Slide 23 of 27
Various physical factors lead to observed intensities being on different scales. among these factors are those related to:
- the incident beam and the camera
e.g. variations in beam intensity at a synchrotron, illuminated volume of the crystal, variations in rotation speed and shutter synchronisation (these latter are disastrous!).
- the crystal and the diffracted beam
e.g. absorption, radiation damage
e.g. incorrect calibration, shadows from beamstop etc.
Scaling models should (if possible) parameterise the experiment so different experiments may require different models. Scales are determined by comparison of symmetry-related reflections, i.e. by adjusting scale factors to get the best internal consistency of intensities. Note that we do not know the true intensities and an internally-consistent dataset is not necessarily correct. Systematic errors will remain despite scaling.
The diagnostics will be dealt with later.