# Finding out ORGX ORGY

The correct mathematical 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 the correct values of ORGY 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:

a) visualize a frame 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 ORGY 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).

b) use adxv for visualization. Otherwise the same as with VIEW. (works for the PILATUS at SLS)

c) 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. Sometimes the X BEAM and Y BEAM from the header are not reliable.

d) use ice rings to find out where the direct beam would be. This should be rather accurate but may be tedious.

The error you make in the determination of ORGX ORGY should be less than half of the distance between two spots. If you have a reasonable estimate, 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.*