Thermal vacuum and balance test of the ESA Solar Orbiter Instrument PHI
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PHI (Polarimetric and Helioseismic Imager) is one of the remote sensing instruments which will fly on board of ESA Solar Orbiter mission. It consists of two entrance filters, an optics unit, an electronics unit and an external harness that links both units. Prior to the instrument delivery to the Spacecraft prime contractor (Airbus UK), PHI was subjected to a thermal vacuum cycling test, which is described in this paper. The test had a threefold objective: the acceptance test of the optics and electronics unit, the thermal model correlation of the optics unit, and the spectral calibration of the instrument. The PHI optics unit is an internally mounted insulated unit, thermally coupled to the spacecraft only through dedicated interfaces and optical apertures towards the Sun, whereas the electronics unit is strongly coupled to the spacecraft through its base. These characteristics required the control of up to 12 interfaces, including the thermal vacuum chamber shroud. For the spectral calibration, it was needed to feed the instrument with natural sunlight. This was done by means of a coelostat that drives the sunlight through the building inside the thermal vacuum chamber. Due to geometrical distribution of baseplate and viewports in the chamber, it was necessary to mount the whole PHI instrument on a high stand 0.5 m above the thermal vacuum chamber baseplate, which was used as a general heat sink for the controlled interfaces. Finally, the entrance filters were monitored through infrared cameras, which could see inside the chamber through dedicated Zinc-Selenide viewports and a mobile stainless steel flat mirror.