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Near-infrared observations of stellar orbits at the Galactic Center provideconclusive evidence for a massive black hole associated with the compact radiosource Sgr A*. The astrometric reference frame for these observations is tiedto a set of red giant stars, which are also detectable at radio wavelengthsthrough SiO maser emission in their envelopes. We have improved the precisionand long-term stability of this reference frame, in which Sgr A* is localizedto within a factor 5 better than previously: ~0.17 mas in position (in 2009)and ~0.07 mas/yr in velocity. This improvement is the result of modeling andcorrecting optical distortion in the VLT/NACO imager to a sub-mas level andincluding new infrared and radio measurements, which now both span more than adecade in time. A further improvement will follow future observations andfacilitate the detection of relativistic orbital effects.

Pinpointing the near-infrared location of Sgr A* by correcting optical distortion in the NACO imager