The Diffractometer Alignment Widget

The following figure displays the Diffractometer Alignment widget. The purpose of this widget is to find reflections by varying the diffractometer angles. If a reflection has been found, it can be stored. The UbMatrix widget uses reflections for the orientation of the crystal.

Figure 8.17: The Diffractometer Alignment Widget

The explanation of the elements:

 Options

 Alignment params
Invokes a widget that selects two parameters:
Peak definition
CMS, y-max or Midpoint.
CV_RATIO
Tells Online when the centering procedure is terminated. The conditions is abs( posMax - posNewMax)/width < CV_RATIO.

 hkl2Angles
A widget that displays the angles, if hkl is supplied.

 Select Signal
A widget which selects the signal.

 Misc

 UbMatrix
Invokes the UbMatrix widget.

 Cursor
Invokes the cursor widget.

 Angles
This frame displays the current angular positions and allows the user to enter new values in the entry fields. The move is started by the 'Move' button.

 hkl
This frame displays the current position in momentum space and allows the user to enter new values in the entry fields. The move is started by the 'Move' button.

 Scans

 Exec Scan
The buttons in this line start various angular scans.

 Range
Defines the range of the angular scans. The range can be enlarged, if no peaks are found during the search.

 To max
After angular scans have been executed the buttons in the line move the corresponding axis to the maximum.

 Centering order
Tell Online in which order the angular scans are executed during the centering procedure, e.g.: OCT means that omega is optimized first, then chi, then two-theta.

 Np
The no. of points in the angular scans.

 N_iter The maximum number of iterations that are used to find the center of a reflection. One iteration includes the optimization of omega, chi and two-theta, if selected. The centering procedure terminates earlier, if there are no significant shift between two consecutive iterations.

 Coarse Scans
The scanning interval is not adjusted.

 Stop
Stop all movements.

 Move
Executes a movement. The destination is supplied on the 'Angles' or 'hkl' frame.

 Center reflection
Starts the centering sequence.

ONLINE performs angular scans to find the maximum of a reflection. The order in which the angles are changed is determined by 'Centering order'. Every angle undergoes a coarse scan first. The coarse scan is used to determine the scan range (2$\times$FWHM). If the fine scan range is less than 50% of the old one, ONLINE repeats the angular scan before it changes the next angle.

If the signal does not go down to half of the maximum, the scan range is doubled. The maximum of a scan is defined to be the CMS, y-max or Midpoint depending on the selection of the 'Alignment Parameters. If the angular scans pass the convergence test, the program terminates. Otherwise the fine scans are repeated N_INTER times.

 Store reflection
Copies the current position, hkl and angles, to the diffractometer server. Reflections are used to orient the UB matrix.