The DISCO beamline at SOLEIL synchrotron radiation facility [1] is a low energy beamline (1-20 eV) with three end-stations. The imaging branch is the lowest energy of the three, ranging between 1 and 5 eV [2], for which ultra high vacuum is not mandatory. Therefore, the branch is separated from the beamline vacuum by a DN63CF suprasyl window protected for colour centre formation by suprasyl plate moveable under vacuum. In order to maximise the reflectivity in the photon energy domain of interest, all the mirrors specific to the branch have been coated with Al-MgF2. Commissioning and first year of exploitation were performed under primary vacuum conditions in the imaging branch line. During that period, the optics got heavily polluted, which induced a strong loss of photon flux. Al-MgF2 optics cannot be cleaned using oxygen plasma without heavy damages and loss of reflectivity. Therefore, the mirrors had to be removed and cleaned ex situ. Following this maintenance, the branch had been re-exploited under high vacuum conditions assured by a turbomolecular pumping to provide what we expected to be better protection of the optical elements. Although the very first photometric measurements showed a gain in flux, very soon the photon flux starts decreasing.