Section 3.7.4.6. Diffraction scans

Data items in these categories are as follows:

(a) DIFFRN_SCAN

(b) DIFFRN_SCAN_AXIS

(c) DIFFRN_SCAN_FRAME

(d) DIFFRN_SCAN_FRAME_AXIS

The bullet () indicates a category key. The arrow () is a reference to a parent data item.

Data items in the DIFFRN_SCAN category describe the parameters of one or more scans, relating axis positions to frames. Each scan is uniquely identified by the value of _diffrn_scan.id. The data items in this category give overall information for the scan. The detailed frame-by-frame data are given in DIFFRN_SCAN_FRAME and DIFFRN_SCAN_FRAME_AXIS. The values of _diffrn_scan.date_start and *.date_end give the starting and ending time for a scan. The original definition of the yyyy-mm-dd data type, which includes date and time, has been extended in the CBF/imgCIF dictionary. This allows the seconds part of the time to include an optional decimal fraction. The approximate average integration time for each step of the scan is given by the value of _diffrn_scan.integration_time. The scan is tied to individual frame IDs by the values of _diffrn_scan.frame_id_start and *.frame_id_end. The number of frames in the scan is given by the value of _diffrn_scan.frames.

Data items in the DIFFRN_SCAN_AXIS category describe the settings of axes for particular scans. Unspecified axes are assumed to be at their zero points. The vector of each axis is not given here, because it is provided in the AXIS category. By making _diffrn_scan_axis.scan_id and _diffrn_scan_axis.axis_id keys of the DIFFRN_SCAN_AXIS category, an arbitrary number of scanning and fixed axes can be specified for a scan. The value of _diffrn_scan_axis.scan_id (a pointer to _diffrn_scan.id) identifies the scan and the values of _diffrn_scan_axis.axis_id (a pointer to _axis.id) associate particular axes with that scan. The steps of each axis are specified by *_start, *_range, *_increment and *_rstrt_incr values for angles or for displacements. The *_start value is the setting of the relevant axis at the start of the scan. The *_range value is the total change in the axis setting through the scan. The *_increment value is the increment in the axis setting for each step of the scan. The *_rstrt_incr value is the increment in the axis setting after each step of the scan.

Data items in the DIFFRN_SCAN_FRAME category describe the relationship of particular frames to scans. The value of _diffrn_scan_frame.frame_id (a pointer to _diffrn_data_frame.id) identifies the frame. The value of _diffrn_scan_frame.scan_id (a pointer to _diffrn_scan.id) identifies the scan of which the frame is a part. Together _diffrn_scan_frame.frame_id and *.scan_id form the category key. The value of _diffrn_scan_frame.date gives the date and time of the start of the data collection for the frame. The value of _diffrn_scan_frame.frame_number gives the number of the frame (starting with 1). The value of _diffrn_scan_frame.integration_time gives the precise time in seconds to integrate this step of the scan.

Data items in the DIFFRN_SCAN_FRAME_AXIS category describe the settings of axes for particular frames. Unspecified axes are assumed to be at their zero points. If for any given frame non-zero values apply for any of the data items in this category, those values should be given explicitly in this category and not simply inferred from values in DIFFRN_SCAN_AXIS. Since the collection for a given frame may involve multiple axes, the frame involved is identified by the value of _diffrn_scan_frame_axis.frame_id (a pointer to _diffrn_data_frame.id) and each axis is identified by the value of _diffrn_scan_frame_axis.axis_id (a pointer to _axis.id). Together _diffrn_scan_frame_axis.frame_id and *.axis_id form the category key. If the axis is an axis of rotation, the axis settings for the frame are given by the values of _diffrn_scan_frame_axis.angle, *.angle_increment and *.angle_rstrt_incr. If the axis is a translation axis, the axis settings for the frame are given by the values of _diffrn_scan_frame_axis.displacement, *.displacement_increment and *.displacement_rstrt_incr. The integration begins at the setting given by the value of _diffrn_scan_frame_axis.angle or of *.displacement. The *_increment value gives the change of axis setting during the scan. At the end of the integration, the axis may need to be repositioned by an additional amount. That amount is given by *_rstrt_incr.