Caught on camera

Years ago, astronomers would use their eyes to look through telescopes. Then came the development of photographic film and suddenly a much more reliable way to measure light from stars and galaxies was available. With the development of electronics, charged coupled devices (CCDs) emerged as an even better way to measure light; they recorded a greater percentage of the photons than film did.

These technological developments, together with larger telescopes (light buckets), have massively improved the amount of light that is recorded. This has lead to fantastically better images. The only trouble, as you will know if you are lucky enough to have your own astronomical CCD, is that it takes time to get an image of a faint object and there is dead time between exposures while the CCD gets read by the computer. This isn't a huge problem but it means that you miss out on events that happen very quickly. That includes eclipses, transits and occultations, not to mention anything that flickers, flares, pulsates, oscillates, outbursts or explodes.

To catch these quick changes in brightness, astronomers from the University of Sheffield, the University of Warwick and the UK Astronomy Technology Centre developed a special CCD camera called ULTRACAM that could be attached to big telescopes. This instrument is actually made up of three separate 1024 by 1024 pixels CCD cameras which are much more sensitive than those you would find in a digital camera. The light from the telescope gets split into three colours - blue (u), green (g) and red (r, i or z) - and each one goes to a different detector. Depending on how they use the instrument, they can get exposure times as short as a millisecond with essentially zero dead-time between exposures. This results in a continuous stream of over 3 MB of data per second. That is as much as 130 GB of data each night!

Since May 2002, ULTRACAM has been used on the 4.2-m William Herschel Telescope (WHT), on La Palma, as well as other telescopes around the world. Two years ago they began preparing it for use on the ESO's Very Large Telescope (VLT) in Chile. First light on the VLT was on 4th May this year and they had 17 consecutive nights of observing.

IMAGE: European Southern Observatory.

During that time, they observed a black hole binary system named GU Muscae. This consists of a black hole and star orbiting each other every 10 hours. What the ULTRACAM team noticed though, was that there was a sudden brightening roughly every 7 minutes. This stable repeating signal is thought to be associated with some stable structure in the disk of matter that surrounds the black hole but nobody is really too sure.

The ULTRACAM team have also observed extrasolar planets, originally discovered by the OGLE project, but now using short exposure times at the three different colours. When these planets pass in front of their parent star, they cause the star to dim as they block out some of its light. This gives a direct indication of the size of the planet as bigger planets block out more light than smaller ones in the same way that someone with a huge hat can block your view of the stage at a concert. Comparing the size of the planet, in the different colours bands, should hopefully say something about the composition of the exoplanets' atmosphere.

Tom Marsh of the University of Warwick hopes that "...ULTRACAM will now become a regular visitor at the VLT, giving European astronomers access to a unique new tool with which to study the Universe".

Posted in astro blog by Stuart on Saturday 11th Jun 2005 (13:51 UTC) | Permalink
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