XMM Users' Handbook

   
Instrument and calibration overheads

The following instrument and calibration overheads apply to XMM science observations. Note that some of them will occur simultaneously, at the start of an observation.

• EPIC setup time (of order 15 minutes per exposure) for offset calculations, during which the amount of optical loading of the CCD chips is determined (which leads to pixel charges that must be subtracted to set the correct zero level for X-ray observations).

• RGS diagnostic mode image (max. ca. 15 minutes) before the start of RGS science observations for background determination.

• Time estimates for calculations of OM field acquisition overheads are mainly driven by the guide star recognition and the acquisition of a reference tracking frame. Both are estimated to be of order 1 minute, and therefore negligible.

• Between different OM science exposures, e.g. when performing filter changes, an overhead of approximately 2-4 minutes is expected.

• No special calibration overheads are foreseen for OM for each individual exposure. The acquisition of OM flat fields does not cause an operational overhead on OM observations, because they will be taken during slews.

In general, instrument calibration overheads refer to exposures, not entire observations. Therefore, if there is no need to change the data acquisition mode during the observations, it is recommended to not interrupt the integration during an observing run. Otherwise, additional overheads will be created, which leads to a decrease in the overall observing efficiency of a program.