The X- and Y- centers mark the coordinates of the center of the iamge. In SAXS mode this is the position where the direct beam would have hit if there were no beamstop, and it is indicated by an X on the false color image. In WAXS mode it is the position where the beam would hit if Detector 2Theta were zero, and it is indicated by an arrow pointing in the direction of increasing 2theta. For a detector with N x N pixels, the X-Center and Y-Center values are often somewhere close to (N/2, N/2), although sometimes data are taken in a mode where the beam zero is close to the edge of the detector, and sometimes the beam zero is completely off the detector.