Sensitivity of the HTPM proper motion in right ascension to radial velocity for HIP 70890 (Proxima Centauri). The sensitivity is linear and has a value C_α = −74.59 μas yr^-1 per km s^-1 (Sect. 4.3). The dashed horizontal line indicates the maximum-tolerable perspective-acceleration-induced proper-motion error caused by an incorrect radial velocity. Since the expected HTPM standard error in right ascension is 97 μas yr^-1 for this star, we set this threshold to 97/10 = 9.7 μas yr^-1. This implies, for a confidence level c = 68.27%, that the maximum-acceptable radial-velocity standard error σ_vr for this object is  km s^-1.