Data products: General - How good are the proper motions?
The proper motions are calculated during target selection. Target selection
provides proper motions by matching each SDSS object against USNOA2.0. By
the rules of target selection, each SDSS object must be processed
independently of other SDSS objects, which forces us to a very simple
matching algorithm. The closest object within 30 arcsec is taken as the match,
and proper motions are calculated using that match. Compare the
description of the astrometric quality and
the astrometry
algorithm
The accuracy of the matching (that is, how many SDSS objects were matched to
the correct USNOA2.0 object) has been calculated by comparison with
a more sophisticated matching algorithm. The following table summarizes the
results for a single scan (scan 752):
inner | outer | Stars | nGood |
---|
(asec) | (asec) | % | % |
0 | 1 | 82.1 | 99.9 |
1 | 2 | 14.3 | 97.1 |
2 | 3 | 2.3 | 82.1 |
3 | 4 | 0.8 | 69.6 |
4 | 5 | 0.4 | 64.6 |
5 | 6 | 0.2 | 60.8 |
The columns in the table are:
inner | - minimum matching radius (arcsec) |
outer | - maximum matching radius (arcsec) |
nStars | - Percentage of all stars matched by target selection over
this range of radii. |
nGood | - Percentage of stars in this matching annulus which were
correctly matched against USNOA2.0. |
Thus, 99.9% of all objects which matched within 1 arcsec were correctly
matched. The number drops only slightly to 97.1% for objects
matched between 1 and 2 arcsec. Thus, out to proper motions of
4-5 arcsec/century (depending on the epoch of the POSS plates), representing
96% of the stars, the contamination of false detections is only 2-3% for those
objects with measurable proper motions (greater than about 1.5 arcsec/century).
The contamination increases to about 35% for the fastest moving 1% of the
objects.
The accuracy of the SDSS positions are better than 100 mas, those of USNOA2.0
about 150 mas, and the difference in epoch about 50 years. Thus, a proper
motion of 1.44 arcsec/century is a 4 sigma detection.
Last modified: Tue Mar 18 18:03:50 CST 2003
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