The William Herschel Telescope (WHT). The WHT is part of the Isaac Newton Group of Telescopes and is the largest telescope of its kind in Western Europe. Image: Rainer Girnstein. European astronomers have used a completely new type of optical detector to simultaneously measure colour and intensity changes in a binary star for the first time.

The scientists, from the European Space Agency's Research and Technology Centre (ESA/ESTEC) in the Netherlands and the UK's Mullard Space Science Laboratory (MSSL) developed the S-CAM camera, testing it on the William Herschel Telescope (WHT) on La Palma in February 1999 and observing the binary system in December that year.

In recent years the main drive in observational astronomy has been to develop ever-larger telescopes, but developing more efficient detectors is still of vital importance. Conventional CCD detectors –the mainstay of astronomy since the early 1980s – are unable to detect very rapid changes in light intensity. With CCDs, the energy (directly related to the wavelength and colour) of the incoming photons is measured by placing a filter or spectrograph into the light path which degrades the efficiency of the instrument. Each detector element on S-CAM is able to measure the arrival of individual photons and measure their energy directly.

The S-CAM detector under development at the ESTEC laboratory. Image: Astrophysics Division, ESA/ESTEC.

A team of astronomers used S-CAM on the WHT to observe the rapidly variable binary star system UZ Fornacis in December 1999. In this system, one of the two stars is a white dwarf in close proximity to its companion. The white dwarf's strong gravitational field accretes material from the neighbouring star, which then flows down the white dwarf's magnetic field lines on to its surface and cools, emitting intense optical, ultraviolet and X-ray radiation. The companion star eclipses the white dwarf once every two hours and covers this emission in a very short time, proving that the area where the material hits the white dwarf's surface is very small - less than 100 km across. S-CAM was used to make a movie of the changing brightness and colour of the UZ Fornacis system as one such eclipse took place.

In the future S-CAM will be used to observe many other objects. 'We have a very powerful instrument for looking at faint astronomical sources which vary rapidly, for example pulsars or binary star systems. It is of particular interest when the light changes its energy distribution, or colour at the same time,' commented Dr. Michael Perryman, who led the investigation team.