Palomar Observatory

A dream and a Pyrex mirror

After the success of Mount Wilson, George Ellery Hale sought money for a telescope of 200 inches, and in 1928 the Rockefeller Foundation granted $6 million, through the California Institute of Technology. The mirror, a 5-metre disk of Pyrex, was cast by Corning in 1934 and ground and polished at Caltech until 1947. Palomar Mountain, in San Diego County, was chosen for its distance from Los Angeles, and the dome and mount were built in the late 1930s; the war interrupted the work.

The Hale Telescope was dedicated on 3 June 1948, and first light came on 26 January 1949. Hale did not live to see it; he died in 1938.

Wide fields and the sky survey

The 18-inch Schmidt camera began operating in 1936, and a 48-inch Schmidt, later named for Samuel Oschin, followed. Its wide field was used for the first Palomar Observatory Sky Survey, completed in 1958 with about 1,900 photographic plates covering the sky north of declination minus 33 degrees. The Survey was for decades the reference atlas of the northern sky, and its plates were digitised for modern use.

With the 18-inch Schmidt, Eugene and Carolyn Shoemaker and David Levy discovered Comet Shoemaker–Levy 9 in 1993; the comet struck Jupiter in 1994.

Quasars, surveys and robots

The Hale Telescope was where Maarten Schmidt measured the redshift of the quasar 3C 273 in 1963, showing that quasars are at cosmological distances. Today the Hale carries adaptive optics and spectrographs, and the Zwicky Transient Facility on the 48-inch telescope scans the sky every night for supernovae, asteroids and other transients. Caltech runs the observatory.

A sky that changed

The Hale Telescope was built for a dark sky, and Palomar was chosen for being some 150 kilometres from Los Angeles. The city grew, and the sky brightened, so that the observatory had to fight for dark-sky protection in the region, and succeeded in persuading San Diego County to adopt outdoor lighting rules. The Sky Survey plates, taken in the 1950s, are thus also a record of a sky that no longer exists in the same form. Its history shows that the quality of a site is not a fixed property, and that it can be protected by local law and by the goodwill of neighbours.

Instruments and technologies

• 200-inch Hale Telescope A 5.08-metre Pyrex mirror, ribbed on the back to reduce weight, in a horseshoe-mounted equatorial telescope with a 'prime-focus cage' large enough for an observer.
• 48-inch Schmidt camera A Schmidt corrector plate corrects the spherical aberration of a spherical mirror over a field of 6 degrees, recorded on curved photographic plates.
• Zwicky Transient Facility camera A 47-square-degree CCD camera on the Schmidt, repeating survey scans of the entire northern sky every two nights.

The people

• George Ellery Hale Conceived the 200-inch; died in 1938.
• Edwin Hubble Used the Hale Telescope in its first years; helped establish its science.

Timeline

• 1928 Rockefeller Foundation grants $6 million for a 200-inch telescope.
• 1936 18-inch Schmidt camera begins operating; dome construction starts.
• 1948 Hale Telescope dedicated on 3 June; first sky survey plates exposed in November.
• 1949 Hale Telescope first light on 26 January.
• 1958 First Palomar Observatory Sky Survey completed.
• 1993 Comet Shoemaker–Levy 9 discovered with the 18-inch Schmidt.

Key lessons for observation today

• Wide-field surveys are a data service The Sky Survey served astronomers for decades. A survey designed as a shared reference outlasts the telescope that made it.
• Let the machine do the repeating The Zwicky Transient Facility scans every night and sends alerts. Automated detection and alerting are central to modern observatories.
• Materials decide scale Pyrex, with its small thermal expansion, made a 5-metre mirror possible. Hardware advances in materials are what set the limits of an observing system.

Sources

• Wikipedia, Palomar Observatory
• Caltech, A History of Palomar Observatory
• Wikipedia, Hale Telescope