Section 11.2.5.1.1. Outlier rejection

It is not unusual for the diffraction pattern to display features other than the Bragg diffraction spots from the crystal of interest. Possible causes are the presence of a satellite crystal or twin component, white-radiation streaks, cosmic rays or zingers. In order to minimize their effect on the determination of the background plane constants, the following outlier rejection algorithm is employed:

The rationale for using a subset of the pixels with the lowest pixel values in step (1) is that the presence of zingers or cosmic rays, or a strongly diffracting satellite crystal, can distort the initial calculation of the background plane so much that it becomes difficult to identify the true outliers. Such features will normally only affect a small percentage of the background pixels and will invariably give higher than expected pixel counts. Selecting a subset with the lowest pixel values will facilitate identification of the true outliers. The initial bias in the resulting plane constant c due to this procedure will be corrected in step (3). Poisson statistics are used to evaluate the standard deviations used in outlier rejection, and the standard deviation used in step (2) is increased to allow for the choice of background pixels in step (1).