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Low dose data: Difference between revisions
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change text to use DATA_RANGE_FIXED_SCALE_FACTOR instead of FIXED_SCALE_FACTOR
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(change text to use DATA_RANGE_FIXED_SCALE_FACTOR instead of FIXED_SCALE_FACTOR) |
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With the current version of XDS, the background of a data frame is used for scaling adjacent frames relative to each other in the INTEGRATE step. | With the current version of 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 [http://homes.mpimf-heidelberg.mpg.de/~kabsch/xds/html_doc/xds_parameters.html# | 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. | ||
Another problem is that below 0.5 counts/pixel, the I/sigma values become overly optimistic. Whether this is due to the intensities being too high or the sigmas too low has not yet been investigated; likely it's the latter. This effect is ''not'' mitigated by use of [http://homes.mpimf-heidelberg.mpg.de/~kabsch/xds/html_doc/xds_parameters.html# | Another problem is that below 0.5 counts/pixel, the I/sigma values become overly optimistic. Whether this is due to the intensities being too high or the sigmas too low has not yet been investigated; likely it's the latter. This effect is ''not'' mitigated by use of [http://homes.mpimf-heidelberg.mpg.de/~kabsch/xds/html_doc/xds_parameters.html#DATA_RANGE_FIXED_SCALE_FACTOR= DATA_RANGE_FIXED_SCALE_FACTOR] . | ||
CC1/2 (printed out in CORRECT.LP since version of March 15, 2012), on the other hand, appears to be more realistic. | CC1/2 (printed out in CORRECT.LP since version of March 15, 2012), on the other hand, appears to be more realistic. | ||