SPM-Hipparcos Proper-Motion Differences

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 >10,000 stars in common in the SPM 2.0.  The differences, in the sense SPM-Hipparcos, are shown above.  The mean differences in µ_RA and µ_DEC are +0.17, +0.43 mas/yr, with standard deviations 3.3, 3.2 mas/yr.  The offsets, particularly in µ_DEC, are larger than expected considering the ~0.2 mas/yr uncertainty in the Hipparcos absolute system.  Note that the differences are highly correlated for stars on the same SPM field and the field-to-field offsets are most likely due to the error in each field's correction to absolute proper motion.  (Therefore, any estimate of the quantitative significance of the non-zero mean of [SPM - Hipparcos] should be based on the number of fields and not the much larger number of stars in common.)