Embargoed until 15.30 BST 21 June 2002
Scientists from the University of Cambridge's Astrophysics Group have today announced a collaboration with teams based in New Mexico, Puerto Rico and at the Naval Research Laboratory in Washington DC to design, install and operate a novel type of astronomical telescope for ultra-high angular resolution observations of stars, galaxies and quasars.
The agreement between researchers based in the Astrophysics (AP) Group at the Cavendish Laboratory and the Magdalena Ridge Observatory Consortium (MROC) marks the first phase of a partnership between AP and MROC to design and commission an ambitious optical/infrared interferometric telescope at the Magdalena Ridge Observatory in New Mexico. Funding for this scientific study is being administered by the Office of Naval Research.
The new facility will comprise a collection of telescopes optically linked to form a single large telescope spread over an area larger than a football stadium. The combined power of the telescopes will provide images of astronomical objects with unprecedented sharpness: features 100 times smaller than the finest detail currently seen in images from the Hubble Space Telescope will be clearly visible.
Dr Chris Haniff, leading the Cambridge team, explained:
"This very high angular resolution will allow astronomers to study the formation of planets around other stars, to watch the final episodes in the lives of dying stars and see close to the hearts of active galaxies."
The telescope array, costing some $40M, will consist of 8 to 10 telescopes, each with a diameter of 1.4m, separated by distances of up to 400m. The signals from the telescopes will be combined in a central laboratory, forming what is known as an interferometric array.
In a process known as aperture synthesis, the combined signal is used to make an image which is equivalent to the image that would be formed by a space-based telescope with a 400m diameter primary mirror - this can be compared with the 2.4m diameter mirror on the Hubble Space Telescope.
The feasibility of this novel type of optical and infrared telescope was first demonstrated by Cambridge scientists in the late 1980's. This same team, then headed by Professor John Baldwin, built the world's first separated-element optical/infrared aperture synthesis telescope - the Cambridge Optical Aperture Synthesis Telescope (COAST) - in the mid 1990's.
This group of scientists have now joined forces with scientists in the USA to develop this technique for a new large-scale facility array.
The first phase of the partnership, announced today, will see both teams working towards a detailed design for the array, and the start of work on some of the critical subsystems for the telescope by researchers on both sides of the Atlantic.
In parallel, both partners will be seeking support for the eventual operation and maintenance budget of the array, which is expected to be available for routine scientific observations in late 2007.
A number of similar optical/infrared interferometer arrays are currently being commissioned elsewhere. The key scientific strength of the new array will be its ability to make true images of complex astronomical objects at a speed which is many times faster than other arrays. This is due to its large number of telescopes, each with sufficient collecting area to grasp the light from faint astronomical sources.
Images and links related to the Optical Aperture Synthesis programme at the Cavendish Laboratory are available from the Cavendish Laboratory website at http://www.mrao.cam.ac.uk/telescopes/coast.
A press release in the US from New Mexico Institute of Mining and Technology can be found at: http://www.nmt.edu/mainpage/news/homepage.html.
The design, fabrication, and current operation of COAST is funded by the UK Particle Physics and Astronomy Research Council (http://www.pparc.ac.uk).
The Optical Long Baseline Interferometry News pages provide a list of resources for students and researchers who are new to the field of stellar interferometry at: http://olbin.jpl.nasa.gov/intro.
For further information please contact:
Astronomers generally rank the angular resolution (i.e. the angular size of the smallest detail discernible) of a telescope as one of its key attributes. This scales inversely with the telescope diameter, so that a 10m diameter optical telescope (the largest size that has been built to date) is, in principle, able to distinguish features as small as one hundredth of a second of arc, roughly equal to the angle subtended by a one penny coin at the altitude of the orbit of the Space Shuttle.
However, in practice, perturbations in the earth's atmosphere, which cause the familiar twinkling of stars, degrade this ability by a factor of between 30 and 100. Space-based telescopes, like the Hubble Space Telescope, suffer no such limitation but are much more expensive than their ground-based counterparts.
An interferometer using 1.4m diameter telescopes and separations of up to 400m, would have an angular resolution of a quarter of a milli-arcsecond, i.e. roughly the angle required to read the writing on the penny in orbit (seeing features of 0.2mm or less). Unlike conventional ground-based telescopes, the angular resolution of interferometric arrays is not compromised by atmospheric fluctuations.
Instead, these limit the sensitivity of the array, and so interferometric arrays provide complementary information to that collected by more conventional telescopes that can investigate fainter, and more distant, astronomical sources.
The technique of interferometry involves the coherent combination of signals from widely separated telescopes to simulate observations that would have been obtained with a single dish as large as the longest inter-telescope separation. Interferometry was first used to make images of astronomical objects just after the 2nd World War when pioneers such as Sir Martin Ryle in Cambridge developed aperture synthesis methods for measurements at radio wavelengths.
This method has recently been extended to work at much shorter optical wavelengths, where the mechanical precision required is some 60,000 times higher. The availability of laser metrology, high-speed electro-optics and cheap computing resources have all contributed to the field's development over the past 25 years.
A typical configuration involves a number of small telescopes which feed their signals to a central "beam-combining" laboratory. There the signals are aligned appropriately, mixed together and detected. The combination of signals from every pair of telescopes in the array measures the contributions in the image that have specific sizes and orientations. Measurements using many pairs of telescopes in a two-dimensional configuration on the ground thus allows the reconstruction of a map or image of the source as though a large single aperture has been used - hence the name aperture synthesis. As more and more pairs of telescopes are used, the interferometric map becomes a closer and closer representation of the true source.
At optical and infrared wavelengths interferometers, like conventional telescopes, are strongly affected by turbulence in the earth's atmosphere. Real-time control of the optical elements in the signal paths are required to stabilize the interferometric measurements. These are typically made at rates of up to several hundred times a second to ensure good data quality.
The combination of large numbers of array elements, active control, and high sensitivity is the key challenge for today's optical/infrared interferometrists, and is the goal for the MROC array.
Currently there are four facility optical/infrared interferometric arrays under construction worldwide. These are the Navy Prototype Optical Interferometer (NPOI - Flagstaff, Arizona), the Center for High Angular Resolution Astronomy array (CHARA - Mt. Wilson, California), the Keck Interferometer (Mauna Kea, Hawaii) and the Very Large Telescope Interferometer (VLTI - Cerro Paranal, Chile).
In all interferometric arrays, the number and size of the array elements has the most profound effect on the type of science that can be pursued. Small numbers of very large collectors are useful for high spectral resolution measurements of simple targets, whereas large numbers of small telescopes favour studies of brighter objects which can, however, be highly structured.
The interferometer proposed for the Magdalena Ridge Observatory has been designed with faint-source imaging in mind, and so uses moderately large collectors but in numbers that exceed those of any of the arrays mentioned above. Since the imaging performance of an optical/infrared synthesis array improves faster than the square of the number of array elements, the quality of the images expected for the MROC-Cambridge array should be unmatched worldwide.
For a general introduction to the principles behind this array see http://www.mrao.cam.ac.uk/telescopes/coast/papers/tyoung.ps.