As for all telescopes, XMM observations are also affected by operational overheads. Proper instrument setup, e.g., requires some calibration exposures in order to provide optimal instrument performance and thereby high-quality science data. As the mission evolves and the knowledge about the instruments improves, additional items may be added or some of those mentioned below might be taken from the list.
We outline here which overheads apply to XMM observation for the users' information only. None of these need to be taken into account in XMM observing proposals, because they will be added ontop of the requested science duration as warranted.
There are two categories of overheads: