A totally new type of optical detector has been used on the William Herschel
Telescope to directly measure intensity and colour changes in a faint,
rapidly variable binary star system, UZ Fornacis, for the first time. A team
from the Space Science Department of the European Space Agency’s Research
and Technology Centre in the Netherlands (ESA/ESTEC), who have developed the
S-Cam camera, were joined by astronomers from the Mullard Space Science
Laboratory (MSSL) in the UK to exploit this advanced instrument.

With conventional optical CCD detectors, very rapid changes in light intensity
cannot be measured. Furthermore, the energy or wavelength of the arriving
photons can only be measured by introducing a filter or spectrograph into
the optical light path, degrading the efficiency. With the new instrument,
advanced detector elements based on superconducting technology register the
arrival of each photon individually, and measure its energy and wavelength

“This cryogenic detector is very efficient, very sensitive, and very fast;
and there is no dark current or readout noise”, explains Dr. Anthony
Peacock, who has led the development of the detector technology from ESA’s
Astrophysics Division. The instrument, built by an ESA technical team led by
Nicola Rando, is cooled to below one degree kelvin to minimise all possible
noise, resulting in an almost perfect detector performance.

Larger and larger telescopes are being built on the ground and in space to
study fainter and more distant objects, but more efficient detectors are
paramount to astronomers’ progress in understanding.

“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”, said Dr. Michael Perryman, who has led this
astronomical investigation.

In the particular binary star system studied, one of the two stars is a
so-called white dwarf, a star in an advanced state of stellar evolution that
collapses slowly under its own gravity. This white dwarf tears gas from the
surface of its nearby companion, which is then engulfed by the white dwarf’s
powerful gravitational field. The material is channelled down highly intense
magnetic field lines onto the surface of the white dwarf, where it emits
prolific amounts of optical, ultraviolet and X-ray radiation as it cools.

Many clues are contained in the very short interval of time in which the
intense light emitted by the infalling material is eclipsed by its larger
but fainter companion. The results show that the diameter of the accretion
spot, where material hits the white dwarf surface, is less than about
100 km.

Dr. Mark Cropper, from the Mullard Space Science Laboratory, has been
studying this type of binary system, known as a magnetic cataclysmic
variable, for more than a decade. “This new detector allows us to look at
changes in the light from the system as it changes over small fractions of
a second. And this is the first time we have been able to study the rapid
colour changes which occur at the same time.”

The results of the observations, performed by an ESA technical team in
support of the ESA/MSSL science team, were made at the William Herschel
Telescope in December 1999, and are reported today in the scientific journal
Monthly Notices of the Royal Astronomical Society.

ESA scientists Dr. Michael Perryman, Dr. Clare Foden and Dr. Anthony Peacock
published the theoretical ideas underlying the new detector in 1993, the
first instrument able to detect the energy of optical photons directly. Dr.
Anthony Peacock and Dr. Peter Verhoeve at ESTEC reported the first detection
of optical photons using this technology in 1996. S-Cam, the instrument
making use of these principles and developed by the same research team, was
commissioned at the William Herschel Telescope in February 1999 (see ING
Press Release ING 0/99,
http://www.ing.iac.es/PR/press/Press_Release_ING099.html) and its scientific
exploitation has been supported by Dr. Fabio Favata and Dr. Alastair Reynolds.

The Isaac Newton Group of Telescopes (ING) is an establishment of the
Particle Physics and Astronomy Research Council (PPARC) of the United
Kingdom and the Netherlands Organisation for Scientific Research (NWO).
The ING operates the 4.2 metre William Herschel Telescope, the 2.5 metre
Isaac Newton Telescope, and the 1.0 metre Jacobus Kapteyn Telescope. The
telescopes are located in the Spanish Roque de Los Muchachos Observatory
on La Palma which is operated by the Instituto de AstrofÌsica de Canarias


Dr. Michael Perryman

Astrophysics Division, ESA/ESTEC

2200AG Noordwijk

The Netherlands

E-mail: mperryma@astro.estec.esa.nl

Phone: +31 71 5653615

Fax: +31 71 5654690

Dr. Mark Cropper

Mullard Space Science Laboratory

University College London

Holmbury St Mary

Dorking, Surrey RH5 6NT

United Kingdom

E-mail: msc@mssl.ucl.ac.uk

Phone: +44 1483 204155

Fax: +44 1483 278312

Mr. Javier Mendez

Public Relations Officer

Isaac Newton Group of Telescopes

E-mail: jma@ing.iac.es

Phone: +34 922 425464, +34 616 464111

Fax: +34 922 425401, 442

More information on the web:

* “High-Speed Energy-Resolved STJ Photometry of the Eclipsing Binary UZ For”
paper on arXiv.org electronic archive,

* The STJ page at ESA Astrophysics
[http://astro.estec.esa.nl/SA-general/Research/Stj/STJ_main.html] includes
details of the ESA superconductor detector programme. S-Cam 2 is the second
prototype camera. More information on S-Cam 2 can be found here,


* Two articles on S-Cam and S-Cam 2 have been published in the ING Newsletter:
“Super Cool Technology” [http://www.ing.iac.es/PR/newsletter/news1/cool.html]
and “S-Cam Update — Novel Capabilities for Resolving Old Problems!”

* More information on cataclysmic variable stars can be found at the web
pages of the visitor centre of the Astrophysics Group of the Mullard
Space Science Laboratory, http://www.mssl.ucl.ac.uk/www_astro/homepage.html

* More information on the Isaac Newton Group of Telescopes:
http://www.ast.cam.ac.uk/ING/PR/ (UK mirror)


[Image 1]

In order to demonstrate the capabilities of S-Cam, the astronomers have created
an animation showing an observation of an entire eclipse of UZ Fornacis. This
was created by splicing together two integrations, with the join between them
near the mid-point of the eclipse. Each step in the animation corresponds to
3 seconds of actual data. The lightcurve in the top of the movie shows the
evolution of the total source intensity across the complete spectral range.
In order to illustrate the energy sensitivity of the device, however, the
36 array pixels in the lower image are first assigned colours based on the
brightness ratio obtained from two energy bands (ranging from blue to red on
a spectral scale), and are then assigned intensities based on the total counts
in that pixel over the 3 second time period. Picture credit: Astrophysics
Division, ESA/ESTEC.

Available formats:

* AVI (9,072 K)

* Animated GIF (499 K)

[Image 2]
S-Cam camera at ESTEC laboratory. Picture credit: Astrophysics Division,

Available formats:

* LZW TIFF (10 cm x 14 cm, 3,695 K)

* JPEG (800 x 1120, 142 K)

[Image 3]

The William Herschel Telescope (WHT). The WHT is part of the Isaac Newton Group
of Telescopes and it’s the largest telescope of its kind in Western Europe.
Picture credit: Rainer Girnstein.

Available formats:

* LZW TIFF (16.5cm x 25.0cm at 300dpi, 11,241K)

* LZW TIFF (10.0cm x 15.1cm at 300dpi, 3,657K)

* JPEG (800 x 1,210 pixels, 331K)

* JPEG (400 x 605 pixels, 58K)