The Chandra X-ray observatory has delivered spectacular first images but astronomers are concerned that some of its detectors are having problems.

Nasa engineers are now studying a loss of sensitivity in the Charge Coupled Devices (CCD) that lie at the heart of one of Chandra's major instruments.

They say the problem is minor but they are concerned.

A billion dollar question

In the $1.6 billion telescope, X-rays from the cosmos are focused onto a CCD detector that converts the image into electrical signals. Chandra has two main sets of CCD's, called "front-side" and "rear-side" detectors.

Since its launch on 23 August, Chandra has been carrying out scientific observations with the front-side system. But engineers have noticed that in eight of the ten detectors there has been an unexplained decrease in their energy sensitivity.

Procedures have been put into action to try to find out what is causing the problem.

No risk

Scientists are keen to point out however that at the moment the telescope's science agenda is not at risk. They say that, even in a worst-case scenario, two of the detectors in the Advanced CCD Imaging Spectrometer (ACIS) are operating effectively and could be used to carry out virtually all planned observations.

If all of the front-side CCD's fail then the rear-side ones could be used, as could detectors on some of Chandra's other telescopes.

All satellites encounter problems as they adapt to space conditions and, generally, the larger and more complicated the satellite, the trickier are the problems. Chandra is one of the most complicated space observatories ever put into orbit and overall engineers are pleased with its performance so far.