Mount Wilson’s Telescopes Expanded Understanding of the Universe

Within the span of about 13 years, between 1904 to 1917, astrophysicist George Ellery Hale built five telescopes at Mount Wilson Observatory in Pasadena, Calif., all of which were the largest in the world when constructed—two were even built simultaneously. 

“Mount Wilson Observatory represents the first example, really in any science, for an enormous amount of money to be dumped into what amounts to an industrial scale operation,” explained Tim Thompson, trustee and science director at the Mount Wilson Institute. “They were steam rolling along up there, and astronomy and the astronomical sciences changed much faster because of Mount Wilson Observatory than it ever has before or since, because of that enormous explosion of building the biggest telescopes in the world.” 

The final of those five, the 100-inch telescope, the first with the ability to see past the Milky Way, was designated as an ASME historic mechanical engineering landmark in 1981. The 100-inch telescope made “many notable advances in structural cosmology” that “revised our ideas about the universe in which we live,” and its “outstanding engineering features” included the use of mercury flotation in the telescope’s mount to reduce friction, as noted on the Society’s landmark plaque.

Years in the making 

Hale’s research interests centered on stellar evolution and the sun. After Hale graduated from MIT in 1890, his father, who made his fortune in the elevator industry in Chicago after the Great Chicago Fire, built the Kenwood Observatory for him on the family estate in Chicago, Thompson explained. It was there that Hale first began to study the sun. 

He went on to become the University of Chicago’s first professor of astrophysics, then co-founded and directed the Yerkes Observatory on Lake Geneva in Williams Bay, Wisc. While there, Hale oversaw the construction of the largest equatorially mounted refracting telescope in the world, which was completed in 1897. 

“But Hale was consistently unsatisfied with what he had. He felt restricted—his telescopes weren’t big enough and he wanted to do infrared astronomy, which nobody was doing at the time really. And you can’t do that with a refractor,” Thompson explained. “But Hale was always one step ahead on the next project.” 

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In fact, Hale’s father ordered glass in 1894 from the Saint-Gobain Glassworks in France that would eventually become the 60-inch telescope’s mirror at Mount Wilson 14 years later. 

“Hale was already plotting to start Mount Wilson Observatory with the largest telescopes in the world before he even had the largest telescope in the world in his own hands at Yerkes,” Thompson said. “So, he kept the mirror. He knew he did not want to put that telescope on Lake Geneva. He came out west.” 

Upon relocating to Pasadena in 1903 and deciding Mount Wilson was the place to build, Hale brought the Snow Solar Telescope over from Yerkes in 1904. With the Snow, Hale proved spectroscopically that sunspots were lower in temperature than the rest of the sun, which explains their relative darkness, Thompson said. 

To avoid some of the issues he encountered when using the Snow telescope, Hale developed a concept for a telescope that featured a high tower and no tube in 1904. Mount Wilson’s 60-foot tower telescope was realized just four years later, followed by a 150-foot tower in 1912. The Snow and two tower telescopes were the largest solar telescopes in the world from 1904 to 1962. “So, throughout the entire first half of the 20th century, that’s where you came to study the sun. A whole bunch of discoveries were made there,” Thompson said. 

Since nighttime observation would be essential to understand the sun as it applies to other stars, Hale began work on the 60-inch telescope at the same time as the 60-foot tower, both of which were completed in 1908. 

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“He immediately went to work—and I mean really immediately—on the 100-inch telescope,” Thompson said. The 100-inch telescope was completed in 1917. 

“Between 1904 and 1917, five telescopes were built on the mountain: three solar, two nighttime, all of which were the largest in the world when they were built,” he said. “Nowhere else in history, before or since, has anything like that been done. Mount Wilson stands very much alone and unique in that respect.” 

Peak engineering 

The dome that houses the 100-inch telescope is about 100 feet in diameter and has a moving weight of about 500 tons, while the telescope itself weighs about 100 tons. 

“You’re in a building that’s designed to rotate and move. And things aren’t supposed to hit each other unless you do something wrong,” added Ken Evans, a retired electrical engineer who previously worked at the Jet Propulsion Laboratory’s Failure Analysis Lab and has been a volunteer at Mount Wilson since 1999. 

“This was designed all by hand,” Evans said. “The building was put together first with nuts and bolts in Chicago, and then dismantled and brought out here, and then reassembled on the mountain permanently with hot rivets.” 

But supporting such a massive telescope required similarly massive structures. The telescope sits on an isolated 33-foot concrete pier, so vibrations won’t affect the instrument.  

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“The 100-inch was so heavy, they had to build an English yoke mount for it to support that kind of weight on two points,” Thompson said. “There are two mercury support systems for the 100-inch—only one for the 60-inch. The 60-inch’s moving weight is 22 tons and for the 100-inch, it’s about 100 tons. Because it was so heavy, it needed some extra flotation.” 

Mercury flotation was used for the first time on the 60-inch telescope. On the 100-inch's polar axle are self-aligning journal bearings, while two steel drums floating in mercury support the telescope’s weight. This solution reduced friction in the support system so well that the entire 100-inch telescope can be adjusted with just one hand. 

Meanwhile, a worm gear system, 18-feet in diameter and made mostly of cast iron, sits at the south end of the polar axle to adjust the telescope’s sidereal rate. 

“The 100-inch is set up as a polar alignment system. This means that the main axis is pointed at the north star,” Evans said. “With this alignment you only need two directions of motion. First is the east/west motion called ‘hour angle’ or ‘right ascension.’ The second is declination, or DEC, and this is a north/south motion.” 

As for the telescope’s mirror, which took more than five years alone to grind and polish into just the right shape, its diameter is what gives the telescope its name. Weighing nearly 9,000 pounds, it remains the largest solid glass mirror in the world. 

Past to present 

Initially, the 100-inch’s engaging motors were nearly all mechanical, and the electrical system was direct current, or DC, Evans explained. In the early sixties, it was converted over to a different type of motor from the DC. Then around 2014, the 100-inch telescope’s control system underwent another upgrade to new electronic assemblies and drive motors, he continued. 

In 2017, the telescope’s old system was removed and the new system installed. But after testing began, the team heard a loud high-pitched noise coming from the DEC slew drive system, Evans recalled.  

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“It sounded like a dry or failing bearing. After studying the problem, it was discovered that the new pulse driven motor had found the resonate frequency of the worm driven gear,” he explained. “This gear is large in diameter and made from brass with a thin web to support the outer gear teeth. To prove this, a large open-ended wrench was bolted to the drive gear and the system tested again and the noise went away.”  

So, the team machined aluminum plate segments to fit the inside of the gear on both sides and installed automotive sound deadening material between the plates and the gear.

“It was an eye opener to me as an electrical engineer, how an electric motor can affect the mechanics of a machine,” Evans said.  

Today, Mount Wilson’s telescopes mostly serve as reminders of the historical achievements realized under these domes. With the help of Mount Wilson’s telescopes, Harlow Shapley was the first to lay out the structure of the Milky Way Galaxy. Meanwhile, Edwin Hubble used the 100-inch to discover the true nature of other galaxies, such as the spiral nebula. 

It wasn’t just scientific advances in astronomy and astrophysics, but also corresponding sociological advances, Thompson explained. “So huge changes in the way people think about themselves, the universe, and everything else come out of Mount Wilson,” he said.  

And although the observatory’s instruments have been left behind in aperture, both the 60-inch and 100-inch telescopes are still available for use. They can even be rented overnight to observe the sky. In addition to daily tours, a monthly concert series is held in the 100-inch dome, taking advantage of the unique acoustics within.  

“We’ve just completed a deal with the Las Cumbres Observatory, which is a worldwide network of telescopes, completely automated, totally robotic, to install one of its telescopes on Mount Wilson,” Thompson said. “And we have plans to bring the 100-inch back into doing some kind of academic research. It’s the largest telescope in all of Los Angeles County, and it’s still the third largest telescope in California.” 

Louise Poirier is senior editor.