Halton Arp was an American astronomer noted for challenging the theory that the large redshifts of quasars and other active galaxies are an indication of great distance. His Atlas of Peculiar Galaxies, published in 1966, contains photographs of 338 peculiar and interacting galaxies taken with the Hale Telescope, and was instrumental in raising the issue of discordant redshifts. Arp received a BS from Harvard (1949) and a PhD from the California Institute of Technology (1953).