The Most Powerful Cosmic Telescopes for Constraining the z > 7 Luminosity Function
Gravitational Lensing by massive clusters ("cosmic telescopes") is a powerful technique for magnifying faint, high-redshift sources into detectability. Cosmic telescopes containing multiple projected halos can theoretically be superior to single clusters due to potentially higher integrated masses and lensing interactions that boost the etendue of the beam. We present two such lines of sight identified from the SDSS with integrated masses of ~4 x 10^15 solar masses within 3.5 arcminutes, surpassing even the most massive single cluster lensing fields. Between two and five virialized halos in each beam are seen in MMT Hectospec spectroscopy of ~1500 field galaxies. The lensing power of these beams is confirmed by the locations and colors of 8 strongly lensed arcs, visible in multi-band HST and Subaru Suprime-Cam imaging. These lines of sight can be used to detect 50-1000% more faint sources at z > 7 than blank field methods with equivalent exposure time. The significant lensing power of these beams makes them important for the fields of cosmology, the study of reionization, and galaxy evolution at high redshift in the era of JWST. Finally, I discuss the extension of the cluster selection technique to catalogs extracted from LSST's deep, multi-band imaging.