see how these spectra have changed over time. The changes in their Doppler shifts will tell us whether the Universe鈥檚 expansion is speeding up or slowing down. And if a decade seems like a long time to wait for this information, don鈥檛 forget that we have wanted to know this for certain since before Einstein formulated his theory of general relativity.

The liquid mirror idea is certainly gaining support among astronomers. Take John Webb of the University of New South Wales in Sydney, who is particularly interested in the fundamentals of physics. In data collected from quasars, he has seen strong hints that some of nature鈥檚 constants, including even the speed of light, may change over time (快猫短视频, 11 May, p 28). It鈥檚 a controversial and potentially far-reaching suggestion, and to back it up Webb needs more and more telescope time. The low cost of building telescopes like LAMA could make that time available. 鈥淚t just comes down to scientific return per dollar,鈥� he says.

If LAMA gets built, it will not be the first of the current generation of liquid mirror telescopes. Researchers at the University of California, Los Angeles, already use a liquid telescope to make measurements of the composition of Earth鈥檚 atmosphere. And until very recently NASA operated a 3-metre liquid mirror telescope near Cloudcroft, New Mexico, which searched the sky for space junk for more than 5 years; Hickson has been borrowing it occasionally to carry out tests and astronomical observations.

Hickson鈥檚 enthusiasm for what he has found has led to a collaboration between scientists at the University of British Columbia, Laval University in Quebec, and the Institute of Astrophysics in Paris. They are building a 6-metre mercury telescope called the Large Zenith Telescope (LZT) 70 kilometres east of Vancouver. The LZT is the first liquid mirror telescope that will do proper star gazing, and represents the technology鈥檚 bid for astronomical credibility. It will begin its tests next month, and regular scientific observations are scheduled to start in January, when it will be used to hunt for undiscovered galaxies and supernovae, for example.

Mercury telescopes are even beginning to shrug off some of their past restrictions (see 鈥淐ome rain, come shine鈥�). Hickson has worked out that the array of mercury mirrors can be made to swing around to point at different places in the sky, without spilling a drop of mercury. In a paper published this year, he showed that the telescope鈥檚 secondary mirror could be adjusted to allow the instrument to scan a circle of sky 8 degrees across, which is equivalent to 16 times the diameter of the full Moon (Monthly Notices of the Royal Astronomical Society, vol 330, p 540). That would mean the array could follow objects for as long as half an hour as they pass overhead.

Michael Bolte, head of the CELT programme in California, is impressed by Hickson鈥檚 work so far, and agrees that liquid mirror telescopes can be very powerful astronomical tools. But these telescopes are definitely not 鈥渄o-everything鈥� facilities, Bolte says. 鈥淟AMA is an interesting idea, but it is different enough from CELT that I don鈥檛 think the two are in competition.鈥�

And Bolte says that cost comparisons with the large 鈥� sorry, extremely large and overwhelmingly large 鈥� steerable telescopes may be misleading. The liquid mirror people should be comparing their project with the cost of building a traditional, fixed-mirror telescope, he says. But even on this basis, LAMA is still a quarter of the price of an equivalent fixed-mirror telescope, Hickson says. Should people be building telescopes with fixed glass mirrors rather than steerable ones? 鈥淭hat depends on the scientific questions that they want to answer,鈥� Hickson says. 鈥淔or us, the choice is clear.鈥�