The telescopes detect radio waves from brilliant but distant galaxies that act like fixed stars in the sky. By continually measuring the positions on the sky of bright radio galaxies, the telescopes in the network can determine their own location in space.
John Conway is professor in observational radio astronomy at Chalmers and director of Onsala Space Observatory.
”The sources that the telescopes measure are distant galaxies, each of which has at its centre a supermassive black hole whose surroundings shine brightly when the black hole consumes material. This is applied astronomy at its best” he says.
The Onsala Twin Telescopes are part of a growing international network of similar telescopes. As part of the global project VGOS (VLBI Global Observing System), they have company all over the world. Measurements of this kind have been carried out over the last few decades, and Onsala’s 20-metre telescope has participated in these. But with a dedicated network of telescopes, observations can now be carried out 24 hours a day, all year round, and will be able to able to make measurements ten times as precise as is possible today.
”With the new network we will be able to measure distances between telescopes to millimetre precision, and almost in real time”, says Rüdiger Haas, professor of space geodesy at Chalmers.
Onsala’s history of geodetic measurements is a long one. In 1968, the observatory’s iconic 25-metre radio telescope became the first in Europe to take part in geodetic VLBI (very long baseline interferometry). The observatory’s 20-metre telescope, inaugurated in 1976, boasts geodetic measurements over a longer period than any other telescope in the world.
The new telescopes and their network meet global needs, as expressed in a resolution which was adopted by the General Assembly of the United Nations in February 2015. The resolution, A Global Geodetic Reference Frame for Sustainable Development, recognised for the first time the importance of coordinating geodetic measurements on a global scale. The resolution strengthened exisiting work in the UN initiative Global Geospatial Information Management (UN-GGIM) and with the Global Geodetic Reference Frame (GGRF).
“Future research on sustainable development and on the Earth as a system will require more reliable, long-term, high-precision measurements. The Onsala Twin Telescopes are a natural continuation of our already long history of such measurements in Onsala”, says Gunnar Elgered, professor of electrical measurements and head of the Department of Earth and Space Science at Chalmers.
Similar telescopes are already in place in the United States, in Hawaii and Maryland, in Wettzell in Germany, in Yebes in Spain, on Santa Maria in the Azores, and on the Arctic Svalbard islands. New telescopes are planned for other locations, among them South Africa and Finland. Operations for the complete network of 16 or more stations are expected to start in 2020.
A reference system with this level of precision is also needed for many applications in Earth Sciences. It will become possible, for example, to measure sea level relative to the centre of the Earth, in order to test models for climate change. Data from the network will also be able to contribute to many other exciting areas of science, for example the movement of Earth’s tectonic plates, the Earth’s changing rotation and axial tilt.
The construction and installation of the Onsala Twin Telescopes has been funded by a generous grant from the Knut and Alice Wallenberg Foundation and Chalmers University of Technology.
Robert Cumming, communicator, Onsala Space Observatory, Chalmers, tel: +46 31-772 5500 or +46 70 493 3114, firstname.lastname@example.org.
Rüdiger Haas, professor of space geodesy, Chalmers, tel: +46 31 772 55 30, email@example.com
Gunnar Elgered, professor of electrical measurements and head of the Department of Earth and Space Science, Chalmers, tel: + 46 31 772 1610 or +46 31 772 5565, firstname.lastname@example.org
1 (top): The Onsala Twin Telescopes will measure the Earth’s movements using distant galaxies. This photo shows the two antennas at with Onsala Space Observatory’s 25-metre telescope, built in 1963, behind them. (Credit: Onsala Space Observatory/R. Hammargren).
2. Onsala Twin Telescopes will become operational during 2017. "First light" for the northern telescope (right) was achieved in early February 2017. (Credit: Onsala Space Observatory/Anna-Lena Lundqvist)
3. Changing the Onsala skyline: the new Twin Telescopes and the 25-metre telescope. (Credit: Onsala Space Observatory/Anna-Lena Lundqvist)
More about the Onsala Twin Telescopes
The Onsala Twin Telescopes are two dish antennas, 13.2 metres in diameter, located 75 metres apart close to the 25-metre telescope at Onsala Space Observatory, in the province of Halland, 45 km south of Gothenburg on Sweden’s west coast. The telescopes have a ring-focus design, the main reflector complemented by a 1.55-metre subreflector. They are designed to be able to observe together in many different modes. Able to move at up to 12 degrees per second in azimuth and 6 per second in elevation, they can slew extremely fast between observations and measure thousands of radio sources every day. The parabolic dishes, accurate to 0.3 mm RMS precision, allow measurements at frequencies up to 40 GHz (wavelength 0.75 cm or more).
Each telescope has its own receiver system with feeds and receivers for radio waves of a common frequency range 3-14 GHz (wavelength 2-10 cm) and two linear polarisations. The northern telescope has a feed with a quadridge design, with four ridges, covering 3–18 GHz (1.7-10 cm) and similar in design to the feed horn Onsala Space Observatory has developed for the international Square Kilometre Array (SKA) project. The southern telescope is equipped with an Eleven feed, a design developed by Chalmers physicist Per-Simon Kildal (1951-2016), which covers 2–14 GHz (2-15 cm). Like the site’s other instruments, the Twin Telescopes have access to Onsala Space Observatory’s hydrogen maser atomic clock. The telescopes are controlled with the help of digital systems capable of speeds up to 128 gigabit per second, located in the control room next to the 20-metre telescope.
The telescopes will be inaugurated on 18 May 2017 by the County Governor of Halland, Lena Sommestad. Details of the inauguration will be provided later.
23rd Working Meeting of the European VLBI Group for Geodesy and Astrometry (EVGA)
NASA’s article about the telescopes’ sister station in Hawaii, USA:
NASA’s animated video about the history of space geodesy and how quasars help scientists measure the Earth: