The Helmholtz Zentrum Berlin HZB builds a 2m long in vacuum Hall probe measuring bench for the characterization of several in vacuum cryogenic undulators currently under development. Aceurate local magnetic measurements need a positioning control of about 5 Jlll. Fabrication tolerances and potentially strong temperature gradients require an active correction of the Hall probe movement along a straight line. The HZBbench employs a system of Iaser interferometers and positionsensitive detectors, which are used in a feed back loop for the Hall probe position and orientatio
