Extracted spectrum of CS Cha b is shown in black, and the stellar residuals at several positions around CS Cha b are shown in blue. The grey line is a binned-down version of the spectrum to enhance S/N. Even after binning, no clear absorption features are present. The spectrum of CS Cha b shows weak Hα emission and two forbidden OI lines at 6300 Å and 6364 Å, which are highlighted on the right. The orange lines show the Gaussian fit to each emission line. The OI 6300 has a line strength that is nearly equal to the Hα emission line. At 7600 Å there are spurious features due the presence of imperfect correction of strong telluric lines. The gap between 5800 Å and 6051 Å is due to the presence of a notch filter, which is centred on the sodium doublet in the LTAO system that blocks the sodium-based laser guide star.