**Table 2:** The MXU multi-slit instrumental set-up.
Run	Run 1 + 2	Run 3 + 4
Telescope	VLT-UT4
Instrument	FORS2
Mode	MXU (i.e. mask multi-slit)
Grism	GRIS_600RI+19
Filter	GG435
Spectral resolution	6.6 Å FWHM
Detector(s)	Site	2 $\times$ MIT
Detector size	$2048 \times 2048$	$2 \times (2048 \times 1024)^{a}$
Pixel size	24 $\mu$ m $\times$ 24 $\mu$ m	30 $\mu$ m $\times$ 30 $\mu$ m^a
Readout noise	5.41 e^-	3.15 e^-
Conversion factor	1.91 e^-/ADU	0.70 e^-/ADU
MXU slit width	1 $.\!\!^{\prime\prime}$ 0	1 $.\!\!^{\prime\prime}$ 0
Spatial scale	0 $.\!\!^{\prime\prime}$ 20/pixel	0 $.\!\!^{\prime\prime}$ 25/pixel^a
Dispersion	1.32 Å/pixel	1.66 Å/pixel^a
Wavelength range^b	5300-8000 Å	5120-8450 Å

^a After binning the CCDs by $2 \times 2$ , which is the default.
^b For a slit at the centre of the field of view.

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