We have constructed a large format mosaic CCD camera for the Sloan Digital Sky Survey. The camera consists of two arrays, a photometric array which uses 30 2048 x 2048 SITe/Tektronix CCDs (24 micron pixels) with an effective imaging area of 720 square cm, and an astrometric array which uses 24 400 x 2048 CCDs with the same pixel size, which will allow us to tie bright astrometric standard stars to the objects imaged in the photometric camera. The instrument will be used to carry out photometry essentially simultaneously in five color bands spanning the range accessible to silicon detectors on the ground in the time-delay- and-integrate (TDI) scanning mode. The photometric detectors are arrayed in the focal plane in six columns of five chips each such that two scans cover a filled stripe 2.5 degrees wide. This paper presents engineering and technical details of the camera.Comment: 67 pages (inc 6 tables), plain TeX, 41 figures (gif), to appear in the Astronomical Journal, December 1998. The figures can be downloaded from http://astro.princeton.edu/~library/prep.html, preprint POPe-774, allfigs.zip, in postscrip

In addition to optical photometry of unprecedented quality, the Sloan Digital Sky Survey (SDSS) is producing a massive spectroscopic database which already contains over 280,000 stellar spectra. Using effective temperature and metallicity derived from SDSS spectra for ∼60,000 F and G type main sequence stars (0.2 < g − r < 0.6), we develop polynomial models, reminiscent of traditional methods based on the U BV photometry, for estimating these parameters from the SDSS u−g and g−r colors. These estimators reproduce SDSS spectroscopic parameters with a root-mean-square scatter of 100 K for effective temperature, and 0.2 dex for metallicity (limited by photometric errors), which are similar to random and systematic uncertainties in spectroscopic determinations. We apply this method to a photometric catalog of coadded SDSS observations and study the photometric metallicity distribution of ∼200,000 F and G type stars observed in 300 deg 2 of high Galactic latitude sky. These deeper (g < 20.5) and photometrically precise (∼0.01 mag) coadded data enable an accurate measurement of the unbiased metallicity distribution for a complete volume-limited sample of stars at distances between 500 pc and 8 kpc. The metallicity distribution can be exquisitely modeled using two components with a spatially varying number ratio, that correspond to disk and halo. The best-fit number ratio of the two components is consistent with that implied by the decomposition of stellar counts profiles into exponential disk and power-law halo components by Jurić et al. (2008). The two components also possess the kinematics expected for disk and halo stars. The metallicity of the halo component can be modeled as a spatially invariant Gaussian distribution with a mean of [F e/H] = −1.46 and a standard deviation of ∼0.3 dex. The disk metallicity distribution is non-Gaussian, with a remarkably small scatter (rms∼0.16 dex) and the median smoothly decreasing with distance from the plane from −0.6 at 500 pc to −0.8 beyond several kpc. Similarly, we find using proper motion measurements that a non-Gaussian rotational velocity distribution of disk stars shifts by ∼50 km/s as the distance from the plane increases from 500 pc to several kpc. Despite this similarity, the metallicity and rotational velocity distributions of disk stars are not correlated (Kendall's τ = 0.017 ± 0.018). This absence of a correlation between metallicity and kinematics for disk stars is in a conflict with the traditional decomposition in terms of thin and thick disks, which predicts a strong correlation (τ = −0.30 ± 0.04) at ∼1 kpc from the mid-plane. Instead, the variation of the metallicity and rotational velocity distributions can be modeled using non-Gaussian functions that retain their shapes and only shift as the distance from the mid-plane increases. We also study the metallicity distribution using a shallower (g < 19.5) but much larger sample of close to three million stars in 8500 sq. deg. of sky included in SDSS Data Release 6. The large sky coverage enables the detection of...

We present evidence for a ring of stars in the plane of the Milky Way, extending at least from l ¼ 180 to 227 with turnoff magnitude g $ 19:5; the ring could encircle the Galaxy. We infer that the low Galactic latitude structure is at a fairly constant distance of R ¼ 18 AE 2 kpc from the Galactic center above the Galactic plane and has R ¼ 20 AE 2 kpc in the region sampled below the Galactic plane. The evidence includes 500 Sloan Digital Sky Survey spectroscopic radial velocities of stars within 30 of the plane. The velocity dispersion of the stars associated with this structure is found to be 27 km s À1 at ðl; bÞ ¼ ð198 ; À27 Þ, 22 km s À1 at ðl; bÞ ¼ ð225 ; 28 Þ, 30 km s À1 at ðl; bÞ ¼ ð188 ; 24 Þ, and 30 km s À1 at ðl; bÞ ¼ ð182 ; 27 Þ. The structure rotates in the same prograde direction as the Galactic disk stars but with a circular velocity of 110 AE 25 km s À1 . The narrow measured velocity dispersion is inconsistent with power-law spheroid or thick-disk populations. We compare the velocity dispersion in this structure with the velocity dispersion of stars in the Sagittarius dwarf galaxy tidal stream, for which we measure a velocity dispersion of 20 km s À1 at ðl; bÞ ¼ ð165 ; À55 Þ. We estimate a preliminary metallicity from the Ca ii (K) line and color of the turnoff stars of ½Fe=H ¼ À1:6 with a dispersion of 0.3 dex and note that the turnoff color is consistent with that of the spheroid population. We interpret our measurements as evidence for a tidally disrupted satellite of 2 Â 10 7 to 5 Â 10 8 M that rings the Galaxy.

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 (j J2000:0 j< 1:266 ) in the right ascension range 20 h 34 m Y 4 h 00 m and with the corresponding r-band (approximately Johnson V-band) magnitudes in the range 14Y22. 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.

We quantify the variability of faint unresolved optical sources using a catalog based on multiple SDSS imaging observations. The catalog covers SDSS Stripe 82, and contains 58 million photometric observations in the SDSS ugriz system for 1.4 million unresolved sources. In each photometric bandpass we compute various low-order lightcurve statistics and use them to select and study variable sources. We find that 2% of unresolved optical sources brighter than g=20.5 appear variable at the 0.05 mag level (rms) simultaneously in the g and r bands. The majority (2/3) of these variable sources are low-redshift (<2) quasars, although they represent only 2% of all sources in the adopted flux-limited sample. We find that at least 90% of quasars are variable at the 0.03 mag level (rms) and confirm that variability is as good a method for finding low-redshift quasars as is the UV excess color selection (at high Galactic latitudes). We analyze the distribution of lightcurve skewness for quasars and find that is centered on zero. We find that about 1/4 of the variable stars are RR Lyrae stars, and that only 0.5% of stars from the main stellar locus are variable at the 0.05 mag level. The distribution of lightcurve skewness in the g-r vs. u-g color-color diagram on the main stellar locus is found to be bimodal (with one mode consistent with Algol-like behavior). Using over six hundred RR Lyrae stars, we demonstrate rich halo substructure out to distances of 100 kpc. We extrapolate these results to expected performance by the Large Synoptic Survey Telescope and estimate that it will obtain well-sampled 2% accurate, multi-color lightcurves for ~2 million low-redshift quasars, and will discover at least 50 million variable stars.Comment: 41 pages, 14 figures, submitted to Astronomical Journa