Abstract
We describe a standard star catalog constructed using multiple SDSS photometric observations (at least four per band, with a median of 10) in the ugriz system. The catalog includes 1.01 million nonvariable unresolved objects from the equatorial stripe 82 (|δJ2000.0| < 1.266°) in the right ascension range 20h34m-4h00m and with the corresponding r-band (approximately Johnson V-band) magnitudes in the range 14-22. The distributions of measurements for individual sources demonstrate that the photometric pipeline correctly estimates random photometric errors, which are below 0.01 mag for stars brighter than 19.5, 20.5, 20.5, 20, and 18.5 in ugriz, respectively (about twice as good as for individual SDSS runs). Several independent tests of the internal consistency suggest that the spatial variation of photometric zero points is not larger than ∼0.01 mag (rms). In addition to being the largest available data set with optical photometry internally consistent at the ∼1% level, this catalog provides a practical definition of the SDSS photometric system. Using this catalog, we show that photometric zero points for SDSS observing runs can be calibrated within a nominal uncertainty of 2% even for data obtained through 1 mag thick clouds, and we demonstrate the existence of He and H white dwarf sequences using photometric data alone. Based on the properties of this catalog, we conclude that upcoming large-scale optical surveys such as the Large Synoptic Survey Telescope will be capable of delivering robust 1% photometry for billions of sources.
Highlights
Astronomical optical photometric data are usually calibrated using sets of standard stars whose brightness is known from previous work
SDSS 2.5 m imaging data are photometrically calibrated using a network of calibration stars obtained in 1520 41:5 ; 41:5 arcmin2 transfer fields, called ‘‘secondary patches.’’ These patches are positioned throughout the survey area and are calibrated using a primary standard star network of 158 stars distributed around the northern sky
Typical causes of substandard photometry include an incorrectly modeled point-spread function (PSF; usually due to fast variations of atmospheric seeing or the lack of a sufficient number of the isolated bright stars needed for modeling the PSF), unrecognized changes in atmospheric transparency, errors in photometric zero-point calibration, effects of crowded fields at low Galactic latitudes, an undersampled PSF in excellent seeing conditions (P0.800; the pixel size is 0.400), incorrect flat-field or bias vectors, and scattered light erroneously included in the flat field
Summary
Astronomical optical photometric data are usually calibrated using sets of standard stars whose brightness is known from previous work. The data obtained by the Sloan Digital Sky Survey (SDSS; York et al 2000) can be used to extend the work by Landolt and Stetson to even fainter levels and to increase the number of standard stars to over 1 million. SDSS has designed its own photometric system (ugriz; Fukugita et al 1996), which is in use at many observatories worldwide This widespread use of the ugriz photometric system motivates the construction of a large standard star catalog with $1% accuracy. As part of its imaging survey, SDSS has obtained many scans in the so-called stripe 82 region, which is defined by jJ2000:0j < 1:266 and right ascension approximately in the range 20hY4h
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