Hi Kay, On Tue, May 12, 2015 at 08:39:24PM +0100, Kay Diederichs wrote: > According to > http://homes.mpimf-heidelberg.mpg.de/~kabsch/xds/html_doc/xds_parameters.html#ROTATION_AXIS= > the definition of a positive value in the ROTATION_AXIS= line is: > > "When looking along the axis, the crystal would rotate clockwise > when proceeding to the next data image."
I find the even better description in that part of the XDS documentation is The direction of the axis is chosen to describe a right-handed rotation. which should follow the right-hand rule a la http://en.wikipedia.org/wiki/Right-hand_rule The "looking along the axis" doesn't clearly define if it is (a) looking from the crystal towards the base of the rotation axis or (b) from the rotation-axis base towards the crystal. > There is of course no right or wrong when it comes to choosing the > direction of rotation. However, conventionally the sense of rotation > is positive; only a small minority of beamlines needs a -1 ("reverse > phi"). Yes: one can always define the rotation axis without the need for a '-1'. But this has an impact on the chosen lab coordinate system and therefore might require a change of INCIDENT_BEAM_DIRECTION= and/or DIRECTION_OF_DETECTOR_{X,y}-AXIS= ... and there might be good reasons for having those defined in a particular way (eg. to avoid a negative value for DETECTOR_DISTANCE= or to have them aligned with the fast/slow changing axis of the image array as X and Y). > The problem is that > a) beamlines do not usually document this on their webpages, and > sometimes change it without notice Indeed. Most beamlines are quite good in providing up-to-date XDS.INP templates that are known to work with data collected on that beamline. Ideally, the {X,Y}-GEO_CORR files for Pilatus detectors should also be placed in a public space (and everything else that might be required). All so that users can process the data again once they are back in the home lab with more time and less stress - trying to reproduce what happened by the automatic processing systems installed on most beamlines (and through that task especially new users will actually learn what entails good data processing practice). > Personally, I wish that beamline designers would be aware of the > potential problem for users; I suspect they often are not. Life > would be easier if all beamlines would use the same convention, and > I'm pretty sure that spindle motors can be > produced/bought/programmed for both directions. Sometimes there are restrictions upon beamlines regarding the choice of coordinate system to be used: this often has to be identical for everything and all beamlines at a given synchrotron. Reaching perfection in an imperfect world ;-) Cheers Clemens -- *************************************************************** * Clemens Vonrhein, Ph.D. vonrhein AT GlobalPhasing DOT com * * Global Phasing Ltd. * Sheraton House, Castle Park * Cambridge CB3 0AX, UK *-------------------------------------------------------------- * BUSTER Development Group (http://www.globalphasing.com) ***************************************************************
