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Strong gravitational lensing by galaxy clusters is a fundamental tool tostudy dark matter and constrain the geometry of the Universe. Recently, theHubble Space Telescope Frontier Fields programme has allowed a significantimprovement of mass and magnification measurements but lensing models stillhave a residual root mean square between 0.2 arcsec and few arcsec- onds, notyet completely understood. Systematic errors have to be better understood andtreated in order to use strong lensing clusters as reliable cosmologicalprobes. We have analysed two simulated Hubble-Frontier-Fields-like clustersfrom the Hubble Frontier Fields Comparison Challenge, Ares and Hera. We useseveral estimators (relative bias on magnification, den- sity profiles,ellipticity and orientation) to quantify the goodness of our reconstructions bycomparing our multiple models, optimized with the parametric software LENSTOOL, with the input models. We have quantified the impact of systematic errorsarising, first, from the choice of different density profiles andconfigurations and, secondly, from the availability of con- straints(spectroscopic or photometric redshifts, redshift ranges of the backgroundsources) in the parametric modelling of strong lensing galaxy clusters andtherefore on the retrieval of cosmological parameters. We find thatsubstructures in the outskirts have a significant im- pact on the position ofthe multiple images, yielding tighter cosmological contours. The need forwide-field imaging around massive clusters is thus reinforced. We show thatcompetitive cosmological constraints can be obtained also with complexmultimodal clusters and that photometric redshifts improve the constraints oncosmological parameters when considering a narrow range of (spectroscopic)redshifts for the sources.

Hubble Frontier Fields: systematic errors in strong lensing models of galaxy clusters - Implications for cosmography