The following timeline covers the early history of Palomar Observatory, from its conception in the early 1900's, through the 200-inch's twenty-one years of construction until it began full-time observing in 1949. All images on this page, except the last one (1949), are copyrighted by the Caltech Archives. Click on any image to enlarge.

1908    1917    1928    1929    1930    1934    1936    1937    1938    1941    1947    1948    1949   

1908: Mt. Wilson 60-inch telescope
Wilson 60-inch, 186K

Under the supervision of George Ellery Hale, and through grants from the Carnegie Institution of Washington, the Mount Wilson 60-inch telescope is completed and sees "first light." It is the world's largest telescope, and with the exceptionally calm and stable atmosphere above the Los Angeles basin, astronomers can see fainter and more distant objects than ever before. Harlow Shapley uses this telescope to measure the size of our galaxy (the Milky Way) and the solar system's position in it.

1917: Mt. Wilson 100-inch telescope
Wilson 100-inch, 167K

While the 60-inch is constructed, Hale and the Carnegie Institution plan a yet larger design. Despite technical challenges in casting and shaping the glass mirror, and difficulties with funding, the 100-inch telescope is completed in 1917. It is a temperamental machine - the large mirror is highly sensitive to temperature variations which cause it to go out of focus easily. Despite such problems, it provides an unparalleled view of the faraway universe. Edwin Hubble uses this instrument to determine the distances and velocities of neighboring galaxies, demonstrating that they are separate "island universes" and not small nebulae contained within the Milky Way, as many astronomers had previously thought. He also discovers the first indications that the universe is expanding. Measurements of more distant galaxies, and fine details of the near ones, are still beyond the reach of the 100-inch.

1928: Rockefeller grant for 200-inch telescope
Hale, 54K

Hale (pictured) secures a grant of six million dollars from the International Education Board, a funding agency endowed by the Rockefeller Foundation, for "the construction of an observatory, including a 200-inch reflecting telescope... and all other expenses incurred in making the observatory ready for use." Unlike the Mt. Wilson observatories, which are operated by the Carnegie Institution, the 200-inch is administered by the recently founded California Institute of Technology (Caltech). Hale and his teams of astronomers, engineers, and opticians set to work.

1929: The Schmidt Camera
Bernard Schmidt, 27K

Bernard Schmidt, an Estonian optician, invents a new telescope design ideal for photographing large regions of the sky. It uses a simple spherically-curved main mirror, with a carefully shaped glass corrector plate at the front of the telescope to compensate for optical distortions. He shows a prototype to Walter Baade, a colleague of Hale's, and the Schmidt design is later utilized for photographic survey conducted on the 18-inch telescope and 48-inch Samuel Oschin Telescope.

1930-34: Site selection
Site from above, 159K

With the increasing light pollution from Los Angeles, Mount Wilson is no longer an ideal site for an observatory. Hale starts a survey of less populated locations for the planned 200-inch telescope. Sites in Arizona, Texas, Hawaii, and South America are considered, but the early favorite and eventual winner is a site at an elevation of 5,600 feet on Palomar Mountain, 100 miles southeast of Pasadena, California. Hale buys one hundred sixty acres of land from local ranchers and the U.S. Forest Service.

1934-36: A Pyrex Mirror
Glass ladle, 26K

After spending over 600,000 dollars, mirror casters from General Electric are unable to make a 200-inch mirror out of fused quartz. Hale approaches the Corning Glass Works of New York with a proposal to instead build the 200-inch mirror out of a new glass blend called Pyrex. Changes in temperature make Pyrex expand and contract much less than ordinary glass, so a Pyrex mirror would be much less prone to the focus and distortion problems that plagued the 100-inch telescope. Corning starts planning how to cast molten Pyrex (left) with the necessary purity and smoothness. On their second attempt, Corning succeeds in casting the 200-inch mirror. The image to the right shows two people standing on the original unpolished surface.

Cast 200-inch mirror, 100K

1934-36: Telescope structure design work
i Blueprints, 140K

Once the mirror specifics are finalized, engineers start designing the telescope's structure. It will weigh hundreds of tons, but must be able to move smoothly and accurately to follow celestial objects as they transit across the sky. While tracking, the mirror must maintain its shape to a few millionths of an inch. Several revolutionary and ingenious engineering concepts are implemented into the design to meet these requirements, including the oil bearing system, the Serrurier truss, and the mirror support cell.

1936: Dome construction
Dome started, 72K
Dome middle, 89K

While the road to the mountain is improved, and water and electricity are installed, construction work begins on the 200-inch dome. Cottages are built for some of the important personnel, while other workers live in barracks that are part of a nearby cattle ranch.

The telescope piers are anchored to the bedrock 22 feet below, while the dome supports go about 7 feet into the overlying granite. During the summer, everyone on the mountain helps pour concrete, including several Caltech undergraduates and the observatory cooks. Work proceeds briskly, and the dome is completed in less than two years.

The finished dome is 41 meters (135 feet) tall, 42 meters (137 feet) in diameter. It is a remarkable coincidence that these dimensions are similar to those of the Pantheon in Rome. The dome weighs approximately 1,000 tons, with a plate steel exterior and aluminum panel interior, separated by four feet to allow for dome venting. Two 125-ton shutters cover the opening seen in the center image and slide open at night to allow light through the slit and into the dome.

The top section of the dome rotates on two circular rails. Many people who have ridden on the rotating dome have commented that it feels smoother than most elevator rides.

For more pictures see the photos of Thomas R. Young, a plumber working in the 200-inch dome or the Kelso photos of a phone crew working on Palomar Mountain in the 1930's.

Dome top, 50K
Dome done, 89K

1936: Mirror transport
Mirror Ribs, 83K

The mirror blank, with only a rough flat front surface, is shipped across the country on a special train from New York to Pasadena, always traveling slower than 25 miles per hour. The telescope project has captured the public imagination, and thousands of people line the train tracks to watch this special cargo. Guards are posted around the mirror during overnight stops to prevent any damage to the disk. The trip takes sixteen days.

Mirror on train, 162K

1936-47: Mirror grinding and polishing
Polishing 200-inch, 110K

In the optics lab at Caltech, the front surface of the mirror is ground to the approximate concave form required. Using successively finer polishing grit, the opticians then carefully smooth the surface, constantly using optical tests to compare it to a perfect paraboloid shape. It is slow and painstaking work. To make the final mirror, almost 10,000 pounds of glass are polished away, including the top two inches which contain "scar tissue" left over from the casting and annealing process.

Polishing machine, 85K

1936: 18-inch telescope in operation
18-inch, 46K

The small Schmidt camera is put into service, used primarily to monitor nearby galaxies for supernova explosions. The performance of the Schmidt design is so good that the project supervisors discuss building a larger Schmidt telescope to photograph the entire night sky. These two telescopes will complement the 200-inch perfectly, since they can quickly take long exposures of large areas of the sky. Until such wide-field images are available, astronomers must make educated guesses about where to look to find new, interesting phenomena.

18-inch, 49K

1937: Telescope construction
Truss, 173K

Components of the telescope are constructed at sites all over the country and then shipped to the mountain for assembly inside the dome. Parts from Westinghouse's Philadelphia factories, Corning's New York glass foundries, and Caltech's and Carnegie's Pasadena labs have to make their way up to the mountain summit. On many occasions, national train routes are rescheduled as these parts travel across the United States. The telescope tube is shipped by boat through the Panama Canal, with the Navy's help.

In the upper left, you see the open telescope tube. On the bottom left are the "arms." The west arm would eventually house the declination axis motor, while the east arm was available for a variety of instruments. In the bottom right image are the two arms and the central component of the giant "horseshoe" (seen in its entirety in the bottom center). In the upper right photo, a welder works near the Cassegrain end of the telescope tube.

Horseshoe, 116K
Welder, 71K
Assembling the tube, 142K  Installing the horseshoe, 76K

1938: 48-inch Schmidt started
48-inch mirror, 59K

Using funds and technology from the 200-inch telescope, work on the 48-inch Schmidt (now called the Samuel Oschin Telescope) begins. Corning casts the main mirror for the telescope and Pasadena opticians make the refractive corrector plate. This smaller telescope, named the Samuel Oschin telescope in 1987, has a wide viewing area (thirty-six square degrees). This field-of-view lets astronomers make detailed maps of the entire northern sky and allows them to systematically select targets of interest for study with the more powerful 200-inch. The drawing to the right was composed by Russell Porter.

Russel Porter drawing of 48-inch, 93K

1941: World War II
Caltech in WWII, 248K

Telescope production halts because most of the engineers and scientists, as well as their laboratories, are reassigned to war-related projects. Not even mirror polishing continues during the war. The 200-inch disk is stored and protected by timbers for three years. After the war concludes, telescope work restarts in September of 1945. After three months of cleaning the labs, mirror polishing resumes. Most of the pre-war telescope workers do not continue with the project, so a new crew must learn the routines.

1947-49: Mirror transport and installation
Mirror to mountain, 77K

The 200-inch mirror is transported from Pasadena to Palomar on November 18-19, 1947. The 40 ton cargo requires three diesel tractors to push it up the mountain. Despite a storm, which nearly aborts the transport, the 125 mile trip is completed in 32 hours.

After removing the concrete disk (now located outside the dome) that was used to test the support structure, engineers install the mirror. Initial imaging results are promising but not ideal. It takes two years to finish polishing, aligning, and adjusting the mirror.

1948: Dedication ceremony
Dedication, 240K

Although the 200-inch telescope is still not yet fully operational, it is dedicated on June 3rd and formally named in honor of George Ellery Hale, who passed away in 1938. Almost one thousand people attend the dedication, including many dignitaries from around the world. The first demonstration of the telescope and dome includes a ride on the dome as it spins. The ride is smooth enough to confuse some into thinking the telescope floor is rotating.

1948: First light on the 48-inch
Hubble using 48-inch, 142K

The 48-inch Samuel Oschin Schmidt Telescope is completed and for many years it would be the largest Schmidt telescope in the world. The first official photograph is taken in September and its image quality is good enough that it is used in Hubble's galaxy atlas. One year later, the 48-inch begins the first Palomar Observatory Sky Survey, which maps the entire northern sky. This catalog would later become the basis for the Guide-Star Catalog used by the Hubble Space Telescope. A second (digitized photographic) sky survey would start in 1985 and finish 15 years later. To the left, Edwin Hubble peers through the finder telescope of the 48-inch in 1949.

1949: Full-time science observing begins
Dome under Milky Way, 40K
Copyright Danner/Hogg

Thirteen years of mirror polishing finally grind the "Big Eye" to the desired form. Edwin Hubble takes the first photographic exposure with the 200-inch in January. In October, the telescope is made available full-time to the astronomers from Caltech and the Carnegie Institution, twenty-one years after the Rockefeller grant. Pictured to the left is a few minute exposure of the open dome underneath star trails and the summer Milky Way.

Palomar, After 50 Years from The Journal of San Diego History

To read a summary of the Palomar Observatory's research results see THE FIRST 50 YEARS AT PALOMAR: 1949-1999 The Early Years of Stellar Evolution, Cosmology, and High-Energy Astrophysics by Allan Sandage

  • Jesse Greenstein Memorial Film and In the Prime Focus cage with Jesse Greenstein

    Images are for personal use only and are copyrighted by the California Institute of Technology's Palomar Observatory.