Finding out ORGX ORGY
The mathematically correct definition of ORGX ORGY is: "the point of the detector (in pixels) closest to the crystal". However, in practice (namely for the standard setup of a beam perpendicular to the detector plane) the best way to come up with good values of ORGX ORGY is to find out where the direct beam would hit the detector.
There are different possibilities to find out where the direct beam would hit the detector:
- visualize BKGINIT.pck (from INIT), or FRAME.pck (from COLSPOT) with VIEW, and click (left-mouse) into the middle of the beamstop shadow (or the attenuated direct beam itself, if it is recorded). Use the pixel coordinates displayed by VIEW as ORGX ORGY (in principle you should add 1 to both numbers, because for VIEW the pixels go from 0 to NX-1 whereas for XDS they go from 1 to NX).
Instead of these files written by XDS, one could directly use a measured frame. However this requires that one enters NX, NY, and the size of the header (which usually is <size of frame in bytes>, minus 2*NX*NY).
- use adxv for visualization. Otherwise the same as with VIEW. (works for the PILATUS detector at SLS)
- use MOSFLM for visualization. It prints out X BEAM and Y BEAM from the frame header, and you may click on the hypothetical direct beam position. However, x and y are swapped in MOSFLM when compared to XDS, and the coordinates are in mm, not in pixels (so one has to divide by QX). Sometimes the X BEAM and Y BEAM from the header are not reliable.
- use ice rings to find out (in VIEW, and with paper and pencil) where the direct beam would be. This should be rather accurate but may be tedious.
- use the Unix command line tools to look at the header of the frame. E.g. "strings <frame-name>|more" would should you the (hopefully correct) information stored in the header, like wavelength, delta-phi, X and Y beamcentre and so on.
The error you make in the determination of ORGX ORGY should in principle be less than half of the distance between two spots. If you have a reasonable estimate (from one of the methods above), but the error is bigger than that, you can still find out the true ORGY ORGY by inspecting IDXREF.LP . This works best if COLSPOT has seen a significant fraction of all frames.