tokens: directions of Sgr A* were 0.056 and 0.060 mas yr −1 respectively. Figure 5. Comparison of the stability of the reference frame as measured by the average differences between the positions and velocities in the radio and IR. The agreements of the maser positions (left) and velocities (right) in the IR are compared to those in the radio. The ellipses represent 1 σ , 2 σ , and 3 σ uncertainties. For the Keck/NIRC2 data, the uncertainty in the distortion correction of 1 mas that dominates the IR error was subtracted so as to show the improvement in the methods used in this paper to construct the reference frame. Using the NACO imager on VLT, Plewa et al. ( 2015 ) reports a stability of ∼0.17 mas in position and ∼0.07 mas yr −1 in velocity. Their analysis used eight masers because their mosaicked field covered 42 arcsec × 42 arcsec. However, their maser sample did not include IRS 7. IRS 7 is a supergiant and its SiO maser features originate from a much larger maser emission region than the other masers used (Reid et al. 2003 ) by a factor of ∼2, which is reflected in larger uncertainties in the star’s radio positions and proper motions. To see how much effect this one star has on the IR reference frame, we construct the reference frame without IRS 7. Excluding IRS 7, we obtain combined positional and velocity uncertainties of 0.17 mas and 0.031 mas yr −1 respectively. The velocity stability of our sample of six masers, without IRS 7, is still a factor of 2 better than the one reported by Plewa et al. ( 2015 ). This is because IRS 7 has less influence on the overall reference frame due to the very large errors assigned to its radio velocities. The maser sample used by Plewa et al. ( 2015 ) also had one less epoch of the radio observations. Furthermore, their IR observations included data through 2013, while our IR data extend four additional years. As explained in Section 5 , the IR data and the method used to create the reference frame presented in this paper were modified significantly compared to those used in Yelda et al. ( 2010 ). The comparison of the reference frame of Boehle et al. ( 2016 ) with that of Plewa et al. ( 2015 ) is likely more appropriate, because both use the maser data through 2013. 5. Discussion We have presented an improved astrometric reference frame that utilizes 13 years of IR observations of radio-emitting SiO masers in the vicinity of the Galactic Center. Since our previous work in Yelda et al. ( 2010 ), several modifications have been made in the construction of the reference frame including: (i) using 13 epochs of maser data (2005–2017) instead of six epochs (2005–2010) used by Yelda et al. ( 2010 ); (ii) using an improved PSF-fitting package, AIROPA, instead of StarFinder v1.6 (Diolaiti et al. 2000 ) used in Yelda et al. ( 2010 ); (iii) using a new method to mosaic 9 or 13 dither positions for each epoch of observations. Yelda et al. ( 2010 ) stitched together one field at a time, building up the starlist by one
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