At the start of each science observation a 20 seconds V-filter image is taken to allow proper identification of the field stars with respect to the uploaded guide stars. Shifts are applied on board, whenever necessary, to correct the window coordinates according to the true positions of the stars. This short exposure is called Field AcQuisition (FAQ). A correct (i.e.: within a few arcseconds) FAQ is indispensable for a proper placing of the Fast Mode windows.
On-flight experience demonstrates that the FAQ is properly working on the overwhelming majority of the fields. However, a few cases have been observed (e.g. very crowded fields or extended objects) where the FAQ fails and consequently no shift is applied.
The XMM-Newton PIs must be aware that no compensation is granted if the FAQ failure yields an impact on the scientific outcome of a given observation.