By Jonathan Amos
BBC News science reporter, in Leicester
Astronomers are getting the most out of Europe's XMM-Newton X-ray observatory by making it work even in those periods that might be considered downtime.
XMM-Newton studies energetic and violent events in the Universe
Like all space telescopes, XMM follows an observing schedule, looking at an object for some hours before swinging away to study the next target.
But scientists have been leaving the "shutter" open as its slews across the sky for some lucky dip observations.
The technique has thrown up thousands of brilliant X-ray stars and galaxies.
"About a quarter of the sky has been covered in this way, and it's all for free - XMM was not designed in the first place to do this," explained Dr Andy Read of Leicester University.
"The telescope is so good at collecting X-rays that even when you look at a point in the sky for 10 seconds, you can find sources," he told the BBC News website.
The XMM-Newton X-ray observatory has been one of the European Space Agency's prestige projects of recent years.
Launched on the first to-orbit flight of an Ariane 5 rocket in 1999, XMM has helped to transform our understanding of the Universe.
The emission of X-rays, or very shortwave light, from distant objects is usually an indicator of violent, energetic events - such as the explosion of a star, or gas falling into a black hole.
XMM, with the huge collecting area of its many nested mirrors and the excellent efficiency of its EPIC X-ray camera-detector, is the most sensitive X-ray observatory ever flown.
As a result, it is able to record unrivalled numbers of X-rays from the cosmos.
But its designers never envisaged the additional viewing mode it has now assumed, and there were doubts at first it would even work.
"When we realised it wouldn't be dangerous, that it wouldn't damage the camera for example, and we thought we could make sense of the data - then we thought 'yes, let's keep the doors open during the slewing time'," Dr Read said.
When XMM turns, it does so very quickly, passing over each point in the sky in only 10 seconds (compared with a normal "pointed" observation of a few hours).
Nevertheless, even this very short exposure is enough time for XMM-Newton to detect thousands of sources in the sky - black holes, quasars, active galaxies and stars - many of which have been observed for the first time.
The survey has been particularly good at highlighting sudden changes.
"I can think of one innocuous-looking galaxy that was studied a few years ago and seemed to be dim and boring, and it now shows up in our survey as very bright.
"The leading explanation is that a black hole at the centre of the galaxy has swallowed something huge. Rare events like that, you're only going to catch in this type of survey."
XMM-Newton is now midway through its mission and is expected to complete a full slew sky survey before being decommissioned.
Scientists would also like to get nearer to real-time observations so that high interest objects seen in a slew could be returned to for a pointed, more intensive study (at the moment, the survey data requires so much processing this is not possible).
Preliminary results from the XMM-Newton Slew Survey are being presented at the UK Royal Astronomical Society's National Astronomy Meeting in Leicester.