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

The 200-inch (5.1-m) Mirror and Coating Chamber

The Hale Telescope is a reflector, that is, a telescope whose primary optical element is a curved mirror. The Hale's primary mirror is a 200-inch (5.1-meter) 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 basic structure of the telescope was completed long before its 200-inch mirror was ready. Initially General Electric was hired to fabricate a 200-inch disk made out of quartz. Just over 600,000 dollars later, that idea was abandoned. The man behind the project, George E. Hale, approached the Corning Glass Works of New York with a proposal to instead cast the 200-inch mirror out of a glass blend called Pyrex. The Corning mirror project was performed under the direction of George McCauley. It took a month to melt the glass needed to cast the mirror.

A failed attempt to cast the mirror disk took place on March 25, 1934. A second disk was cast on December 2, 1934. The disk remained in the oven at pouring temperature for just over a month and then was gradually cooled over a period of ten months. McCauley personally checked on the cooling operation daily.

On March 26, 1936, the mirror blank began its 16-day trip by rail from Corning to the Caltech optical shop in Pasadena. The telescope project captured the public’s imagination, and all across the country thousands of people lined the train tracks to watch it pass. Under the direction of John A. Anderson and Marcus H. Brown at Caltech, the mirror disk was ground and figured into the proper shape. It began as a 20-ton disk and finished at 14.5 tons. Including the delays from World War II, the disk was in the optical shop for 11 ½ years.

The mirror finally arrived at Palomar on November 19, 1947. It was transported on the back of a flatbed truck with two additional trucks behind it pushing it up highway S-6 (South Grade Road, then called the “Highway to the Stars”) and to the observatory site.


The Hale Telescope’s metaphoric heart is the 200-in/5.1-m primary mirror. This massive optic is literally the entrance pupil for the telescope, much as our pupils define the light that enters our human eyes.

Like the Hale itself, the primary mirror was an extraordinary feat of engineering research and development, taking more than twenty years to complete from original conception to astronomical use. The mirror glass was manufactured by the Corning Glass Company in New York State, taking two years to pour, cool, package, and deliver to Caltech in Pasadena in 1936. The mirror material is Pyrex, a form of glass widely used in kitchen applications for its heat tolerance. The favorable thermal properties of Pyrex help the Hale primary keep its parabolic shape even with changes in mirror temperature. After eleven years of mirror shaping and polishing at Caltech, the mirror was carefully transported from Pasadena to Palomar and the Hale dome in November 1947, and saw first astronomical use roughly 18 months later.

The 14-ton mirror rides in a protective steel cell bolted to the bottom of the telescope structure. The mirror glass is so massive that it sags under its own weight—changing its shape when the telescope is pointed away from the zenith. To minimize sagging and the optical distortions it would create, the mirror is supported in the cell at 36 support points, each with its own dynamic support mechanism patterned after a lever and fulcrum system. As the telescope points away from the zenith, gravity pulls weights down on one end of the lever, and that force is transferred into an opposite upward force on the mirror back structure to oppose mirror shape and optical distortions.

To make the mirror glass highly reflective a thin layer of aluminum is deposited onto the front surface. This aluminum layer is very thin—3-4 millionths of an inch (roughly 100 nm) in thickness—and in order to function properly it must be very uniform across the mirror surface. The coating degrades with time and exposure to air, so the mirror is periodically resurfaced. As one might imagine we don’t send a 14-ton optic out to an offsite provider for servicing—the recoating must be accomplished here in the Hale dome. Just behind the telescope and to the right is the primary mirror recoating chamber. The Hale primary and cell are decoupled from the telescope and lowered onto the rail-supported cart. After removing the old coat and cleaning the optic, the recoating chamber top is lowered to enclose the mirror and the new aluminum coat is vapor-deposited—a process developed for the Hale by Caltech engineer John Strong back in 1930. After successful quality assessment the mirror and cell are recoupled to the structure and the telescope is returned to service.

Upcoming guided tour.