Follow-Up Imaging and Spectroscopic Observations of Comet C/2023 A3

From 22 to 26 October 2024, we carried out spectroscopic follow-up observations of comet C/2023 A3 on four nights with the Échelle spectrograph FLECHAS, which is installed at the Nasmyth port of the 90 cm reflector telescope of the University Observatory Jena. The instrument was operated in its 2x2 binning mode, which provides a spectral resolving power of R~7400. In addition to the scientific data, flatfield and arc lamp spectra were always taken before each comet observation to calibrate the scientific spectra. Total integration times on target between 45 and 60 minutes were used. Throughout the observations, the fiber of the spectrograph, which has a diameter of 3.9 arcsec in the sky, was centered on the coma of C/2023 A3, to which the telescope was guided to correct for the rapid angular motion of the comet. In the wavelength range between 5000 and 7000 Å we have detected several emission lines in the spectra of C/2023 A3, with those of Na, [OI], NH₂ and C₂ being the most prominent ones. Due to the velocity of the comet relative to the observation site, the emission lines are redshifted (by about 54 and 58 km/s for 22 and 26 October 2024, respectively), which agrees well with the expected RV of the comet based on its current orbital solution. While the emission lines of [OI], NH₂ and C₂ remain stable at the 3σ level, the equivalent width of the Na lines decreases significantly by about 2/3 during the observation period. In addition to the emission lines, the spectra of C/2023 A3 also show absorption lines due to the reflection of sunlight by dust particles in the comet's coma. In order to measure the apparent brightness of C/2023 A3, its position in the sky and to study its morphology, the comet was also imaged before and simultaneously to the spectroscopic observations in the R-band with the CCD camera CTK-II, which is operated at the observatory's 25 cm Cassegrain auxiliary telescope.