Low dose data: Difference between revisions
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This seems to work very well down to an average of 0.5 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 [http://homes.mpimf-heidelberg.mpg.de/~kabsch/xds/html_doc/xds_parameters.html#DATA_RANGE_FIXED_SCALE_FACTOR== DATA_RANGE_FIXED_SCALE_FACTOR] has been introduced to handle this situation. | This seems to work very well down to an average of 0.5 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 [http://homes.mpimf-heidelberg.mpg.de/~kabsch/xds/html_doc/xds_parameters.html#DATA_RANGE_FIXED_SCALE_FACTOR== 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 [http://homes.mpimf-heidelberg.mpg.de/~kabsch/xds/html_doc/merge2cbf_program.html merge2cbf] program of the XDS package. This is currently only relevant for the Pilatus detector; 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; [http://dx.doi.org/10.1107/S0907444910007262])." | |||
Data frames which have been collected too finely/weakly may be added together using the [http://homes.mpimf-heidelberg.mpg.de/~kabsch/xds/html_doc/merge2cbf_program.html merge2cbf] program of the XDS package. | |||
See also: [[Difficult datasets]] | See also: [[Difficult datasets]] | ||
Revision as of 18:41, 2 March 2015
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 very well down to an average of 0.5 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 detector; 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