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75 Years Later, Palomar Observatory Still Shaping Astronomy

"It's on the money," said Paul Nied, one of three telescope operators for the 200-inch Hale Telescope at Caltech's Palomar Observatory near San Diego. Nied was sitting in a large black chair at the helm of the controls for the massive telescope, which is nestled on Palomar Mountain. He had just directed the telescope to point at a star on the sky while astronomer Zach Vanderbosch watched nearby as the target moved into view on a screen in front of him.

"The atmosphere is pretty turbulent right now," Vanderbosch said as he peered at the screen from the chilly control room, a wool hat on his head. "The star we are observing is very far south. At lower elevations, you have to look through more atmosphere, which smears out the stars from small pinpoints of light to large fuzzy blobs."

Vanderbosch, a Caltech postdoctoral scholar in the research group of Tom Prince, the Ira S. Bowen Professor of Physics, Emeritus, was working through the night to observe a handful of dead stars called white dwarfs that he believes are circled by the pulverized remains of planets. He had observed some of the same objects the night before and was hoping to compare the two datasets to better understand the nature of the suspected planetary debris.

The telescope quietly tracked Vanderbosch's white dwarfs as they slid across the sky. Every time Nied guided the telescope to a new target, the giant machine made a whining noise and the rotating dome rumbled.

The Hale Telescope, known affectionately by many during its development as the Big Eye, is celebrating is 75th "first-light" anniversary this month. On January 26, 1949, renowned astronomer Edwin Hubble took the telescope's first picture, a celebratory moment in astronomy known as first light. The image, which shows a billowing nebula dotted with stars, marked the start of what was to be the world's largest effective optical telescope for the next 44 years, until the W. M. Keck Observatory opened on Maunakea in Hawaiʻi in 1993. (The Russian Large Altazimuth Telescope, which is larger than Palomar, opened in 1975 but never operated at its theoretical limits).

Back in Palomar's early days, the astronomers studied the dark skies from an elevated platform known as the prime focus cage, which could be as high as 78 feet above the observing floor. This is where light from the 200-inch mirror is focused. A lift would carry the scientists up the side of the dome to the cage, where they could peer into an eye piece to help find their target of interest. Shri Kulkarni, the George Ellery Hale Professor of Astronomy and Planetary Science, and the director of Palomar Observatory from 2006–18, remembers what it was like in the cage.

"It was cold, and, in the winter, you would wear a heated suit," says Kulkarni, who led the development of several instruments for Palomar telescopes. "You go all over the sky, and sometimes you are lying down or leaning forward."

Instrument Specialist Keith Matthews (BS '62), who has also built many instruments for the Palomar Observatory, says he had a good parka and snowmobile boots for observing nights and could easily stay up in the cage for 12 hours. "The mirror is big enough that you could see stars at the level of the prime focus over the edge of the cage. They look like they are floating there in the air," he says.

Today, many of the telescope control systems are automated, and astronomers often observe from the comfort of either their homes or a conference room at Caltech while communicating via Zoom with telescope operators on site. Andy Boden, the deputy director of Palomar, refers to observing from home as "pajama observing," and says that about 40 percent of researchers now make a trip to Palomar Mountain to observe in person, down from about 50 to 60 percent before the COVID-19 pandemic.

Some astronomers like Vanderbosch say they still like to observe in person. A trip to Palomar includes a stay at the astronomers' dorm, a simple white building nicknamed the Monastery tucked among a forest of fir trees. Vanderbosch, who likes to hike in the area, says he finds it rewarding to stay up all night in the control room with the operators, noting that his addiction to coffee started on these observing nights.

Photos of Palomar staff members: Greg Van Idsinga, Carolyn Heffner, Diana Roderick, Kathleen Koviak, and Paul Nied.
Pictured here are just some members of Palomar's staff who keep the observatory running smoothly. The full on-site staff consists of more than 20 Caltech employees. From left to right are: Greg Van Idsinga, Carolyn Heffner, Diana Roderick, Kathleen Koviak, and Paul Nied. Credit: Caltech

Professor of Astronomy Mansi Kasliwal (MS '07, PhD '11), who was a graduate student under Kulkarni and has also developed instruments for Palomar, still likes to make her way to the 5,600-foot mountaintop when she can. She loves the food served by on-site chefs at the Monastery. "They make me Indian food and spoil me," she says. Kasliwal adds that Palomar is the "best playground for new ideas in astronomy. Even as a grad student, you can do innovative work there."

Now, 75 years after its first light, Palomar is still going strong. "They built an extremely good telescope," says Nied, who, along with a staff of more than 20 Caltech employees, spends his nights not only taking care of the 200-inch telescope but also several others at the site. One of these, the Samuel Oschin Telescope, a workhorse also known as the 48-inch, recently celebrated its own 75th anniversary (its first light occurred in September 1948).

A vital part of Palomar's continuing success is its evolving array of instruments such as the wildly successful Zwicky Transient Facility (ZTF), a robotic camera currently attached to the 48-inch. ZTF's discoveries include the "green comet," the first asteroid known to reside inside the orbit of Venus, the first glimpse of a star eating a planet, and more than 10,000 supernovae candidates.

"They are doing things routinely now that were completely out of reach before," says Robert Brucato, who served as the assistant director of the observatory from 1982–2003 . "We have robotic telescopes like ZTF, and we can handle much more data than before due to processing with computers and machine learning. Palomar is continually renewed with new instrumentation."

The Big Eye

Before Palomar Observatory was built, the world's largest telescope was the 100-inch Hooker Telescope perched above Pasadena at the Mount Wilson Observatory. The 100-inch, along with its sister telescope, the 60-inch, are owned by the Carnegie Institution for Science. Both telescopes were developed by Caltech co-founder George Ellery Hale, widely considered to be one of the greatest telescope builders of all time.

"Hale was a one-man show who had a hunger to make things happen," said Kulkarni in a Caltech news story about Hale. "The telescopes he built are just magnificent."

George Ellery Hale
George Ellery Hale Credit: Caltech Archives

Using Mount Wilson's 100-inch telescope, Hubble discovered our Milky Way galaxy is not alone but adrift in a sea of other galaxies. Hubble also discovered that galaxies are flying away from us, indicating the expansion of space itself, a finding that provided the first hints of the explosive Big Bang that produced our universe.

These discoveries led to burning questions about the birth and fate of our cosmos—questions that could not be answered without a much larger telescope. Hale took up the fight, traveling by train back and forth to the East Coast, to discuss the telescope project with influential philanthropists. In 1928, he persuaded the Rockefeller Foundation to provide $6 million to fund the 200-inch observatory's construction. At that time, Los Angeles and its city lights had begun to spread, and Mount Wilson was no longer the ideal place from which to peer into dark skies. Ultimately, Palomar Mountain northeast of San Diego was chosen as the observatory's site, but it would be another 20 years—a decade after Hale's death—before the observatory finally opened.

The sheer size of the Hale Telescope, a 530-ton structure containing a 200-inch, first-of-its-kind Pyrex mirror, led to engineering challenges that took years to overcome. The creation, or casting, and polishing of the great mirror was plagued by one problem after another. During one crucial stage in casting the mirror, which took place at Corning Glass Works in New York, a heavy storm hit the area, resulting in floods and power outages. Corning employees and firefighters worked through the night to restore power and save the mirror.

The design and construction of the huge Hale telescope tube and mounting also presented a challenge. The steel structure is so big that the Westinghouse Corporation, which made battleship components for the U.S. Navy, was hired to produce it.

As the Big Eye slowly came together, the whole world seemed to root for the project. During the mirror-making process at Corning, onlookers watched and cheered from a platform in the building as searing-hot molten glass was poured ladle by ladle into the mirror cast. When the mirror made its way across the country via train in 1936, thousands of people came out to gaze at the odd-shaped cargo and cheer.

"The great telescope, an achievement of American science and technology in the midst of the most terrible depression anyone could remember, had become a part of the American consciousness, a symbol of pride and achievement," wrote Ronald Florence in The Perfect Machine, a history of the building of Palomar.

"Anybody alive during that time knew about the Big Eye," Brucato adds.

The grinding and polishing of the delicate mirror took place in Caltech's Optical Shop (known later as the Synchrotron building) over a period of more than 11 years. The tedious process required fastidiously clean conditions. The workers wore immaculate white uniforms, and the floors were washed daily. At times, the walls were coated with cedar oil to make them sticky enough to catch stray grains. A speck of dust under a polishing tool could make a scratch that would lead to major setbacks. During World War II, work on the mirror was largely paused as the Optical Shop was turned over to war-related research. By 1947, the mirror was finally ready to be driven up Palomar Mountain.

The 200-inch pictured in Caltech's Optical Shop.
The grinding and polishing of the 200-inch mirror took place in Caltech's Optical Shop over a period of more than 11 years. Credit: Palomar Observatory/Caltech

In June 1948, about 1,000 people gathered below the grand machine for its dedication ceremony. Several months later, the telescope saw first light, and it has not stopped operating since.

"In a quiet moment at Palomar, I stop and pinch myself because I feel so incredibly lucky to be there," Boden says. "I am from a generation that grew up reading about Palomar as this icon. The first time I walked on the catwalk that circles the dome, I knew I was hooked."

Breaking Open the Cosmos

Some of the most significant discoveries from the 200-inch telescope involve studies of distant objects. The giant eye allowed astronomers to see farther back into space than ever before—and farther back in time, since light from those distant objects can take billions of years to reach us. In 1963, Caltech astronomer Maarten Schmidt stunned the world with the discovery of the first-known quasar, an extremely luminous source of light that we know is generated by an actively feeding black hole.

"Many people didn't believe it and thought these objects must be in our Milky Way galaxy," said Richard Ellis, a former Caltech professor and director of the Palomar Observatory from 2000–05, in an obituary for Schmidt. "His discovery created this excitement that the 200-inch could look back at the evolution of our universe."

Building on the findings of Hubble and astronomer Walter Baade of Carnegie Institution, Allan Sandage (PhD '53) of Carnegie Institution used the 200-inch to study the size and expansion rate of the universe. In 1958, Sandage measured a value for the expansion rate, also known as Hubble's parameter, of 75 kilometers per second per megaparsec, which is not far from its modern value of 71 kilometers per second per megaparsec (one megaparsec equals 3.26 million light-years). He also studied the large-scale structure of the universe, declaring that it is essentially similar in all directions. "Sandage's work at Palomar made observational cosmology a real science," Boden says.

Data from the 200-inch was also used to help show how elements are synthesized by stars, a landmark finding from the 1950s known as B2FH after its authors: Margaret Burbidge, Geoffrey Burbidge, and William Fowler (PhD '36) of Caltech, along with Fred Hoyle of the University of Cambridge. The finding was based in large part on observations made by Caltech's Jesse Greenstein, an authority on the evolution and composition of stars, who spent more than 1,000 nights observing at Palomar.

"A major portion of the Infrared Army's arc also took place at Palomar," Boden says, referring to pioneers in infrared astronomy who, among other achievements, created new instruments for Palomar Observatory that captured previously unseen wavelengths of infrared light. The pioneers include Gerry Neugebauer (PhD '60), director of Palomar from 1980–94; Tom Soifer (BS '68), the Harold Brown Professor of Physics, Emeritus; and Keith Matthews. The Big Eye also enabled pioneering submillimeter observations led by Caltech's Tom Phillips. Neugebauer's graduate student Andrea Ghez (PhD '92), a Nobel laureate now at UCLA, used Palomar for her Caltech PhD thesis, which showed that most young stars in dense star-forming clouds form in pairs.

Triaging the Stars

The success of the 200-inch also rests on the shoulders of its smaller sister telescope, the 48-inch. Built at the same time as the 200-inch, with urging from astronomers Fritz Zwicky of Caltech and Walter Baade, the 48-inch telescope has a much wider field of view than the 200-inch. Astronomers recognized in the 1940s that such a telescope could scan the skies in search of the most interesting objects, which the 200-inch could then study in more detail. Zwicky used the 48-inch (and an 18-inch prototype) to discover more than 120 supernovae, holding the record for the most supernova discoveries by a single astronomer until 2009.

"This idea—surveying the sky with dedicated smaller telescopes and following up the interesting objects with a large telescope—is still very effective today," said George Djorgovski, professor of astronomy and data science at Caltech, in a story about the 70th anniversary of the 48-inch. "The 48-inch really pioneered the modern sky surveys." (Palomar's 60-inch telescope, which opened in 1970, would later join in this hierarchical scheme to follow up on discoveries made by the 48-inch.)

In its more than 75 years of operations, the 48-inch has run several sky surveys. The first, now called the Palomar Observatory Sky Survey, or POSS I, took place from 1949–58 and was funded by Caltech and the National Geographic Society. The second, POSS II, took place from 1985–2000 and was funded in part by Eastman Kodak.

Jean Mueller, who worked as a telescope operator at Palomar for nearly 30 years, exposed thousands of photographic plates for the POSS II survey. In her spare time, Mueller would scan the plates for stars that appeared in and around galaxies and mark the galaxy with a red felt pen. She would then compare that galaxy to an earlier epoch to see if a star had newly appeared. If it had, she would carefully measure the position of the star, and then an astronomer would confirm her discovery on the 200-inch. This meticulous work enabled Mueller to discover more than 100 supernovae in addition to more than two dozen comets and asteroids.

"All these photographic sky surveys have been digitized, which extends their scientific utility for decades, and makes the data freely available to anyone in the world," Djorgovski says.

After the POSS II survey, modern digital cameras were installed on the 48-inch, and several additional surveys followed. Other discoveries from the 48-inch include that of the dwarf planet Eris by Mike Brown, Caltech's Richard and Barbara Rosenberg Professor of Planetary Astronomy, in 2005. Eris's size—it is slightly heftier than Pluto—resulted in the former planet's infamous demotion.

Today, the 48-inch is home to ZTF, a robotic camera that, like its predecessors, scans the skies, but at a much faster rate. The ZTF camera scans the entire accessible sky every two nights, which means it can find bursting, exploding, and other transient objects in near real time.

The Zwicky Transient Facility (ZTF) instrument at the 48-inch Samuel Oschin Telescope at Palomar Observatory.
The Zwicky Transient Facility (ZTF) instrument at the 48-inch Samuel Oschin Telescope at Palomar Observatory. Credit: Palomar Observatory/Caltech/Justin Belicki

"You take an image and then come back the next night and compare the two images. The things that change pop out," Kulkarni says, who developed ZTF and its predecessor, the Palomar Transient Factory (PTF). "The sheer volume of data means we need machine-learning algorithms to find the objects and classify them. The ultimate goal is to automate the discovery. But you can't complete the discovery process without ancillary telescopes to pursue the newfound objects, including the 60-inch, the 200-inch, and Keck."

Kasliwal adds that ZTF has had a huge impact on astronomy. "Astronomers have already written nearly 1,000 papers involving ZTF data in just the five years that it's been in operation."

Keeping an Aging Observatory Relevant

Another factor that contributed to Palomar's longevity was the modernization of its control systems. Boden says Neugebauer put a lot of effort into updating the telescope's electrical infrastructure and control systems in the 1980s and early 90s, keeping it at the top of its game. Perhaps what best keeps Palomar "young," however, is the ever-changing suite of innovative instruments at its heart. Currently, eight instruments are regularly shifted in and out of the 200-inch telescope to fit astronomers' observing schedules. In addition, the 200-inch serves as the receiving end of NASA's Deep Space Optical Communications experiment, which recently transmitted a cat video via lasers down to the telescope, demonstrating a new way to beam high-bandwidth data from space.

One tried-and-true instrument in use since the 1980s, the Double Spectrograph, will soon be replaced with the state-of-the-art Next Generation Palomar Spectrograph. These instruments are used to spread light apart into its different wavelengths and reveal clues about the composition and other characteristics of cosmic objects.

"The new instrument will have modern, much faster optics and a high level of automation. What we could do in one hour will take 15–20 minutes," says Jonas Zmuidzinas, the Merle Kingsley Professor of Physics at Caltech and director of Palomar Observatory from 2018–23.

Other mini domes housing new kinds of instruments also reside at the Palomar site, including the infrared sky surveys called Palomar Gattini-IR and WINTER (Wide-field INfrared Transient ExploreR), developed by Kasliwal and her students. Like ZTF, both telescopes are designed to look for cosmic fireworks and other rapidly changing objects. However, they will do so using infrared wavelengths, allowing them to sleuth out objects, such as dusty supernovae, hidden in optical light.

The WINTER telescope at Palomar Observatory, with the 200-inch Hale Telescope dome behind it.
The WINTER telescope at Palomar Observatory, with the 200-inch Hale Telescope dome behind it. Credit: Matt Dieterich for Caltech

"Palomar is unique in hosting experimental instruments like these that are shaping real-time, or transient, astronomy," Kasliwal says.

Christopher Martin, the current director of the observatory and the Edward C. Stone Professor of Physics, says that the observatory remains a platform for trying risky new ideas both for science observations and creating instruments. "It's also a place for educating instrument builders," he says, noting that he and his students built the state-of-the-art Cosmic Web Imager for Palomar, a predecessor to the Keck Cosmic Web Imager (KCWI) now at Keck.

Using the Double Spectrograph, Zach Vanderbosch searched for metal pollution around his target white dwarfs—the presence of heavy metals is a sign of pulverized planets—while a camera called CHIMERA (Caltech HIgh-speed Multi-color camERA) developed by professor of astronomy Gregg Hallinan helped characterize suspected clumps of debris circling the white dwarfs. When stars like our Sun die and evolve into white dwarfs, the orbits of planets become jostled. Nearby planets that creep too close to their stars could be become shredded by the gravitational forces of a white dwarf, leaving a circling cloud of debris.

Zach Vanderbosch observing in the 200-inch dome.
Zach Vanderbosch observing in the 200-inch dome. Credit: Mike Romo for Caltech

Vanderbosch is not yet sure if this phenomenon occurred around the stars he is studying but says it is "more fun to tackle projects that are less guaranteed. There's a higher risk and reward." Although clouds and atmospheric turbulence threatened to derail his latest observ run, "In the end, the data are pretty good," he says.

One final and very important reason for Palomar's endurance is the telescope itself, a marvel of engineering that still runs smoothly 75 years later.

"To this day, I cannot believe how well they built that telescope," Jean Mueller says. "Sure, things go wrong, but the telescope always weathers it."

Written by Whitney Clavin

Whitney Clavin
(626) 395-1944