Independent Determination of the Systematic and Random Errors of the Proper Motions in the PPMXL Catalogue Using Quasars
Using a sample of 117,053 quasars identified in the PPMXL Catalogue, the systematic errors of the proper motions in the PPMXL Catalogue are estimated independently as � 2:0 and � 2:1 masyr � 1 in μα cos δ and μδ, respectively. For objects with SDSS r magnitude between 16.0 and 21.0 mag and SDSS gr color between � 0:4 and 1.2, there is no obvious magnitude and color dependence in the systematic errors of the proper motions in the PPMXL Catalogue. The random errors of the proper motions in the PPMXL Catalogue change from ∼4:0 to ∼8:0 masyr � 1 , along with the increase of the magnitude of the objects. No color dependence in the random errors of the proper motions in the PPMXL Catalogue is found. There is obvious right-ascension α dependence of the systematic errors in both components of the proper motions in the PPMXL Catalogue. For quasars with declination δ > 0, the systematic errors of μδ in the PPMXL Catalogue increase linearly with δ with a slope of 0:07 masyr � 1 deg � 1 . For comparison, using a subsample of quasars identified in both the PPMXL and SDSS DR7 catalogs, the systematic errors of the USNO-SDSS proper motions in the SDSS DR7 Catalogue are derived as 0.9 and 0:5 masyr � 1 in μα cos δ and μδ, respectively. There is no obvious magnitude, color, and α dependence in the systematic errors of the USNO-SDSS proper motions. For quasars with declination δ > 0, the systematic errors of μδ in the SDSS DR7 Catalogue increase linearly with δ with a slope of 0.03. The random errors of the USNO- SDSS proper motions change from ∼2:5 to ∼6:0 masyr � 1 , along with the change of the magnitude and color of the objects. Combining the 2MASS or SDSS observation data can significantly reduce the systematic errors of the proper motions in the initial photographic catalog.