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.
