OAN - Tonantzintla was established in 1948, when observing conditions at the OAN location in Valley of Mexico became too degraded by light pollution to be useful. OAN was first established on the balcony of Chapultepec Castle in Mexico City in 1878. It was moved to Tacubaya, then on the outskirts of the city, to a building that was started in 1884 and completed in 1909. OAN completed the next move to Tonantzintla in 1951. By the mid-1960s, the night sky over the observatory became so polluted that research was hampered. OAN began looking for a new location 1966, and determined that Sierra de San Pedro Mártir would be an excellent site. The first telescope at the new observatory was installed in 1969 using a mirror polished by OANTON and UNAM.