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Mapping Compound Cosmic Telescopes Containing Multiple, Projected Cluster-Scale Halos

Lines of sight with multiple, projected, cluster-scale halos have high total masses and complex lens plane interactions that can boost the area of magnification, or etendue, making detection of faint background sources more likely than elsewhere. To identify these new "compound" cosmic telescopes, we have found lines-of-sight with the highest integrated mass densities, as traced by the projected concentrations of Luminous Red Galaxies (LRGs). We use 1151 MMT Hectospec spectra to derive preliminary magnification maps for two such lines of sight with total mass exceeding ~ 3 x 10$^{15}$ Msun -- J0850+3604 (0850) and J1306+4632 (1306). We identify 2-3 group- and cluster-scale halos in each beam over 0.1 < z < 0.7, all of which are well-traced by LRGs. In Subaru Suprime-Cam imaging of beam 0850, we discover serendipitously a candidate multiply-imaged V-dropout source at z = 5.03, whose location is consistent with the critical curves for a source plane of $z_s$ = 5.03 predicted by our mass model. Incorporating the position of the candidate multiply-imaged galaxy as a constraint on the critical curve location in 0850 narrows the 68% confidence band on lens plane area with mu > 10 for a source plane of $z_s$ = 10 to [1.8, 4.2] square arcminutes, comparable to that of MACS 0717+3745 and El Gordo, two of the most powerful known single cluster lenses. The 68% confidence intervals on the lens plane area with mu > 10 for 1306 are [2.3, 6.7] square arcminutes. The significant lensing power of our beams makes them powerful probes of reionization and galaxy formation in the early Universe.