The ATLAS catalog is an all-sky reference catalog comprised of approximately 1 billion stars to a limiting magnitude of 19.  It incorporates data from PanSTARRS DR1, ATLAS PathFinder, ATLAS re-flattened APASS, SkyMapper DR1, APASS DR9, Tycho-2, and the Yale Bright Star Catalog.  The astrometry is sourced from Gaia DR2.  It was developed by Tonry et al in 2018 and serves as a very robust catalog for photometry.

For conducting photometry, it is recommended that the native magnitudes of the chosen catalog be used, which in the case of the ATLAS catalog is the r' (Sloan R) magnitude.

The acknowledgements for this catalog, to be used when publishing a paper, are as follows:

This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. ATLAS is primarily funded to search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area.  The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen's University Belfast, the Space Telescope Science Institute, and the South African Astronomical Observatory.

References:

Tonry, J.L., Denneau, L., Flewelling, H., Heinze, A.N., Onken, C.A., Smartt, S.J., Stalder, B., Weiland, H.J., Wolf, C. (2018). "The ATLAS All-Sky Stellar Reference Catalog." Astrophys. J. 867, A105.

Kostov, A., Bonev, T. (2017). "Transformation of Pan-STARRS1 gri to Stetson BVRI magnitudes. Photometry of small bodies observations." Bulgarian Astron. J. 28, 3 (AriXiv:1706.06147v2).