XMM Users' Handbook

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Dithering mode

XMM also does not have a dithering mode. This means that users must, in order to overcome inhomogeneities in the coverage of a FOV (different net exposure times, e.g., because of chip boundaries), conduct observations of slightly offset positions on the sky. However, since XMM does not have a raster observing mode either (see above), this leads to a sequence of separate observations that are, from the standpoint of scheduling, treated as independent and might thus not be scheduled back-to-back.

As a secondary point, users should be aware that when proposing such partly overlapping observations to cover a field of view larger than 30', one should make sure that within a single observation the required OM filter coverage is achieved (because the position angle of the spacecraft might have changed by the time a neighbouring observation is scheduled, which would lead to different coverages on the sky).

European Space Agency - XMM Science Operations Centre