Matching X-ray source, optics and detectors to protein crystallography requirements

A review of the requirements for collecting X-ray diffraction data from pro tein crystals is given, with an emphasis on the properties of the crystal a nd its diffraction pattern. The size, unit-cell dimensions and perfection o f the crystals can all be related to the required size and divergence of th e incident X-ray beam, together with the size and spatial resolution of the detector. The X-ray beam causes primary radiation damage, even in frozen c rystals. If the incident beam is very intense, temperature rises and gradie nts could occur in the crystal. The extent to which these problems can be o vercome is also discussed.