Uncertainties in Ground‐Based Visual Double Star Measures—<scp>II</scp> Accuracy of Measures

ABSTRACT Letchford and White 2025, Paper I, determined the uncertainties in ground‐based historic measures of double stars. In this paper, the accuracy of 26,205 ground‐based measures of 857 visual double stars, made since 1780, is determined relative to space‐based measures based on positions and proper motion from the HIPPARCOS (via ASCC) and Gaia DR3 missions. We determine the accuracy of measures for the epoch of observation, the size of the telescope, the different attached instruments, and the filter applied. We find that the accuracy of the measures is improving with time; however, the typical telescope used for double‐star observations has been, and continues to be, relatively small. Modern instrumentation such as speckle interferometry and lucky imaging has advanced the accuracy by a factor of ten over traditional micrometre measurements.