MACS J0717 is the most massive and extended of the Hubble Frontier Field clusters .
It is one of the more difficult clusters to model , and we argue that this 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 at least 10 ^ { 13 } M _ { \odot } more mass than that based on nearby sources , z _ { s } < 2.6 , and attribute the excess mass to a putative LoS , which is at least 75 ^ { \prime \prime } from the cluster center .
Furthermore , the lens-model fitted z _ { s } ’ s of the recent Kawamata et al .
reconstruction are biased systematically low compared to photometric z _ { 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 the presence of 100-200 images , lens-model adjusted source redshifts can conceal the presence of LoS , demonstrating the existence of degeneracies between z _ { s } and ( sub ) structure .
Also , a very good fit to image positions is not a sufficient condition for having a high fidelity mass map : Kawamata et al .
obtain an rms of 0.52 ^ { \prime \prime } for 173 images of 60 sources ; our Grale reconstruction of the exact same data yields a somewhat different map , but similarly low rms , 0.62 ^ { \prime \prime } .
In contrast , a Grale model that uses reasonable , but fixed z _ { s } gives a worse rms of 1.28 ^ { \prime \prime } for 44 sources with 126 images .
Unaccounted for LoS can bias the mass map , affecting the magnification and luminosity function estimates of high redshift sources .