The Correction to Absolute Proper Motion in SPM 2.0

The SPM proper motions are derived in a two-step process:  Relative proper motions are determined using numerous faint, anonymous stars to tie together the 1st- and 2nd-epoch plates.  Then, all well-measured galaxies in the field are used to determine a constant offset that, once applied to the relative proper motions, will force the (weighted) mean motions of the galaxies to zero and, hence, yield absolute proper motions.

The scarcity of measurable galaxies in the Zone of Avoidance precludes the second step for fields near the Galactic Plane.  For this reason, fields near the Galactic Plane are, in general, not included in SPM 2.0, but will be left for a future version of the Catalog.  The one exception to this is Field 576, (r.a. = 16^h 20^m,  dec. = -25°  B1950), which we nevertheless include due to its interesting nature, (the field contains three separate globular clusters).  For this field, and this field only, the correction to absolute was determined using the ~100 Hipparcos stars present in the field and forcing their mean motion to match that of the Hipparcos Catalog.  The absolute proper-motion system for the remainder of the SPM 2.0 is based on external galaxies and is independent of the Hipparcos proper-motion system.

There are a handful of fields, those nearest to the Galactic plane, which also have a relatively small number of measurable galaxies and these have been examined on an individual basis to determine if the derived correction to absolute required adjustment.  In these cases, comparison with adjacent overlapping fields was used to make any such adjustment.