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Long-term performance of the VLT UT active optics system

For large telescopes, like the Very Large Telescope (VLT) unit telescopes, it is compulsory to use an effective and reliable Active Optics system in order to guarantee the optimal optical performance. The active optics ensures that the actual wavefront aberrations introduced by the telescope itself are kept as low as possible. In order to evaluate the longterm performance of this system, an extended timeseries data analysis for all four unit telescopes (UT) was performed. The results presented in this paper demonstrate that the VLT UT active optics system works very stable and reliable with no significant performance degradation over time.