HATPI Preperihelion Time-series Photometry of the Interstellar Comet 3I/ATLAS

The Hungarian-made Automated Telescope PI Steradians (HATPI) is a recently commissioned time-domain facility at Las Campanas Observatory, Chile, that uses 64 wide-angle, 9.6 cm diameter lenses and back-illuminated CCDs, yielding a mosaic field-of-view of 7100 square arcdegrees, observing the night sky at a cadence of 45 s and a spatial scale of 19. ″7 pixel⁻¹. In this paper, we present moving object time-series photometry with this facility, focusing on the interstellar comet 3I/ATLAS, which was first robustly recovered by HATPI on the night of 2025 July 2 (one night after its discovery) at a Gaia G-band magnitude of G = 17.796 ± 0.082 mag (±0.030 mag systematic uncertainty). The comet then increased in brightness to G = 14.071 ± 0.073 mag ± 0.030 mag by 2025 September 13, after which it became unobservable by HATPI as it approached perihelion. Before 3I/ATLAS achieved a brightness of G = 16.396 ± 0.029 mag ± 0.030 mag on 2025 August 6, it could be detected when stacking all HATPI observations from a single night, while after this date it is sufficiently bright to detect in individual 45 s exposures. We do not detect evidence for significant short-time-scale variations in the brightness of 3I/ATLAS after August 6. Compared to other light curves in the literature, the HATPI photometry exhibits a somewhat steeper rise in brightness with decreasing heliocentric distance, r _H . The HATPI magnitudes are well-fit as a power law function of r _H , with an exponential index of n = 5.167 ± 0.095, over the range 2.14 au <r _H < 4.44 au, compared to n = 3.94 ± 0.10 when fitting together with other observations in the literature. We find that the phase function is constrained to β = 0.0552 ± 0.0032 mag deg⁻¹.