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 : { \sim 0.17 \mathrm { ~ { } mas } } in position ( in 2009 ) and { \sim 0.07 \mathrm { ~ { } mas~ { } yr } ^ { -1 } } 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 .