The instrumental background also requires attention, particularly because of its variability. Most of the time the background is low enough for the RGS to be more-or-less photon limited for the detection of point-source spectra over most of its bandwidth once the proper event selections have been made. However, solar flares and other particle events have caused changes of two or more orders of magnitude in the background rate on time scales short in comparison with the typical length of an observation. In such cases the signal spectrum can be completely swamped but an entire observation is seldom affected. It is imperative, therefore, to make an assessment of the background's behaviour during an observation and exclude any periods that are too badly contaminated following, for example, the procedure described below in § 5.7.2.