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MACS J0717 is the most massive and extended of the Hubble Frontier Fieldclusters. It is one of the more difficult clusters to model, and we argue thatthis is in part due to the line of sight structure (LoS) at redshifts beyond 2.We show that the Grale mass reconstruction based on sources at 3<z_s<4.1 has atleast 10^{13}M_sun more mass than that based on nearby sources, z_s<2.6, andattribute the excess mass to a putative LoS, which is at least 75" from thecluster center. Furthermore, the lens-model fitted z_s's of the recent Kawamataet al. reconstruction are biased systematically low compared to photometricz_s's, and the bias is a function of images' distance from the cluster center.We argue that these mimic the effect of LoS. We conclude that even in thepresence of 100-200 images, lens-model adjusted source redshifts can concealthe presence of LoS, demonstrating the existence of degeneracies between z_sand (sub)structure. Also, a very good fit to image positions is not asufficient condition for having a high fidelity mass map: Kawamata et al.obtain an rms of 0.52" for 173 images of 60 sources; our Grale reconstructionof the exact same data yields a somewhat different map, but similarly low rms,0.62". In contrast, a Grale model that uses reasonable, but fixed z_s gives aworse rms of 1.28" for 44 sources with 126 images. Unaccounted for LoS can biasthe mass map, affecting the magnification and luminosity function estimates ofhigh redshift sources.

Evidence for the line of sight structure in the largest Hubble Frontier Field cluster, MACSJ0717.5+3745