Success of long-term preventive maintenance on telescope subsystems using the example of the VLT adapter-rotators at the ESO Paranal Observatory

More than 11 years have passed, since the first of the four Unit Telescopes of the VLT on Cerro Paranal has entered into operations. To keep four such complex telecopes at a high level of availability with only around 3 percent of technical down time does not only depend on a good and robust design and manufacturing process, but long term also on a sound preventive maintenance plan and program. In this paper the Instrument Adapter-Rotators, twelve of which are installed at the observatory, have been chosen to show how a preventive maintenance plan has been developed, implemented and executed and what the results are. In the first part the most common problems are shown and some larger interventions are described and listed. It explains the tests that have been developed to follow the status of the systems by measuring key parameters such as position error, motor current, torque and encoders status in order to detect at an early stage any degradation in performance parameters. Depending on the test results preventive actions can be planned well ahead of serious failures, making optimum use of scheduled technical time periods and consequently reducing loss of observing time. Finally some statistic charts show how problems have been reduced as a result of the preventive maintenance plan