![]() ![]() ![]() As the astrometric accuracy of the SDSS is about 0.1” even for relatively faint stars, we can consider it essentially an “absolute” reference for our purposes (Pier et al. We only used a NUV-selected sample as the final astrometry for each image in the GALEX pipeline was determined using the positions of stars on the NUV detector. These stars are bright enough such that the statistical errors in their positions are small. The resulting sample consists of 9,000 stars in the MIS and 24,000 in the AIS. We have further restricted the sample to stars detected in the NUV with aperture magnitudes in the range 15 < NUV 18, r-band PSF magnitude errors less than 0.05, PSF r-band magnitudes less than 20, and that are not saturated in the SDSS. We have matched sources using a 4 arcsec maximum search radius from the Medium Imaging Survey (MIS) and All-sky Imaging Survey (AIS) in the GALEX GR6 data release with objects identified as stars in the SDSS-DR7 catalogs (type = 6). This section briefly describes the absolute astrometric uncertainties of GALEX data. However, at the bright end, systematic uncertainties in the positions dominant the errors. For faint sources, this is the dominant source of error. The positional errors listed in the GALEX photometric catalogs are the errors in the coordinates due simply to counting statistics. ![]() This chapter describes in more detail the astrometry GALEX of images, the GALEX point spread function, and the various artifacts present in GALEX images. ![]()
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