Data products: General - Proper Motions

In DR1, proper motions were calculated by matching SDSS positions against USNO-A2.0 (Monet et al. 1998). USNO-A2.0 provides positions at a single epoch; no proper motions are provided. Thus, for each SDSS object, the closest matching object in USNO-A2.0 was assumed to be a valid match and the proper motion calculated from the pair of detections, with a typical time baseline of 50 years. For motions greater than roughly 40 mas/year, corresponding to separations between the SDSS and USNO-A2.0 positions of greater than 2 arcsec, contamination by false matches becomes significant and rises with increasing motion/separation.

For DR2, the SDSS positions were matched against USNO-B1.0 (Monet et al. 2003), which provides positions and proper motions. Each SDSS object is matched to the nearest USNO-B1.0 object within 1 arcsec, after first converting the USNO-B1.0 positions to the epoch of the SDSS observations. This eliminates nearly all of the false matches, yielding much cleaner samples of high proper motion stars. The USNO-B1.0 proper motion is then given for each matching SDSS object.