Photocenters were computed using synthetic aperture radii ranging from 2.0 to 6.0 pixels in steps of 0.1 pixels, so the extrapolation from 2.0 to 0.0 pixels represents 50 percent of the range actually fit. An average value for the tailward bias was computed from the trend seen in multiple exposures on each night (see Table 4). By default, the symmetric fitting function computes the background level from an annulus surrounding the synthetic aperture with inner and outer radii of 2.5 and 5.0 times the radius of the synthetic aperture, respectively. For a pixel size of 0.439 arcsec, the background annulus was superposed on the coma of the comet, so some of the light from the comet was being subtracted from the area inside of the synthetic aperture. As such the reported photometry underestimates the true brightness of the comet for that aperture size. However, the goal here was accurate astrometry, not accurate photometry. We conservatively weighted these observations at 0 . 3 ′′ to 0 . 5 $^{′′}$, depending on the size of the bias corrections and the number of observations in the same observing night (Farnocchia et al., 2015). Though solution 101 provided successful ephemeris predictions for the HiRISE close approach observations of C/2013 A1, our attempt to extend the data arc revealed a clear discrepancy between the post-encounter data and the orbital predictions. For instance, when fitting A$_{1}$ , A$_{2}$ , and A$_{3}$ to the full data arc, we find A$_{3}$ = ( − 0 . 77 ± 0 . 15) × 10 − 9 au/d $^{2}$, which is not statistically compatible with the much stronger negative acceleration measured before 3.2. Discrepancy between pre-encounter orbit and post-encounter data 21