J. Tonry, L. Denneau, M. R. Alarcón, A. Clocchiatti, Nicolas Erasmus, A. Fitzsimmons, J. Licandro, K. J. Meech, Robert J. Siverd, H. Weiland
Abstract We present calibrated Asteroid Terrestrial-impact Last Alert System (ATLAS) photometry of the interstellar comet 3I/ATLAS (C/2025 N1) from March 28 through 2025 August 29, obtained with the five-site, robotic ATLAS network in the c (420–650 nm), o (560–820 nm), and Teide w (420–720 nm) bands. Stacked difference images yield reliable light curves measured in four fixed apertures that capture the evolving coma. We observe 3I/ATLAS transitioning in color from red ( c − o ) ≈ 0.7 before MJD 60860 to near-solar ( c − o ) ≈ 0.3 after MJD 60870, coincident with the appearance of a prominent antisolar tail. The absolute magnitude curve H ( t ) shows a slope break near MJD 60890 at r ∼ 3.3 au from −0.035 to −0.012 mag day −1 , or in terms of coma cross section as a function of heliocentric distance, r −3.9 to r −1.1 . We release the aperture photometry with geometry and uncertainties to enable cross-instrument synthesis of 3I/ATLAS activity and color evolution.