  We present a strong lensing analysis of the galaxy cluster PSZ1 G311.65-18.48
(z=0.443) using multi-band observations with Hubble Space Telescope,
complemented with VLT/MUSE spectroscopic data. The MUSE observations provide
redshift estimates for the lensed sources and help reducing the
mis-identification of the multiple images. Spectroscopic data are also used to
measure the inner velocity dispersions of 15 cluster galaxies and calibrate the
scaling relations to model the subhalo cluster component. The model is based on
62 multiple images grouped in 17 families belonging to 4 different sources. The
majority of them are multiple images of compact stellar knots belonging to a
single star-forming galaxy at z=2.3702. This source is strongly lensed by the
cluster to form the Sunburst Arc system. To accurately reproduce all the
multiple images, we build a parametric mass model, which includes both
cluster-scale and galaxy-scale components. The resulting model has a r.m.s.
separation between the model-predicted and the observed positions of the
multiple images of only 0.14''. We conclude that PSZ1 G311.65-18.48 has a
relatively round projected shape and a large Einstein radius (29'' for z_s =
2.3702), which could indicate that the cluster is elongated along the line of
sight. The Sunburst Arc source is located at the intersection of a complex
network of caustics, which explains why parts of the arc are imaged with
unprecedented multiplicity (up to 12 times).
