State-of-the-art Facility Begins Survey of the Southern Sky
Mount Stromlo Observatory had just refitted the historic Great Melbourne Telescope as an automated sky survey instrument when the 2003 Canberra bushfires swept through the area. These devastating fires destroyed the Great Melbourne Telescope along with many other instruments. Although a research setback, this also presented an opportunity to design a state-of-the-art replacement survey telescope - SkyMapper. Growing light pollution in Canberra and improvements in communications technologies led to the decision to site SkyMapper under the pristine skies of Siding Spring Observatory near Coonabarabran.
Skymapper represents the state of the art both in terms of the 1.35m diameter f4.79 modified Cassegrain optics and the 268 megapixel detector designed and built at The Australian National University. The instrument also has the advantage of being designed as a stand alone automated survey instrument from the outset. So the design isn’t compromised by requirements to perform other kinds of astronomy.
The aspheric mirrors, and a sophisticated mirror alignment system give the telescope a wide field of view free of distortions, such as coma and spherical aberration that would normally render such a fast optical system useless. “Normally when a telescope looks at the sky, it looks at a narrow patch which is about a hundredth the size of a full moon,” Project leader Professor Brian Schmidt explains. “SkyMapper will look at a piece of sky 40 times larger than the full moon. In addition, there will be huge digital cameras behind them that are 100 times more sensitive than normal cameras.”
Data will be transmitted at a rate of 100 Megabytes a second to The ANU supercomputer facility for processing. The telescope will be fully automated, with the astronomers working from the Mount Stromlo Observatory.
SkyMapper’s main task will be to conduct the first ever systematic survey of the entire southern sky to produce a detailed digital map.
Since SkyMapper will be sensitive enough to pick up some of the most distant and faintest objects, the chart will have a deep time dimension. Because of the time it takes light waves to reach Earth, the Southern Sky Survey will enable astronomers to look back to the time soon after the Big Bang when the first stars’ nuclear fusion reactions set the primeval universe ablaze. This was the time when stars were beginning to manufacture the heavy elements from hydrogen, including iron and the element that billions of years later would form the basis of life on Earth.
As well as information on brightness, position and shape of celestial objects, SkyMapper’s survey will also record the spectral types of stars. To achieve this, a series of six glass filters each about 300mm square will be automatically inserted in sequence during each exposure. The filters isolate various wavelength bands from ultra-violet to the near infra-red. Of course design of this system is more complex than it at first sounds because each filter must have superb uniformity in order for images recorded at different times and orientations to be meaningfully compared. “This wavelength discrimination facility will enable scientists to pinpoint stars with low concentrations of heavy elements,” says ANU astronomer Stefan Keller. “Successive generations of stars return heavy elements to the interstellar gas and this is incorporated in future stars, SkyMapper will search for stars with the lowest proportion of heavy elements to find the oldest stars. These will give us insight into the way the universe was first lit up by stars and how galaxies form. This is the real niche of SkyMapper. These stars are very rare – about 100 in a billion.”
Another project will centre on objects almost on our doorstep – lumps of rock heavier than Pluto and variously catalogued as planetoids, trans-Neptunian objects and dwarf planets.
“Most observations of our solar system so far have focused on the orbital plane of the planets,” says Stefan Keller. “Skymapper will look far beyond that and has the potential to detect the biggest planetoids.”
A manageable distilled version of the Southern Sky Survey will provide a publicly available set of images of all the stars, galaxies, and nebulae in our celestial hemisphere, as well as a database containing the accurate colour, position, brightness, variability, and shape of every one of the billions of objects in the southern sky.
The SkyMapper survey will be used by astronomers across Australia and around the world to undertake a multitude of projects including:
• Uncovering the most distant objects known in the universe – the first quasars that we think formed when the universe was 3% of its current age.
• Discovering large dwarf planets like Pluto in the outer solar system.
• Obtaining a comprehensive map census of the stars in our Galaxy, providing the temperature, composition, and size of more than a billion objects.
• Providing our best map of the invisible material (known as dark matter) which makes up the majority of our galaxy using samples of very rare stars uncovered in the survey.
• Pinpointing the first stars that formed in our galaxy 13 billion years ago by their chemical composition.