× 0. Palomar Observatory
M. Visitor Center (Museum)
H. Hale Dome

The 200-inch (5.1-m) Hale Telescope and Dome

Dedicated in 1948 and the largest effective telescope in the world until 1993, the 200-inch Hale Telescope is a workhorse of modern astronomy and contributes to a wide range of astronomical research including Solar System studies, the search for extrasolar planets, stellar population and evolution analysis, and the characterization of remote galaxies.

The Hale Telescope is a reflector, that is, a telescope whose primary optical element is a curved mirror—there are no lenses in the telescope itself. The Hale's primary mirror is a 200-inch (5.1-m) in diameter Pyrex disk that weighs 14.5 tons (13 tonnes). Its polished surface, covered with a thin layer of aluminum, is concave. The mirror's thickness varies between 19 ⅝ inches (49.8 cm) at the center and 23 ½ inches (59.7 cm) at the outer edge.

The 200-inch mirror and instruments are supported by a steel equatorial mount, which allows for east-west and north-south motion. Moving this 530-ton (481-tonne) telescope must be done precisely if astronomical observations are to be possible at all. For slewing (rapid movement) it uses two small motors: a 3-hp motor for right ascension and a 1-hp motor for declination. For tracking (keeping up with Earth’s rotation during long exposures) it is moved by a 1-hp step motor—this replaced the original 1/12-hp tracking motor after almost 65 years of continual use. The Hale Telescope is kept in perfect balance. As such, when instruments are changed the balance of the telescope must be adjusted. The piers for the Hale Telescope are anchored to the bedrock 22 feet (6.7 m) below, while the dome supports go about 7 feet (2.1 m) into the overlying granite. The dome is 135 feet (41 m) tall and 137 feet (42 m) in diameter. It is a remarkable coincidence that these dimensions are similar to those of the Pantheon in Rome.


The Hale Telescope began scientific operation in 1949, and was widely viewed as the world’s most prominent astronomical facility for the next 45 years. The Hale has a moving mass of 530 tons, and is enclosed in a dome with a moving mass of over 1000 tons. Motions of these massive structures are controlled to the exacting tolerances necessary to collect and direct incident starlight into a diverse instrument suite that makes measurements in visible, ultraviolet, and near-infrared light.

The telescope is in a so-called “equatorial” mount: the telescope mount outer axis is aligned with Earth’s rotational or north polar axis. Motion of this outer structure allows the telescope to view sources east and west of Palomar, and track them as they appear to move east-to-west across the sky with Earth’s rotation. Astronomers call this direction right ascension. Supporting the telescope’s 530 tons in the right ascension structure are a set of pressurized oil bearings—during operation the telescope literally rides on a thin layer of oil with no mechanical contact to the underlying foundation. The inner telescope truss holds the telescope optics and instrumentation. This truss mass is roughly 100 tons, and is supported on mounts that allow the telescope to point north and south of Palomar’s latitude along what astronomers call declination. This declination truss is supported with a large set of roller bearings and spokes that both look and function like a familiar bicycle wheel. The surrounding dome has a slit that opens to expose the telescope to the night sky, and can rotate in azimuth on a set of rails and wheels reminiscent of railroad construction. Sophisticated control mechanisms coordinate telescope and dome motions during astronomical observations.

The main telescope optics are all reflective—curved mirrors that use aluminum on their front surfaces to direct incident starlight into instruments at three different telescope foci. The 200-inch/5.1-m primary mirror weighs roughly 14 tons and is cradled in a protective steel cell bolted to the bottom of the telescope tube. Larger instruments that often separate incident light into its constituent colors—spectroscopy—are typically mounted below the primary mirror cell at the so-called Cassegrain telescope focus. Smaller imaging cameras can be mounted at the top of the telescope tube at the so-called primary telescope focus. Astronomers rarely “look” through the Hale telescope, but instead control and collect data with these instruments from the safety and comfort of the nearby data room, just behind the visitor gallery south wall.

Upcoming guided tour.