SPM-Hipparcos Proper-Motion Differences

SPM-Hipparcos proper motions

As an external check of the SPM proper motions and their error estimates, we compare the absolute proper motions to those from the Hipparcos catalog for the ~2000 stars in common in the SGP region.  The differences, in the sense SPM-Hipparcos, are shown above.  The mean differences in µRA and µDEC are +0.39, +0.76 mas/yr, with standard deviations 3.3, 3.1 mas/yr.  The offsets, particularly in µDEC, are larger than expected considering the ~0.2 mas/yr uncertainty in the Hipparcos absolute system.  This may be due to undetected residual magnitude equation still lurking in the SPM motions.  This is currently under study using additional SPM fields.