The Herschel Space Observatory Telescope is the first of its kind to cover the 60-670 μm far infrared spectral band. Its optical characterization, performed in the visible range, was a true technological challenge requiring very large dynamic range coupled to very high accuracy. A specific Hartmann Wavefront Sensor (HWFS) was designed to meet the demanding specifications of the measurement.

The metrological system used by the EADS Astrium team to characterize the silicon car-bide based telescope will be presented as well as the main features of the specifically developed HWFS. The large expected wavefront error was measured in a double path set-up using the HWFS positioned in an extra-focal plane and a point source in the focal plane. The auto-collimation was carried out thanks to several liquid mirrors covering the M1 pupil plane and located in the conjugation plane of the HWFS sub-apertures.

The results on the wavefront error obtained at the Centre Spatial de Liege (CSL) in Belgium will be shown as well as the simulated Point Spread Function to be compared to the real PSF obtained during on flight measurements. The thermally induced focal length variations are also presented as the telescope is meant to operate at 70°K in space.