A proportion of detected events are `minimum ionising particles' (MIPs). During each ccd frame, pixels occupied by a MIP are not free to detect an xray; hence MIPs lead to xray flux loss. The fraction of affected pixels is stored in the FRACEXP column of the EXPOSUnn extensions (the `nn' indicating the ccd/node) of the event list. Note that, although this column has the same name as that used to store the total exposure in the output TS, their values are not closely related.