Protein Function Discovery
and Department of Biomedical and Molecular Sciences Molecular Modelling and Crystallographic Computing Facility 


Calculation of DeltaChi (optimal chi offset for oscillation data collection)In order to collect a complete data set, the crystal should be positioned, such that the cusp (the angular region of reciprocal space that does not cross the Ewald sphere during rotation of the crystal) does not lie too close to a crystallographic axis. After taking a single frame of data, the program strategy can be run to predict the optimum oscillation (phi) range and starting point.If strategy suggests that complete data cannot be obtained, even with a 360degree rotation of the crystal, then look at the values for rotz and roty. It is likely that they are too close to 0 (zero), or too close to 90 degrees if the axis defined in the spindle command in the denzo integration input file is not really aligned along the spindle. In the latter case it might be useful to use xdisp to determine which axis is closest to the spindle axis (pick spot predictions in the zoom window to see the reflection indices). The following calculation is an attempt to define what the minimum offset of the crystal should be from the fully aligned position. If the value for deltachi is less than the ideal calculated chi value, which is determined only by the maximum resolution, then it would be advisable to try to adjust the arcs of the goniometer head in order to tilt the crystal away from its alignment. Chi is calculated using the formula: chi = sin^{1}(lambda/(2*res))and DeltaChi is calculated using the formula: dchi = cos^{1}(cos(roty)*cos(rotz)), whereIn the case of the RAxes (sic), the vertical axis is actually horizontal. Last revised: Tuesday, 16Sep2003 10:34:49 EDT 