This report describes a new method for monochromator energy calibration, wh
ich is based on the freedom to change the fix focus constant c(ff) in the p
lane grating monochromator at BESSY. If the monochromator is not perfectly
aligned, spectra taken at different c(ff)-values show energy shifts (of 1-2
%). These can be attributed to offsets in the grating angles. Measuring spe
ctra at different c(ff)-values, one can determine these offsets by employin
g a fitting procedure. If these offsets are inserted, c(ff) variation measu
rements lead to relative energy scale changes of less than a few 10(-4). (C
) 2001 Published by Elsevier Science B.V.