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  : The scale lengths and the fluorescence efficiency of different molecules at 1 au scaled by r$_h^{-2}$.

Spectroscopy (UVES/VLT)
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We obtained one spectrum of comet 21P with the Ultraviolet-Visual Echelle Spectrograph (UVES) mounted on the Unit 2 telescope (UT2) at ESO’s VLT on September 18, 2018 (a week after perihelion, r$_h$=1.01 au and $\Delta$=0.40 au) under Director’s Discretionary Time. We used the UVES standard settings DIC\#1 346+580 covering the range 3030 to 3880 Å in the blue and 4760 to 6840 Å  in the red. We used a 0.44$^{\prime\prime}$ wide slit, providing a resolving power R$\sim$80 000. We obtained one single exposure of 3000 s at 8h35 UT with a mean airmass of 1.7. This exposure provided two different spectra, both of them covering one of the above mentioned spectral ranges.

The ESO UVES pipeline was used to reduce the spectra in the extended object mode, keeping the spatial information. The spectra were corrected for the extinction and flux calibrated using the UVES master response curve provided by ESO. One-dimensional spectra were then extracted by averaging the 2D spectra with simultaneous cosmic ray rejection and then corrected for the Doppler shift due to the velocity of the comet 