Absolute energy calibration for plane grating monochromators

This report describes a new method for monochromator energy calibration, which is based on the freedom to change the fix focus constant cff in the plane grating monochromator at BESSY. If the monochromator is not perfectly aligned, spectra taken at different cff-values show energy shifts (of 1-2%). These can be attributed to offsets in the grating angles. Measuring spectra at different cff-values, one can determine these offsets by employing a fitting procedure. If these offsets are inserted, cff variation measurements lead to relative energy scale changes of less than a few 10-4