metric measurements. To minimize the error from coma asymmetries, we used a small 2-pixel astrometric aperture in all cases. Because of the high quality of the HST images, the uncertainties of the astrometric positions were within 0.05 $^{′′}$. However, when fitting these data we found residuals as large as 0.25 $^{′′}$. The cause of these larger residuals is the limited accuracy of the HST ephemeris. The geocentric coordinates of HST that we used are obtained from JPL’s Horizons system (Giorgini et al., 1996) and are derived from the Two-Line Element (TLE) 4 orbit solutions provided by the Joint Space Operation Center $^{5}$. TLEs are not Keplerian elements and are fits to a particular dynamical model called SGP4 (Hoots and Roehrich, 1980). We analyzed the day-to-day variations by comparing the ephemerides of the HST TLE releases between 2014 October 2 and 17. The difference between the analyzed solutions suggest that the accuracy of the HST ephemeris is no better than tens of km. Therefore, we weighted the HST observations at 0.3 $^{′′}$. To extend the observed arc and include astrometry after the Mars flyby, we observed C/2013 A1 from the Mauna Kea observatory from February to April 2015. While the astrometric positions we reported to the Minor Planet Center are from a 2.0 pixel (0.88 $^{′′}$) radius aperture, here we adopted the same linear extrapolation of the trend to a theoretical zero-sized aperture to compute the position of the cometary nucleus as desciribed in Sec. 2.1. 20 $^{4}$http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/SSOP Help/tle def.html $^{5}$https://www.space-track.org/auth/login