Rough Calculation of Maximum Delta-Phi

(oscillation angle for data collection)

With longer cell axes, reflections that are in a line almost parallel with the X-ray beam can become overlapped on one image as the crystal is rocked, if the oscillation angle is too large. The true maximum oscillation angle (delta-phi) depends on the orientation of the crystal because the longest cell axis may not be oriented in such a way that it affects the spacing of reflections in "phi-space". After taking a single frame of data, the program strategy can be run to predict the optimum oscillation (phi) range and starting point, but it does not predict the maximum delta-phi. Mosflm on the other hand can calculate both the optimal oscillation starting point, range and angle. The optimum delta-phi can vary with the actual phi value (crystal orientation). For those that use HKL and strategy, I offer the following simple calculation.

Delta-Phi is calculated using the formula:

dphi = tan^-1(res/cell) - mos, where
res = the high resolution limit
cell = the longest cell dimension that is not parallel to the rotation axis
mos = the estimated mosaicity of the crystal

In the case of the R-Axes (sic), the vertical axis is actually horizontal.

Enter values below:
resolution	Å	cell length	Å	mosaicity	degrees

Delta-Phi:	degrees	Delta-Phi:	degrees

To Do Additional Calculations: Change the appropriate values above and press CALCULATE again.

Last revised: Thursday, 25-Apr-2002 13:21:24 EDT