Table 4: Astrometric corrections for the Mauna Kea post-encounter astrometry. These corrections are obtained by extrapolating to the limit of zero-sized aperture and were added to the positions reported to the MPC. the encounter (see Table 3). To quantify the discrepancy between the pre-encounter solution and the post-encounter data, we measured how much extending the data arc after the encounter changes the orbital solution. As shown by the fourth row in Table 5, there is a statistically unacceptable > 10 σ correction from the pre-encounter best-fit solution to the full arc best-fit solution. The observed discrepancy points to the need of refining the nongravita- tional perturbation model. The Marsden et al. (1973) model does a good job at fitting the pre-encounter observations of C/2013 A1, actually none of the other models discussed below improves the fit to the pre-encounter arc. How- ever, this model proves inadequate as the trajectory is further constrained by post-encounter data. 3.3. Dynamical models 22 Date East bias North bias 2015-02-17 0.175 ′′ 0.025 ′′ 2015-02-26 0.175 ′′ 0.00 ′′ 2015-02-27 0.16 ′′ 0.00 ′′ 2014-03-19 0.145 ′′ 0.04 ′′ 2015-03-20 0.155 ′′ 0.035 ′′ 2015-03-24 0.13 ′′ 0.045 ′′ 2015-03-26 0.145 ′′ 0.055 ′′ 2015-04-22 0.10 ′′ 0.12 ′′ 2015-04-28 0.06 ′′ 0.125 ′′