  In the context of the Large Binocular Telescope project, we present the
results of force actuator calibrations performed on an adaptive secondary
prototype called P45, a thin deformable glass with magnets glued onto its back.
Electromagnetic actuators, controlled in a closed loop with a system of
internal metrology based on capacitive sensors, continuously deform its shape
to correct the distortions of the wavefront. Calibrations of the force
actuators are needed because of the differences between driven forces and
measured forces. We describe the calibration procedures and the results,
obtained with errors of less than 1.5%.
