Pinpointing the near-infrared location of Sgr A* by correcting optical
  distortion in the NACO imager
  Near-infrared observations of stellar orbits at the Galactic Center provide
conclusive evidence for a massive black hole associated with the compact radio
source Sgr A*. The astrometric reference frame for these observations is tied
to a set of red giant stars, which are also detectable at radio wavelengths
through SiO maser emission in their envelopes. We have improved the precision
and long-term stability of this reference frame, in which Sgr A* is localized
to 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 and
correcting optical distortion in the VLT/NACO imager to a sub-mas level and
including new infrared and radio measurements, which now both span more than a
decade in time. A further improvement will follow future observations and
facilitate the detection of relativistic orbital effects.
