Aluminisation

A lot of effort was spend on preparing and planning for the aluminisation of the primary and secondary mirrors. Special arrangements were made with the ING to limit the amount of downtime of the telescope as much as possible, and it has now been possible to have the mirrors back from the aluminisation plants after 2 nights. The mirrors were aluminized at the beginning of July and during the third night reassembly of the mirrors was finished. During the first observations after the installation of the mirrors significant coma was seen which was found to be caused by an improper alignment of the secondary mirror. However, still some observations could be done and misalignment of the secondary was correct during the following day. Measurements of the telescope aberrations the following night showed everything to be as good as before, possibly with a slight improvement in the amount of coma over the pre-aluminisation results.

Checks of the reflectivety of the mirrors using reflectometers and measurements of the overall efficiency of the telescope using standard stars have shown a clear increase, but the measured increase in efficiency is lower than expected ($\sim$5-10% in the visual and NIR compared to $\sim$15%). However, starting shortly before the aluminisation there have been extended periods of calima and it has been difficult to obtain reliable measurements, but an analysis seems to indicate that the aluminisation itself was not so successful. One specific thing to note is the relatively high level of light scattering that are seen in reflectometer measurements taken some time after the aluminisation which are at similar levels to those shortly before the aluminisation, but that might be related to the calima. We are currently looking to what extend things can be improved through cleaning the mirrors with CO$_2$ and washing.

As a result of dismounting and mounting the mirrors there were some changes in the set-up parameters, in particular to default telescope focus for the various instruments. Both the documentation and the relevant observing scripts were updated. It was noted that the relative focus offsets between the instruments are very stable, and re-measuring the optimal focus for one instrument provided a good indication for the optimal focus for all the other instruments.

It was noted that to check the reflectivity of the telescope mirror (or any other mirror) it is always a hassle to get people from the ING to come with their meter, while our own hand-made meter does not work very well. We will check if we can use and/or share on a regular basis a reflectometer from another telescope (e.g., LT or GTC).