The EUV detector covers the 55-155 nm wavelength range. A CsI (Cesium Iodide) photocathode is used in order to remove at best stray-light from longer wavelengths.
This CsI photocathode is sensitive to ambient air and moisture. For this reason, a small vacuum chamber is built around the EUV detector focal plane. Until the launch, the focal plane was maintained under vacuum (less than 1.10-1 mbar) thanks to a pumping ground system equipment developed by LATMOS PHEBUS team.
Moreover, considering the fact that no material is transparent at the very short wavelengths of the EUV range, the EUV chamber is equipped with a MgF2 (Magnesium Fluoride) window which shall be open at least four weeks after launch. The door will be open thanks to a one-shot mechanism (similar to the ALICE instrument case on Rosetta). This action is expected to be done on December 7th 2018 by activating an actuator (Starsys) that will unlock the door.
Note : The FUV detector is very similar to the EUV one and operates within the range 145-315 nm. As the EUV detector, the FUV detector is maintained under vacuum. As the MgF2 is not opaque at the FUV wavelengths (above 115 nm) the FUV window is made of that material and seals the vacuum chamber without requiring neither a mechanism to open it nor a pumping system. A solar blind photocathode of CsTe (Cesium Telluride) is used in order to protect the detector against stray light.