MOSFIRE spectrum of the Einstein ring. Right: Zoom-in on the [OII] doublet (S/N = 5.7) and the best-fit solution. The doublet was fitted using two Gaussian profiles separated by 3Å, with similar dispersion, and with an amplitude ratio bound between 0.35 and 1.5. The best-fit redshift, line-spread-function-corrected dispersion, and integrated [O II] flux are indicated on the bottom right. The inset shows the slit configuration (yellow box), the location where the spectrum is extracted (red line), and the 2D spectrum.