Metrology Characterization of the PLATO Telescope Optical Units

PLATO (PLAnetary Transits and Oscillation of stars) is the ESA M-size mission aimed to the discovery of exo-planets hosted by bright, nearby stars. The launch of PLATO spaceship is scheduled for 2026. It will orbit Sun-Earth L2 for 4 years, looking towards deep space and covering a wide (2132 deg 2 total) area of space. To do so, PLATO will use 26 fully dioptric designed cameras, each composed of a telescope optical unit (TOU) and a focal plane array (FPA). For manufacturing purposes, the FPA will be assembled at ambient temperature, although the best image plane (BIP) of the TOU will be determined during test at cryo-vacuum conditions (-80 degrees C). This poses a metrology challenge at TOU manufacturing and testing and at FPA integration facilities. To this extent, the procedures for the precise characterization and localization of the BIP during TOU tests are presented. In order to circumvent the thermal effects of contraction of the fixture, which in part operate at ambient and in part are cooled at -80 degrees C (operation temperature), the final measurement will be derived by the repeated assessment of the coordinates of a set of fiducials at ambient and operation temperature. Compliance with a strict error budget is achieved by using a combination of sensors (confocal, focus-variation), mounted on very precise and repeatable hexapods. Hartmann masks are also used to co-register the position and orientation of mounting of the FPA between facilities.