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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 can cause changes of two or more orders of magnitude in the background rate on short time scales as compared with the typical length of an observation. In such cases, the signal spectrum can be completely swamped, while the entire observation is seldom affected. Therefore, it is imperative 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.