tractions, performed Molecfit corrections, and forced Reflex to output a fourth data product: the reduced and calibrated sky observation datacubes. We used these sky observations to help isolate cometary [OI] emissions from atmospheric [OI] emissions. This method is necessary due to the high levels of noise introduced when Reflex subtracted the strong [OI] sky emissions. Therefore, we needed to perform our own more nuanced subtraction, first outlined in Opitom et al. (2020) and described further in subsection 3.3.1. This constituted our secondary science dataset, which preserved the cometary [OI] signal.

2.2. Advanced Datacube Processing This section describes the post-reduction processing we conducted on each datacube, such as telluric absorption correction, dust and solar continuum corrections and subtraction, molecular species isolation, forbidden oxygen line [OI] extraction, relative spectral slope fitting, and various rebinning and coaddition procedures. Mentioned in subsection 2.1, we leveraged Molecfit to perform corrections of strong telluric absorption features that were heavily contaminating our datacubes, mainly from ∼7000 to 9350 ˚A. Telluric absorption is caused by the interaction of inbound photons with gaseous molecules, aerosols, and trace gas species in Earth’s atmosphere. In our affected spectral region, the primary contaminants are H2O and O2 absorption bands (Smette et al., 2015), however, O3 Chappuis ozone absorption bands do influence transmittance in the sub-7000 ˚A regime (Chappuis, 1880). We corrected for this by feeding Molecfit an extracted spectrum corresponding to the spaxels within ρ = 104 km from the comet optocenter for each datacube. We then executed the Molecfit radiative transfer routines to model synthetic atmospheric transmission spectra of the aforementioned molecular species at each time of observation. The final products consist of a telluric absorption corrected (TAC) spectrum, as well as the stand-alone atmospheric transmission model spectrum. To correct all of the ∼96,000 spectra in each datacube, we assumed that the telluric absorption was isotropic (Li et al., 2018; Mill´an et al., 2024) over the region of sky subtended by MUSE’s FoV, and divided each spaxel by the corresponding ρ = 104 km atmospheric transmission model. If we consider the bulk contribution of H2O and O2 telluric absorption occurs within one atmospheric scale length of Earth (8.5 km in height), we assume isotropy over the physical size of the field of view at the top of this scale height, 4.4×4.4 m. Similarly, the O3 contribution peaks at ∼25 km, roughly subtending 7.3×7.3 m. Finally, if we assume an entire column of the atmosphere, 100 km high, we assume isotropy over 24.3×24.3 m. Through these assumptions, we achieved clean spectra in the final datacubes, as seen in the top panel Figure 2. The main contribution to each spectrum in our datacubes comes from the dust coma, which dominated the observed flux. The dust is non-radiative in the visible regime, so the bulk dust continuum originates from reflected solar light. This reflected solar light still carries signatures intrinsic to the Sun, such as a broad blackbody peak around 6000 ˚A and solar absorption features like the well-documented Fraunhofer lines (Fraunhofer, 1817; Kirchhoff and Bunsen, 1861). To correct for this dust and solar continuum, and isolate the 9

CN
C2 [OI]1S [OI]1D NH2