Europe’s billion-star surveyor ready for launch

Europe’s billion-star surveyor will be launched into space on 19 December, when it will embark on its mission to create a highly accurate 3D map of our galaxy.

By repeatedly observing a billion stars with its billion-pixel video camera, the Gaia mission will allow astronomers to determine the origin and evolution of our galaxy whilst also testing gravity, mapping our inner solar system, and uncovering tens of thousands of previously unseen objects, including asteroids in our solar system, planets around nearby stars, and supernovae in other galaxies.

Gaia will map the stars from an orbit around the Sun, near a location some 1.5 million km beyond Earth’s orbit known as the L2 Lagrangian point. The spacecraft will spin slowly, sweeping its two telescopes across the entire sky and focusing their light simultaneously onto a single digital camera, the largest ever flown in space.

Edinburgh and Gaia

"Gaia is an ambitious mission employing a complex instrument orbiting in deep space. Accurate calibration of the instrument is required to make the precise angular measurements that astronomers wish to achieve in order to do the revolutionary science for which Gaia is designed.

"Within our small team of scientists and software engineers at the IfA we are pleased to be contributing calibration software in key areas of the on-ground processing systems for the Gaia data." Nigel Hambly, Institute for Astronomy, University of Edinburgh

UK scientists and engineers played key roles in the design and build of Gaia, including astronomers and software developers in the School's Institute for Astronomy (IfA) who made a significant contribution to the ground data-processing system. The IfA team members are Ross Collins, Nick Cross, Michael Davidson, Nigel Hambly and Alex Ouzounis. Dr Hambly explains the team's involvement.

Gaia's imaging system

"At the heart of Gaia is an imaging system that employs charge-coupled devices (CCDs) consisting of arrays of optical light-sensitive elements. These CCDs are specially designed versions of the same kind of detector commonly found in digital cameras, but they operate in a particular mode and within the harsh environment of deep space. However, this results in anomalies in the measured signals, and the Edinburgh team is tasked with dealing with two specific aspects of this problem, along with associated calibration issues.

"The first problem concerns the way the CCDs “see” the star field being scanned. In a domestic digital camera, we point at the scene of interest and then click the shutter to get an exposure of usually some fraction of a second. The CCD is then very quickly read out to capture an image of the scene as a stream of data numbers that reflect the electronic charge in each pixel produced by incident light from the scene. A clue to the way in which a CCD is read out is in the name: each pixel is “charge-coupled” to its neighbours such that once exposed, the array of charge measurements can be moved across the pixel array and into a line of light-insensitive, but similarly charge-coupled pixels known as the read-out register which is itself clocked to shift charge to produce a sequence of data numbers. Hence the CCD is read by shuffling each line of pixels in turn into this serial register.

"However in Gaia, rather than using the familiar point-and-shoot mode we commonly employ when using a digital camera, the CCD is continually clocked in what is known as time-delay integration (TDI) mode. As the satellite scans the sky, the CCD is continually clocked such that the scene shuffles across the device at a rate that exactly counteracts the scan. So, rather than observing individual pointed frames, a long image strip is imaged by each CCD.

Simulation of Gaia imaging

"A (very) short simulated example of such a strip image can be seen on the European Space Agency's website (picture credit: Dr Michael Davidson, IfA Edinburgh). This simulation shows several interesting features of Gaia imaging.

"For orientation, Gaia is scanning from right to left, while the CCDs are clocked to shuffle charge from left to right in order to compensate for the image motion. Ignoring the vertical stripes for the moment, we can see star images on a flat background with the familiar diffraction spikes introduced by Gaia’s imaging system of mirrors.

"However if we look closely on the left hand (trailing) side of each image, we can see that charge has been smeared away from the centre. This effect is known as charge-transfer inefficiency (CTI) and is the result of radiation damage to the CCD pixels. Incident radiation (primarily solar protons in this case) damages the crystalline structure of the CCD pixels, inducing traps that grab charge from passing signals as they are shuffled along the columns of pixels. This trapped charge is then released a short time later, but after the signal has passed. In this way, the image of each star is distorted a small but significant amount, and this distortion will get worse as the radiation dose received by Gaia increases throughout its mission.

"Note that the bright vertical lines in the images are periodic, artificial “charge injections” that will be employed in the Gaia CCDs to fill up charge traps as much as possible so that passing images are distorted less. The dark vertical lines correspond to transient reduced exposure times triggered by the transit of very bright stars to avoid CCD pixels becoming unusably soaked with charge in those images. While primarily beneficial, these phenomena create additional complications in the processing of Gaia data.

"The second problem concerns the way Gaia data is acquired on board the satellite as a result of practical limitations in the volume of data being handled and transmitted back to Earth. In the image example above, most of the pixels see just sky rather than astrophysically interesting objects like stars. During read-out, the Gaia CCDs are configured to “window” the useful data such that most of the sky is excluded, save a small amount around each object. Scientifically interesting pixels within defined windows are digitised relatively slowly for accurate measurement of incident light intensity, while sky pixels are relatively quickly flushed and discarded in the read-out process. This quick-slow-quick read-out induces a rather complex pattern of systematic errors in the digitised data that have to be mimicked on-ground by reconstructing the read-out process and modelling the size of the effects.

"Working with scientists and engineers within ESA and the Gaia Data Processing and Analysis Consortium, a small team of scientists and software engineers based at the IfA in the School of Physics & Astronomy at Edinburgh is designing and implementing solutions in software to these challenging problems. The software will run in data processing pipelines running at various Gaia Data Processing Centres across Europe including Cambridge, Madrid, Barcelona and Paris."

UK participation in the mission is funded by the UK Space Agency. The UK Science and Technology Facilities Council (STFC) funded the early development of the project, including the set-up of the data applications centre. STFC’s current support involves the UK exploitation of the scientific data to be yielded from the mission.