Low dose data

The average of the background counts per pixel are printed in INIT.LP, or can be estimated by visualizing the frames with XDS-Viewer or adxv.

In XDS, the background of a data frame is used for scaling adjacent frames relative to each other in the INTEGRATE step.

This seems to work well down to an average of 0.05 counts/pixel. If, however, the crystal is exposed so shortly/weakly that the average drops significantly below this value, scaling becomes impossible - the ratio of averages which are around zero is obviously undefined. The keyword DATA_RANGE_FIXED_SCALE_FACTOR has been introduced to handle this situation.

The following problem was fixed in the March 1, 2015 version of XDS: "Below 0.5 counts/pixel, the I/sigma values become overly optimistic. Data frames which have been collected too finely/weakly may be added together using the merge2cbf program of the XDS package. This is currently only relevant for the Pilatus and Eiger detectors; a typical low background on other types of detectors is more on the order of 10 counts/pixel (Holton and Fraenkel (2010) Acta Cryst. (2010). D66, 393–408; [1])."

See also: Difficult datasets