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.

Even if this feature is used, DEFPIX may stop with the (misleading) error message

!!! ERROR !!! WRONG DETECTOR PARAMETERS

If this happens, omit DEFPIX from the JOB= list of parameters, and copy BKGINIT.cbf to BKGPIX.cbf (DEFPIX would have to write BKGPIX.cbf, but this has to be done manually).

See also: Difficult datasets