Observations with XMM-Newton are only possible under a number of celestial constraints, for example Sun, Earth limb, and Moon avoidance angles. As a consequence, not every source is visible at all times. Therefore, it is mandatory that observers study the visibility of a target, in particular if the target should be observed under a specific position angle, on a certain date (see Section 5.7.1) or simultaneously with other telescopes (see Section 5.7.2). The XMM-Newton SOC provides a web-based tool which allows an online target visibility check for given coordinates and observation times, the ``XMM-Newton Target Visibility Checker''. For details see:
For programs aiming large amounts of observing time of a particular area on the sky observers should ensure that the total requested exposure time does not exceed approximately 40% of the cumulative visibility of the area during the AO. This 40% is typically 15 XMM-Newton revolutions for most parts of the sky, but observers are encouraged to check the visibility. An exception to this rule may apply for ``Multi-Year Heritage Programmes'', see Section 5.4 for details.
Several scientific questions might be addressed equally well with several different targets or areas on the sky. In this case it is strongly recommended that the proposer selects the target or area with the longest cumulative visibility.