LRS2 PSF shape and estimation of its FWHM based on the DIMM seeing. Left panel: Distribution of the FWHM PSF ratio, Q(FWHM) = FWHM_X/FWHM_Y, measured on the standard stars observed with LRS2. The PSF is on average consistent with being circular with a mean FWHM ratio of 0.97±0.08. Second left to right: Comparison between the DIMM seeing at the telescope and the FWHM (mean between the x- and y-axis of the elliptical Moffat function, see Fig. B.2) measured on the standard stars (Sect. B.2) at the middle of the LRS2-B wavelength window (∼ 5300 Å), at the Mg II and at the [O II] wavelengths, respectively. The dashed line shows the one-to-one relation. The DIMM seeing provides a relatively good estimation of the PSF FWHM near ∼ 5300 Å but overestimates it at Mg II wavelengths (≈ 3780 Å). The red line shows the estimated PSF FWHM based on the DIMM seeing calculated using Eq. B.1 (see Sect. B.2).