Photometric Analysis of the Interstellar Comet 3I/ATLAS During its Pre-perihelion Period

Abstract We present a photometric study of the interstellar comet 3I/ATLAS based on observations obtained between 2025 July 3 and September 22 from two observatories in Spain (Z10 and Y71) using small-aperture telescopes. The comet was monitored in five photometric bands ( CV, C, R, G , and B ), revealing an apparent brightness range from 19.10 to 13.50 mag. The observed variability reflects differences in observing site and filter bandpass, as well as significant scatter caused by background stellar contamination in crowded fields. Throughout the observing interval, comet 3I/ATLAS exhibited weak activity, with only a faint tail detected. A consistent brightening trend was measured in all filters, with average rates ranging from 0.033 to 0.066 magnitudes per day, indicating a progressive increase in activity. Measurements of coma size, tail length, and tail position angle were obtained for selected epochs. Derived Af ρ values suggest that comet 3I/ATLAS was undergoing a phase of increasing activity associated with its approach to perihelion.