The “Big Eye” on Palomar Mountain

Past a highway sided by orange groves, avocado stands, and an occasional patch of prickly pear cactus, and up a road that winds through the campgrounds and hiking trails of the Cleveland National Forest, sits the famed 200-inch Hale Telescope on Palomar Mountain in north San Diego County, California.  The 200-inch was the largest effective telescope in the world for 44 years, and unquestionably the most famous until the Hubble Space Telescope.  It’s still an impressive sight up close and remains a workhorse of modern astronomy.

The Basics

• The 200-inch telescope was conceived and planned by George Ellery Hale, an astronomer and visionary who made possible four successive telescopes that were the world’s largest: the 40-inch refractor at Yerkes, the 60-inch and 100-inch reflector at Mount Wilson, and the 200-inch at Palomar that now bears his name.

• Plans for the telescope began in 1928 after $6 million in funding was secured through the Rockefeller Foundation.  The glass blank for the mirror was created by the Corning Glass Works out of a then-new material called “Pyrex”.  Grinding, figuring, and polishing of the mirrors removed 5 tons of glass and took 13 years, including a 4-year gap caused by World War II.

• The telescope was opened in mid-1948 at a meeting attended by dignitaries from all over the world.  Some took a slow spin on the telescope’s massive dome.  Edwin Hubble himself took the first image with the 200-inch in January 1949.

• After an prodigious but exhausting career, Hale died in 1938 and never saw the completion of his greatest work.

The Dome of the 200-inch Hale Telescope on Palomar Mountain

A Deeper Look

• The telescope is a Cassegrain with aperture of 5.1 meters, figured at a focal ratio of f/3.3.  Until recently, astronomers actually sat in a cage at the prime focus of the big mirror at the top of the telescope and stayed there all night to take images and spectra.  Today, most observations are done at the Cassegrain focus at the bottom of the tube with electronic equipment so the astronomers stay warm in the control room.

• The 200-inch was the largest telescope in the world from 1949 until 1976, when a 6-meter Russian telescope was commissioned.  But the 6-meter had significant problems with its optics, and never had the utility or cachet of the Hale, which was not surpassed in resolving power until the 10-meter Keck I telescope came online in 1993.

• While now dwarfed by larger telescopes such as the Keck, Gemini, and Subaru, the 200-inch is still a workhorse of astronomy.  Caltech, which runs the observatory, gets 1/2 the observing time, while Cornell University and the Jet Propulsion Lab get 1/4 each.

• The telescope has been updated with adaptive optics to cancel out the turbulent effects of the atmosphere.  The “Big Eye”, as it was called when it was finished, is still used for astronomical research some 290 clear nights each year.

Good To Know

The word Palomar means “pigeon house” in the language of the La Jolla indians.  While many call the mountain “Mount Palomar”, the location’s proper name is “Palomar Mountain”.  The mountain also houses a 60-inch reflector, the famous 48-inch Schmidt Camera used for the Palomar Sky Survey, and an 18-inch Schmidt Camera.

Personal View

It was a treat to see the 200-inch up close during a recent pilgrimage to the observatory on a cold snowy day in late March (snow seems to follow me everywhere).  While I knew the specifications of the telescope, I was struck by its immensity.  Sure, it’s far from the biggest telescope in the world.  But it’s still a massive machine… the truss tube is 60-feet long and weighs 70 tons.  Yet it’s so well balanced and engineered that a 1/10 horsepower motor can move the whole thing.

This telescope and the astronomers who used it made huge advances in understanding the structure and size of the universe, and it did me good to stand in its presence.  You can see the Hale telescope for yourself at the Palomar webcam.