Observational properties of the observed C¹⁸O and ¹³CO (2−1) lines as a function of the total column density observed along the Musca filament: (upper panel) optical depth of the C¹⁸O (2−1) lines; (mid panel) C¹⁸O total column density; (lower panel) observed (blue solid squares) and opacity corrected (orange diamonds) T_mb(¹³CO)/T_mb(C¹⁸O) line ratio. The gray dashed line in the upper panel corresponds to the characteristic detection S/N = 3 line detection threshold for a spectrum with an rms value of 0.14 K, in T_mb units. The derived C¹⁸O column densities are consistent (within a factor of two) with the typical abundances obtained in clouds like Taurus and Ophiuchus (solid red line in the mid panel; Frerking et al. 1982). The red horizontal line in the lower panel indicates the canonical T_mb(¹³CO)/T_mb(C¹⁸O) = 7.3 line ratio expected in an optically thin regime.