Counter arc IV observed with HST ACS/F606W (left) and VLT/MUSE NFM after collapsing the datacube along the indicated wavelength intervals (middle and right images). The ground-based AO-assisted observations of arc IV outperform those from space (see narrow-band imaging after collapsing the cube in the wavelength ranges 6500-8000Å and 8000-9000Å), providing further evidences for the compactness of the star-forming region and in particular the LyC star cluster 5.1 (in this case, the image 5.1n has been targeted). The right most figure shows the FWHM of the OGS (On-axis Guide Source), measured within a 1.5″ region, as a function of the wavelength. The black line show the measurements in the final stacked data-cube, whereas the blue region indicates the scatter (68% confidence level) of the measurements in each single exposure. The FWHM is < 60 mas at λ > 7000Å and we assume a similar resolution is achieved on arc IV (middle).