Section 11.2.2.2. Detector parameters

Detector calibration is essential for high data quality. Both the spatial distortion and the non-uniformity of response of the detector must be accurately known, and it is equally important that these corrections are stable over the timescale of the experiment (and preferably for much longer).

Finally, the crystal-to-detector distance, the detector orientation and the direct-beam position must be refined and continuously updated during integration, using observed spot positions. The crystal-to-detector distance can vary during data collection if the crystal is not exactly centred on the rotation axis, and the direct-beam position can move after a beam refill at a synchrotron. For image-plate detectors with two (or more) plates, the direct-beam position and detector distance often differ slightly for different plates.

With appropriate care, it is normally possible to predict reflection positions on the detector to an accuracy of 20–30 µm, or a fraction of the pixel size, particularly for highly collimated X-ray beams available at synchrotron sources. This level of accuracy is necessary to minimize possible systematic errors, particularly in the case of profile fitting.