The Hale Telescope is the largest telescope at the Palomar Observatory, named after astronomer George Ellery Hale. The , 3.3 telescope was the largest operating telescope in the world from its completion in 1948 until the BTA-6 became operational in 1975. However the BTA-6 had large defects in its mirror, and the resolving power of the Hale, 0.025 arc seconds, was not exceeded until the completion of the Keck 1 telescope in 1993.
Hale supervised the building of the telescopes at the Mount Wilson Observatory with grants from the Carnegie Institution of Washington: the 60-inch (1.5 m) telescope in 1908 and the 100-inch (2.5 m) telescope in 1917. These telescopes were very successful, leading to the rapid advance in understanding of the scale of the Universe through the 1920s, and demonstrating to visionaries like Hale the need for even larger collectors.
In 1928 Hale secured a grant of US$6 million from the Rockefeller Foundation for "the construction of an observatory, including a reflecting telescope" to be administered by the California Institute of Technology (Caltech), of which Hale was a founding member. In the early 1930s, Hale selected a site at 1,700 m (5,600 ft) on Palomar Mountain in San Diego County, California, USA as the best site, and less likely to be affected by the growing light pollution problem in urban centers like Los Angeles. The Corning Glass Works was assigned the task of making a mirror out of a new glass blend called Pyrex. Construction of the observatory facilities and dome started in 1936, but because of interruptions caused by World War II, the telescope was not completed until 1948.
The Hale and the BTA-6 are the largest telescopes made with mirrors of single pieces of glass, and are likely to remain so. The Hale's 14.5 ton mirror was a major technological achievement of the 20th century, but it is close to the maximum size that can be made rigid enough for a telescope mirror. A larger mirror would sag slightly under its own weight as the telescope is rotated to different positions, changing the precision shape of the surface, which must be accurate to within a millionth of an inch. The larger telescopes built in recent years use a different mirror design to solve this problem, with clusters of smaller flexible mirrors whose shapes are controlled by a computer servo system.
It continues to be used every clear night for scientific research by astronomers from Caltech and their operating partners, Cornell University, the University of California, and the Jet Propulsion Laboratory. It is equipped with modern optical and infrared array imagers, spectrographs, and an adaptive optics system.