The 2021 outburst of RS Oph. A pictorial atlas of the spectroscopic evolution: the first 18 days

A pictorial atlas of the spectroscopic evolution at optical wavelengths is presented for the first 18 days of the 2021 outburst of the recurrent nova RS Oph, prior to the emergence of high ionization emission lines. The spectra presented here have been obtained at daily cadence with the Asiago 1.22m + B&C (3200-7900 Ang, 2.3 Å/pix) and Varese 0.84m + Echelle telescopes (4250-8900 Å, resolving power 18,000). The spectra have been fully calibrated in IRAF, absolutely fluxed, and heliocentric corrected. The Echelle spectra have been also corrected for telluric absorptions.