Reflectivity

The main mirror was twice wet cleaned. It first was cleaned in June, but only a few weeks later the mirror got dirty again (spots) when the telescope was not closed in time during rain (the humidity was low and the rain was at the beginning mostly sporadic big drops which did not trigger the rain alarm), and the mirror was washed again in mid-September. In addition, the mirror was cleaned with CO$_2$ three times between the washing.

A more effective procedure was developed to dry the mirror. We do not dry the mirror any more with air from the mirror compressor, but paper is carefully place on the mirror and we use a soft painting roll to press it on the mirror and absorb the water. This procedure is much faster and leaves less water drop marks on the surface.

Some reflectivity measurements were made using a device from the TNG which do not show strong changes from previous measurements but those were made with a different device (which is now broken). Observations of standard stars show scatter of a few percent, but the average zero points in different filters are basically the same as a year ago, and at most a few percent lower than just after the last aluminsation and there does not seem a strong need for a re-aluminisation. A more careful monitoring of the reflectivity will be made using the device from the TNG to get a better feeling of the average value and any change with time.