The Online UB Calculation Routine

This module allows to calculate the UB matrix from either two reflections and the cell constants or standalone from three reflections. A little aid for indexing is implemented too. As usual when dealing with calculations the common rule: shit in, shit out holds true. In this case this means that the quality of the UB matrix obtained increases when:

Reflections have been centered as accuratly as possible, i.e. with slits closed and collimators in.
Two theta has been calibrated against a standard beforehand.
The crystal under investigation has been properly centered.
Cell constants and wavelength are accurate.

This module is for classic four circle diffraction only.

A range of commands allows for reflection and data input and calculations. For the following discussion it is assumed that the UB matrix calculation object has been configured into the system under the name ubcalc.

ubcalc ref1, ref2, ref3 h k l two_theta omega chi phi

Reflection input for up to three reflections. Miller indices must be given. When angles are not given, the current position of motors stt, om, chi, phi is read.

ubcalc cell a b c alpha beta gamma

Input of cell constants

ubcalc ub2ref

Calculate the UB matrix from ref1, ref2 and the cell constants.

ubcalc ub3ref

Calculate the UB matrix from the three reflections ref1, ref2 and ref3.

ubcalc listub

Print the calculated UB matrix

ubcalc cellub

Caclulate and print the cell constants as calculated from the UB matrix.

ubcalc activate

Copies the UB matrix to the hkl module. The positions of new reflections can then be calculated with hkl.

ubcalc index two_theta

Makes suggestions for possible miller indices matching two_theta. If two_theta is omitted, the current value of two theta is read from the motor stt. A brute force search through the space of possible indices is undertaken using the cell constants given and the wavelength from the hkl module. This routine is controlled by three parameters within ubcalc:

difftheta, The maximum permissible difference in two_theta
maxindex, The maximum value for miller indices in any direction.
maxlist, The maximum number of suggestions to list.

ubcalc difftheta, maxindex, maxlist value

Inquire or set the above parameters. For inquiry, give no value, for setting the parameter give tha name and the value.