alomar Mountain Observatory

One popular tourist destination in the San Diego Area is a trip to Mount Palomar to tour the Mount Palomar Observatory, home to the George Hale 200-inch (5m) reflecting telescope. The construction planning for the Observatory began in 1928 when astronomer George Hale secured an initial grant from the Rockefeller foundation. After its completion in 1948, its 18-foot wide reflecting mirror was the largest in the world until the USSR built the BTA-6, a 6-meter reflecting telescope located on Mt Pastoukhov, in the Caucasus in 1976. It was not until the late 1990's, when a large number of new telescopes were built, that any other optical telescopes surpassed the Hale Telescope in size.

The construction of this single telescope and observatory was an epic task.

The 200-inch mirror alone took a decade to construct. In fact, making the mirror required an entirely new glass making technology. The mirror blank was to be a pyrex cylinder eighteen feet wide and four feet thick, weighing tens of thousands of pounds. No one had ever molded a glass blank of this size before. Not only that, it had to be made of extremely pure glass that could be ground into a nearly perfect optical surface to serve as the telescope's mirror. The Corning Glass Works in New York had to build a special mold and invent new glass pouring technology to make the blank.

One of the vital processes in making the blank was cooling the molten pyrex very slowly over a period of several months, in a process called "annealing". Unfortunately, during the annealing of the first blank, heavy rains caused massive flooding in upstate New York. Despite the best efforts of the Corning workers to prevent it, the cold water flooded the annealing chamber, and the rapid cooling caused massive cracks in the pyrex. An extra year was required to produce another pyrex mirror blank.

After the blank was completed in 1936, the mirror had to be transported across country by train to Pasadena, California, where optical grinding was conducted. Grinding the surface of the glass blank to optical perfection and mirroring the surface took another eleven years, including the four years where work had to be halted during World War II. 10,000 pounds of glass were polished away during this process, which finally ended in 1947.

The huge weight of the 200-inch mirror and the steel telescope and scaffolding that support it also required another technical innovation to make it possible for motors to turn the telescope. The entire telescope and mounting were installed on an "oil bearing", so that, instead of resting on a flat surface, the telescope rests suspended on a very thin later of oil. This allows the telescope to be moved easily, without its massive weight and friction gluing it in place.

Tenth planet discovered at Palomar Observatory!

The Hale Telescope is still a working scientific instrument, despite the increasing light pollution from Los Angeles and San Diego. The Palomar Observatory supports the research of the Cal-Tech faculty and students, as well as researchers from other institutions.

The grounds of the observatory are open every day, and there are walking paths and a shaded picnic area adjacent to the observatory dome. The observatory itself is open from 9:00 a.m. to 4:00 p.m. daily, and visitors are allowed to view the telescope from the observation gallery. There is also a visitor's center containing displays of pictures taken by the Hale Telescope, as well as a full-sized representation of the telescope's massive 200-inch mirror. A small gift shop is open daily from mid June through Labor Day and only on weekends for rest of the year.

Because the Hale Telescope is still a working scientific instrument, nighttime visits are generally prohibited. There is, however, one exception. The Reuben H. Fleet Science Center hosts a series of special guided tours of the Palomar Observatory. The tours include lectures on topics of current interest in astronomy and (weather permitting) viewing through the 60-inch telescope. Contact the Fleet Science Center for more details. Click here to get more web information on the Palomar Observatory.