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."
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").
The problem is that
a) beamlines do not usually document this on their webpages, and sometimes
change it without notice
b) the headers of the frames usually do not document this (exceptions are
d*trek and Nexus headers, it seems)
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.
So when a dataset from an unknown beamline is processed, both possibilities
need to be tried, and one of them then turns out to be correct. Sorry that I
did not earlier think of this, in your case. Usually, the beamline support
provides suitable templates for XDS processing.
best,
Kay
