Re-commissioning of the VLT/UT4 telescope after the Deformable Secondary Mirror installation

During the 2nd semester of 2016, a new Deformable Secondary Mirror (DSM), part of the Adaptive Optics Facility Project at Paranal Observatory, was installed on the Unit Telescope 4. Starting end of November 2016, we then re-commissioned the telescope and the following three instruments : HAWK-I, MUSE and SINFONI. It is important to understand how a DSM, even if used in non-adaptive optics mode (i.e., in its rest/flat position), can impact the operations and the quality of the observations. We discuss here the results of this telescope and instrument re-commissioning, the challenges and problems we met, and compare the new performance to the ones obtained previously with the traditional Dornier Secondary Mirror.