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 5.4)
or simultaneously with other telescopes (see Section 5.5).
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:
http://xmm2.esac.esa.int/external/xmm_sched/vischeck/AO8/
In order to reach a maximum efficiency, the start time of
``Fixed Time'' observations should not be constrained below
a margin of 
15 ks (see Section 5.4-5.5).
For large surveys of a particular area on the sky, the observer 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% being about 15 revolutions for most parts of the sky.
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.