... Up: Sky Surveys . ... The resultant serendipitous source catalogues have made a significant contribution to our knowledge of the X-ray sky and our understanding of the nature of the various Galactic and extragalactic source populations. ... Figure 1: Example XMM-Newton X-ray image. ... As well as the potential for building up large samples, the brighter sources in the XMM-Newton catalogue also provide the prospect of obtaining high quality X-ray spectra and time series data (and morphology). ...