Inverse beam

Question: We are trying to automate XDS processing for data being collected at the beamline. We see a lot of people collecting data using inverse beam anywhere in the range of 90 to 120 degrees for the first wedge with the starting angle say zero. A corresponding inverse wedge with the above mentioned range gets collected starting at frame 181 (they use 1° frames). So now they have two wedges of data for the same crystal but with missing frame numbers and corresponding missing wedges of data. How does XDS handle such situations?

Answer: as long as XDS can correctly calculate the phi value from the frame number, processing works "out of the box", which means with a single XDS run you can process both wedges. XDS will print out an error message for each missing frame, but these messages can simply be ignored - there is no harm associated with them.

But this _requires_ that there is e.g. no "off-by-one" problem (second wedge should start at frame 181, not at frame 180), or other offset! If you collect 0.5° frames, of course the second wedge should start at framenumber 361.

In case the frames of the second half have frame numbers which do not fit, then an easy way is to create a directory with symlinks to the frames of both wedges, and to give the symlinks the appropriate frame numbers. Of course this needs a script!

In general, however, there is no real advantage (that I can think of) to a _single_ XDS run; you might as well have two runs and merge the resulting files together, using XSCALE. Use REFERENCE_DATA_SET=<firstwedge_xdsdirectory>/XDS_ASCII.HKL for the second wedge; that ensures consistent indexing.