Reflectivity

The last aluminisation of the mirrors in 2009 did see an improvement in reflectivity, but lower by at least 5% compared to previous aluminisations. This effect seems to be limited only to M1. We are seriously considering if a new aluminisation of M1 should be made. An analysis of ALFOSC zero point measurements suggest that the reflectivity in the V, R and i-bands are still similar to the ones measured just after the last aluminisation, while in the U- and B-bands the reflectivity has gone down by a few per cent. The plan is to do a proper washing of M1 and measure the reflectivity of the mirrors directly with a meter owned by the ING (this also includes a measurement of the roughness of the mirror) before coming to a definite decision about aluminising M1 in the coming summer.