  Observing the vibrational/rotational lines in a comet's optical spectrum
requires high-resolution spectroscopy, as they are otherwise seen as a blended
feature. To achieve this, we have obtained medium and high-resolution (R
($\lambda/\Delta \lambda$) = 30000 and 60000) spectra of several comets,
including C/2015 V2 (Johnson), 46P/Wirtanen, 41P/Tuttle-Giacobini-Kres\'ak and
38P/Stephan-Oterma, using the Hanle Echelle Spectrograph (HESP) mounted on the
2-m Himalayan Chandra Telescope (HCT) in India. The spectra effectively cover
the wavelength range 3700 - 10,000 {\AA}, allowing us to probe the various
vibrational bands and band sequences to identify the rotational lines in the
cometary molecular emission. We were also able to separate the cometary Oxygen
lines from the telluric lines and analyse the green-to-red (G/R) forbidden
oxygen [OI] ratios in a few comets. For comets C/2015 V2, 46P, and 41P, the
computed G/R ratios, 0.04$\pm$0.01, 0.04$\pm$0.01, and 0.08$\pm$0.02
respectively, point to H$_2$O being a major source of Oxygen emissions.
Notably, in the second fibre pointing at a location 1000 km away from the
photocenter of comet 46P, the G/R ratio reduced by more than half the value
observed in the first fibre, indicating the effects of quenching within the
inner coma. We also measured the NH$_2$ ortho-to-para ratio of comet 46P to be
about 3.41$\pm$0.05 and derived an ammonia ratio of 1.21$\pm$0.03 corresponding
to a spin temperature of $\sim$26 K. With these, we present the results of the
study of four comets from different cometary reservoirs using medium and
high-resolution optical spectroscopy, emphasising the capabilities of the
instrument for future cometary studies.
