Follow up observations of interstellar comet 3I/ATLAS
ATel #17275; M. Minev (IA and NAO, BAS, Bulgaria), A. Kostov (IA and NAO, BAS, Bulgaria), A. Mutafov (IA and NAO, BAS, Bulgaria)
on 6 Jul 2025; 21:14 UT
Credential Certification: Andon Kostov (akostov@astro.bas.bg)
Following the current alerts from ATel#17263 and ATel#17264 we perform 2 hours of observations of the newly discovered interstellar comet 3I/ATLAS. The observations were carried out at two of the telescopes of the National Astronomical Observatory - Rozhen - 1.5m ASA AZ1500 telescope, and 50/70cm Schmidt telescope. Between 20:50:46 UT and 22:54:36 UT at 2025-07-04 a set of 275 images in filter R were obtained with the 1.5m ASA AZ1500 telescope, and 60 more with the 50/70cm Schmidt telescope. Co-adding the images reveal clearly cometary activity of 3I/ATLAS. The isophote contour map of the resulted co-added image shows relatively symmetrical shape, with slight elongation along the axis of the heliocentric velocity vector of the comet. The outermost contour, which is set at 1 sigma_sky above the sky level is extended at ~ 42500 km along the X axis (the X axis and the heliocentric velocity vector is nearly parallel), while along the Y axis, the contour encompass approximately 38000 km at the comet geocentric distance of 3.4 AU. The step of the contours is 0.05 mag.
This contour map can be found here: https://nao-rozhen.org/kostov/C2025N1_contour_map.jpg
The picture of the 275 co-added images, each one 22s exposure can be found here: https://nao-rozhen.org/kostov/libra.astro.bas.jpg
Additionally we calculate the R magnitude of the comet to be 17.46 +/-0.02 [mag], and the phase angle corrected Afrho of 413 +/-6 [cm]. The calculations were done with photometric aperture with radius 3 arcsec, which corresponds to 3000 km at the comet position.
The research that led to these results was carried out with the help of infrastructure purchased under the National Roadmap for Scientific Infrastructure, financially coordinated by the Ministry of Education and Science of the Republic of Bulgaria. It is also partially supported by Bulgarian Scientific Research Fund of the Ministry of Education and Science under the grants KP-06-N78/5 / 2023 and KP-06-H88/5 "Physical properties and chemical composition of asteroids and comets - a key to increasing our knowledge of the Solar System origin and evolution."
