Comet 3I/ATLAS: Post-perihelion spectroscopic results from PRL Mt Abu Observatory
ATel #17502; Shashikiran Ganesh (PRL), Goldy Ahuja (PRL), Arvind B.(PRL,), Anil Bhardwaj(PRL)
on 19 Nov 2025; 16:03 UT
Credential Certification: Shashikiran Ganesh (shashi@prl.res.in)
Referred to by ATel #: 17561
Observations of the comet 3I/ATLAS, also known as C/2025 N1 (ATLAS), were obtained using the instruments on PRL's 1.2m telescope at the Mount Abu Observatory during November 12 to 17 in the late hours of the night, just before dawn. Initially, when the comet was visible for only a short time, observations were carried out using an imager in the R filter (see false colour image in the online link below).
On the later nights, we carried out spectroscopic observations on the PRL 1.2m telescope. The observations were conducted using the LISA instrument, which provides spectral coverage from 3800 to 6800 Å. At the time of observation, the comet was at a heliocentric distance of around 1.51-1.53 AU and a geocentric distance of 2.08 - 2.05 AU. The slit was placed on the photocenter of the coma of the rare interstellar object, and exposures of 15 minutes were obtained with the telescope following the fast-moving object in non-sidereal tracking mode. The spectrum was extracted using a rectangular aperture of 32.60 × 1.76 arcseconds, corresponding to approximately 48707 km × 2617 km on the coma. The spectrum was flux-calibrated and solar continuum corrected to remove the dust contribution. The spectra show typical emission lines and bands from CN, C2 and C3, as are also usually seen in Solar system comets. The spectra are available in the link below.
To our knowledge, these are the first post-perihelion spectra of Comet 3I/ATLAS. The spectra show it to be a comet similar to those belonging to the Solar System. Our value for the log(C2 / CN) production rate ratio is approximately -0.73 (observation of November 16, 2025), which indicates the comet to be similar to the depleted class of comets of the solar system in terms of carbon composition. Observations with longer exposure times will help confirm the production rates and ratios.
We are conducting further observations with PRL's Mount Abu telescopes and encourage continued monitoring of 3I in the days to come.
Work at the Physical Research Laboratory is supported by the Department of Space, Govt. of India.
Observations of the comet 3I/ATLAS from PRL Mt Abu Observatory
