Since the beginning of the year 2000, we have implemented a program of regular washing (every 6 months) of the primary mirrors of the 4m, 1.5m and 0.9m telescopes. The results are so good that we are now planning to aluminize these mirrors every 4 years only (instead of every 2 years up to now). The 4m M1 is washed in-situ with a system of inflatable seals. We are able to peform the operation in a minimum amount of time (typically 2-3 hours at the 4m) without specific engineering time scheduled. Since August 2000, we are also using an IRIS 908RS device to measure scattered and reflected light from the mirrors (the IRIS replaces the Minolta instrument). In August 2000, we have also installed a grounding cable for the 4m M1 coating in the hope to minimize the accumulation of dust by electrostatic attraction.

Besides, we perform fortnightly C02 snow cleaning of the 4m and 1.5m primary mirrors. The secondary mirrors keep clean much longer and are typically cleaned with CO2 twice per year.