The ATLAS All-Sky Stellar Reference Catalog

Abstract

The Asteroid Terrestrial-impact Last Alert System (ATLAS) observes most of the sky every night in search of dangerous asteroids. Its data are also used to search for photometric variability, where sensitivity to variability is limited by photometric accuracy. Since each exposure spans 7.6 • corner to corner, variations in atmospheric transparency in excess of 0.01 mag are common, and 0.01 mag photometry cannot be achieved by using a constant flat field calibration image. We therefore have assembled an all-sky reference catalog of approximately one billion stars to m ∼ 19 from a variety of sources to calibrate each exposure's astrometry and photometry. Gaia DR2 is the source of astrometry for this ATLAS Refcat2. The sources of g, r, i, z photometry include Pan-STARRS DR1, the ATLAS Pathfinder photometry project, ATLAS re-flattened APASS data, SkyMapper DR1, APASS DR9, the Tycho-2 catalog, and the Yale Bright Star Catalog. We have attempted to make this catalog at least 99% complete to m < 19, including the brightest stars in the sky. We believe that the systematic errors are no larger than 5 millimag RMS, although errors are as large as 20 millimag in small patches near the galactic plane.

SkyMapper Southern Survey: Second data release (DR2)

“…The predominant differences are close to the Galactic plane and follow Galactic structure, and so are likely due to the treatment of reddening and/or issues of source density in crowded fields (cf. Tonry et al, 2018). The mottled effect in the (g − r) map (which is shown at higher contrast than the others) is primarily due to residual flatfielding imperfections in the SkyMapper images.…”

“…Stars are only used as calibrator stars when (i) Gaia DR2 reports a parallax of > 1 milliarcsecond, meaning the stars are closer than 1 kpc, to limit the potential impact of dust extinction, (ii) Gaia photometry is considered reliable and not a blend of sources, and (iii) the star is in a suitable colour range for linear colour terms; for details, see Tonry et al (2018). We further require that either the integrated E(B − V ) reddening in the all-sky map of Schlegel, Finkbeiner, & Davis (1998, hereafter SFD) is below 0.3 mag or the star has a meaningful (i.e., non-zero) value for the A G extinction estimated in Gaia DR2.…”

“…Focusing on a small halo region around RA= 180, we find RMS Figure 10. Comparison of SkyMapper DR2 photometry with Pan-STARRS1 DR1 photometry in g, r, i, z after applying bandpass transformations by Tonry et al (2018): plotted are the median magnitude differences per deg 2 , restricted to objects with PSF magnitudes between 14.5 and 17.5. The large offsets at the Southern edge of the PS1 coverage are where PS1 becomes unreliable.…”

SN 2019ehk: A Double-peaked Ca-rich Transient with Luminous X-Ray Emission and Shock-ionized Spectral Features

“…SN2019ehk was also observed with ATLAS, a twin 0.5 m telescope system installed on Haleakala and Mauna Loa in the Hawai'ian islands that robotically surveys the sky in cyan (c) and orange (o) filters (Tonry et al 2018a). The survey images are processed as described in Tonry et al (2018a), and immediately are photometrically and astrometrically calibrated using the RefCat2 catalog (Tonry et al 2018b). Template generation, image subtraction procedures, and identification of transient objects are described in Smith et al (2020).…”

THOR 42: A touchstone ∼24 Myr-old eclipsing binary spanning the fully-convective boundary

“…The rest of the data reduction proceeded as for DR2: suppression of high-frequency interference noise, overscan subtraction, 2D bias subtraction, per-row bias structure removed by principal components analysis (PCA), flatfield correction, background equalisation between the two amplifiers on each CCD, and a PCA-based subtraction of detector fringing. Photometric zero-points were based on the ATLAS All-Sky Stellar Reference Catalog (Tonry et al 2018b) after applying Pan-STARRS1-to-SkyMapper bandpass transformations (for details see Onken et al 2019). In a further modification from DR2 processing, an individual photometric data point consisted of a PSF magnitude determined by a 2D model created from well-measured stars across each CCD using the PSFEX software package (Bertin 2011), where the model was allowed to vary linearly with (x, y) position on the CCD.…”