Among the most important questions in science right now is how the universe evolved from a dense, opaque state into a transparent state rich with galaxies. Despite its vast importance in cosmological history, this transition, known as reionization, is poorly understood. The first stars and galaxies likely reionized the universe, but confirming this has been difficult. A survey of z>~7 galaxies is needed. However, such sources are extremely faint due to their high redshift and the increased optical depth during reionization. The magnification gain achieved using galaxy clusters as gravitational telescopes is often needed to study the otherwise unobservable first galaxies. Magnification maps of these telescopes are required to convert observed properties of galaxies into intrinsic ones. I am leading our group's effort to design these maps for a large sample of galaxy clusters in three large programs using NASA's space telescopes: the Hubble Space Telescope (HST) Frontier Fields (HFF), the Spitzer Ultra Faint Survey Program (SURFS UP), and the HST Grism Lens-Amplified Survey from Space (GLASS). I have produced lens models using ancillary data of the 6 HFF clusters. I recently submitted a first author publication for which I developed an improved lens model of the HFF cluster MACSJ0416.1-2403 using GLASS spectroscopic data. I will use the lens models I create for the remaining clusters, coupled with high resolution ground based spectroscopy I will collect in the remaining year and a half of my PhD, to measure the stellar properties of the first galaxies likely responsible for reionization.