Expanding Horizons
First Light
Gentleman Astronomers
More Glass
1917: Discovering the Universe
1931: 'Listening' to the Stars
1939: Telescopic Teamwork
1946: Aiming for the Stars
1948: The Giant Eye
Sky High

Telescope History

More Glass

Diagram of a star and planet orbit a common center of mass.

If astronomers had adopted a catchphrase during the first half of the 20th century, it probably would have been “bigger is better.” Giant new telescopes sprang up like wildflowers on a sunny spring day, providing astronomers with ever more impressive views of the universe.

The key to this growth industry was the ability to cast, mold, and install giant glass mirrors.

The task is much more difficult than it sounds. The mirror must be as thin as possible to hold down weight and prevent the mirror from slumping as it is tilted. The raw glass must be melted and then cooled slowly to keep the glass well mixed and prevent bubbles from forming, because any flaws in the glass itself could cause the mirror to crack, ruining years of work. Grinding and polishing to just the right shape takes years more, because a deviation of just a few millionths of an inch in the mirror’s curved shape will ruin its focus, giving it a fuzzy view of astronomical objects. And the reflective coating must be applied evenly and smoothly. Despite problems and delays — and World War II — telescope makers created some extraordinary scientific instruments, most of which are still in use today. They capped their run with the creation of the great telescope at Palomar Observatory, with a mirror that’s big enough to hold two automobiles side by side.

Using these giant instruments, astronomers discovered that the universe consists of a multitude of galaxies, which are rushing away from each other as the universe itself expands. They deciphered the mysteries of what makes stars shine and discovered new bodies in our solar system.

And all because in the case of a telescope, bigger really is better.


Discovering the Universe

Mount Wilson domeThe dedication of the 100-inch telescope at Mount Wilson, California, sets the stage for Edwin Hubble to prove that the universe consists of a multitude of galaxies.


'Listening' to the Stars

A search for static leads to a new field of study: radio astronomy.


Telescopic Teamwork

82-inch in 1939McDonald Observatory joins the game with the completion of its 82-inch reflector.


Aiming for the Stars

Spitzer space telescopeLyman Spitzer proposes launching a telescope into space.

Lyman Spitzer

June 26, 1914
Toledo, Ohio

March 31, 1997
Princeton, New Jersey


• First to propose space-based telescope

• Leader of early space-based ultraviolet telescope, Copernicus

• Namesake of Spitzer Space Telescope

As World War II ended, the U.S. Army was considering possible uses for rockets, like the German-built V-2. A young astronomer and physicist, Lyman Spitzer, was asked to study the possible scientific uses of orbiting satellites.

In his report, "Astronomical Advantages of an Extra-Terrestrial Observatory," Spitzer proposed launching small ultraviolet telescopes to study the Sun and some of the hottest and most powerful objects in the universe. Earth's air blocks ultraviolet light (which causes sunburn and other problems), so it can be studied only from well above the atmosphere.

At the same time, Spitzer proposed launching an optical telescope (which views the wavelengths of light visible to the human eye) with a primary mirror 5 to 10 meters in diameter. Free of Earth's obscuring atmosphere, such a telescope would see far more clearly than any ground-based instrument, and it could be far more stable than ground-based telescopes.

Copernicus satellite

The Copernicus satellite is prepared for launch. [NASA/GSFC]

It was a daring recommendation. The giant Palomar telescope, with a 5-meter mirror, had not yet been completed, and its mirror alone would weigh about 10 tons. What's more, rockets of the day could not reach orbit, and even their brief one-way trips into space carried a total payload of a few hundred pounds. The technology to implement his plan seemed almost like a science-fiction dream.

Yet Spitzer was convinced that space-based telescopes would work, and he set out to convince his colleagues, too. He eventually succeeded.

In 1965, Spitzer chaired a National Academy of Sciences committee that studied possible research projects for a large space telescope. Its report formed the basic outline for the research plan for Hubble Space Telescope, which NASA began developing in the 1970s. Spitzer was an enthusiastic campaigner for the telescope, helping convince both colleagues and politicians of its value for American science.

In 1972, Spitzer was the lead scientist for NASA's fourth Orbiting Astronomical Observatory, which was named Copernicus after it reached orbit. As Spitzer had recommended 26 years earlier, it was equipped with a small ultraviolet telescope. During nine years of operations, it scanned more than 500 ultraviolet sources, including massive stars and white dwarfs.

Because Spitzer was still living when NASA launched its Large Space Telescope in 1990, the agency could not name the telescope in his honor, so it picked Edwin Hubble instead. A few years after Spitzer's death, though, NASA launched another of its "Great Observatories," to study the infrared sky. It named this orbiting telescope for Spitzer, honoring the man who had championed space telescopes for more than a half century.

Spitzer space telescope Spitzer Space Telescope [NASA]

More Information

Lyman Spitzer, Jr.

Lyman Spitzer Jr.

Lyman Spitzer and the Space Telescope


The Giant Eye

Palomar Observatory in California dedicates the world’s largest and most famous telescope.

In the 1950 sci-fi flick Rocketship X-M, the Air Force knows just where to turn for help when its loses contact with a Moon-bound rocket: Palomar Observatory in California, home of the 200-inch Hale Telescope. Stock footage shows the art-deco dome, the complex control system, and the giant telescope itself, with an astronomer riding in a cage at the top.

An astronomer rides in the observing cage. [Palomar Observatory/Caltech]

For the two decades between its conception and completion, the 200-inch telescope was revered as one of the wonders of modern science and technology. During the lean years of the Great Depression, people bought tickets to watch technicians grind and polish its mirror, which was known to press and public and "The Giant Eye." Even today, it remains the perfect example of a research telescope, with a massive but beautiful structure filling a capacious dome.

The Hale Telescope's scientific accomplishments match its beauty. Since Edwin Hubble snapped the first photograph through the telescope in early 1949, astronomers have used it to help decipher the lifecycles of stars, map the distribution of galaxies, and discover the brilliant beacons at the edge of the universe known as quasars.

The nearly completed 200-inch mirror blank [Palomar Observatory/Caltech]

The telescope was the creation of its namesake, George Ellery Hale, who had already spearheaded construction of the 40-inch refractor at Yerkes and the 60- and 100-inch reflectors at Mount Wilson. Completing the 100-inch exhausted Hale both mentally and physically, so he left administration to pursue his own research.

Yet even as the 100-inch neared completion, Hale had dreamed of even bigger telescopes to probe deeper into the universe. Such behemoths were necessary to address a new field of science, known as cosmology, which studies the birth, evolution, and fate of the universe. Such grand ideas required observations of vast numbers of galaxies at the greatest possible distances, and the ability to detect objects and phenomena that no one had yet dreamed of.

The telescope is dedicated on June 3, 1948. [Palomar Observatory/Caltech]

In 1928, Hale convinced the forerunner of today's Rockefeller Foundation to spend $6 million to build the giant new telescope.

But finding the money was easier than actually building it. Large telescope mirrors are heavy, so as they move around they can sag and deform under their own weight, ruining their view. In addition, no one had ever cast such a large slab of glass to the level of perfection required by the new telescope, and many optics experts feared the task was impossible.

And at first, it looked like they were right.

The 100-inch mirror for the Mount Wilson telescope was cast from conventional glass, like that used in bottles. Although the mirror was good, it expanded and contracted too much as it heated during the day then cooled at night. A larger mirror would face even greater difficulties.

At first, Hale contracted with General Electric to make a mirror of fused quartz. But after several failed attempts, he turned to New York's Corning Glass and its new glass material, Pyrex, which changed little as it heated and cooled.

The first casting — conducted under constant public view — failed. But a second succeeded, and in 1937 the 40,000-pound mirror blank was transported to Pasadena, California, where opticians would grind its surface to the perfect shape.

The glass disk itself was not completely solid. Instead, it used a "honeycomb" structure on the back to reduce weight, making the mirror less likely to deform. After it was completed, a series of counterweights and pressure points would help the mirror maintain the precise shape required to provide sharp views of astronomical objects.

World War II shut down work on the mirror, but after the war, technicians completed grinding it to within a few millionths of an inch of the proper curved shape. In all, they ground about 10,000 pounds away from the original disk.

The mirror was mounted in the telescope tube in 1947, and dedicated in 1948. The telescope is about 85 feet long, and with its unique yoke-shaped mounting weighs more than 500 tons.

The 200-inch telescope reigned as the world's largest until 1976, when the Soviet Union completed a 236-inch instrument. Its vision was never as sharp, though, so no telescope outperformed it until the Keck Telescope was completed in the 1990s. Today, improved instruments and control systems and an adaptive optics system to compensate for some of the blurring of Earth's atmosphere help keep The Giant Eye at the forefront of astronomical research.

More Information

The 200-Inch Hale Telescope

History of Palomar Observatory

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

National Park Service: Palomar Observatory 200-Inch Telescope

The First 50 Years at Palomar, 1949-1999

PBS Program: Journey to Palomar

The Perfect Machine, by Ronald Florence (HarperCollins, 1994)

Palomar dome

Summertime view of the telescope dome [Damond Benningfield]

Next: Sky High