Mirror cleaning

The mirror has been washed and has been CO$_2$ cleaned a few times. In one case, some bird droppings had to be removed. Although the CO$_2$ cleaning provides some improvement, the washing seems to work much better. However, washing the mirror is a more complex operation while it is also more dependent on the weather and in the long run we intend to make this easier and less work intensive such that little time and planning is needed.

It has been found that the overall efficiency of the secondary mirror has decrease significantly by $\sim$1.5-2% with respect to a test mirror and we are considering to aluminize this mirror in the coming summer. However, the long-term variation in reflectivity does not show a well-defined trend and we plan to do a washing of M2 and look at the result before deciding about a possible aluminisation.