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There were 3 faults, spread over 5 fault reports, causing 2 hours or more downtime during period 51.
Due to a grism not being installed properly, the wheel got stuck when observations were started. This has occurred before and requires manually turning a shaft which moves the wheel, and which takes a fairly large amount of time. However, beyond identifying the problem itself, most time was lost in finding the detailed description how to remedy the problem in the fault data base. To avoid this in the future, the specific instructions what to do if this occurs has been added to the general trouble shooting information, but also the facility to search in the data has been upgraded to allow for a more targeted search.
During an unusual lightning storm in the summer, the general electrical power line to the observatory was damaged. Due to the very bad weather, it took several days for the local electricity company to repair the damage and restore power. This included a significant amount of time which would normally have been used for observing, which was reported as downtime. This is not a specific problem with any part of our observing system, and as such should in principle not be considered as part of technical downtime (I understand that at other telescopes this has been counted as weather downtime). However, it does affect the use of the telescope, and I have included it in the general overview given in the Table 2, and also shown the effect of including this downtime in Table 1. It should also be noted that if our technician would not have been on sick leave, we likely would have had our passive cooling system fully automated, and we would have been able to observe normally just using our emergency power generator.
Some problems where found with the optical quality of imaging data, showing varying distortions of the star images, not allowing the telescope to be focused properly and effecting observations. It become soon clear that the problem was with the adaptive optics system of the main mirror, but which part (electronic, mechanic or pneumatic) of the system was not clear. Some first adjustments of the system remedied the problem somewhat, but only after consultation with our technician the problem was localised to the individual bellows which support and shape the main mirror. A full check of all the bellows revealed that the pressure regulators of 2 bellows were failing, and they were replaced. These are only the second and third of the regulators which have failed since there installation, and it is actually suspected that one of the regulators already failed earlier, explaining some of the problems we encountered since re-installing the mirrors after their aluminisation last year.
One of the issues in identify the problem was that the pressure regulators do have a sensor to check if they work, but these sensors are very unreliable, and have been deactivated for a long time. We could replace the regulators, but they are rather expensive and replacing them would require a lot of work, and telescope downtime. To avoid future problems, a system has now been set up to regularly check the pressure regulators manually.
Thomas Augusteijn 2016-05-05