More times than she can remember, Jean Mueller stood on the catwalk of the 200-inch Hale Telescope at the Palomar Observatory scanning the night sky, trying to time the exact moment to close the dome.

An hour and a half northeast of San Diego, the Palomar Observatory is owned and operated by Caltech, and as telescope operator, Mueller was responsible for protecting its instruments from the weather. Inside the structure, a 200-inch mirror captured light from distant stars in a time window crucial to the observing astronomer’s research. But when a fog bank rolled closer, Mueller had to make the call.

“I would get the dome closed within a minute or two of the fog actually hitting it,” said Mueller. “We are vigilant for anything that might damage the mirror. You don’t want acid rain on the mirror because that’s going to eat the aluminum coating. Ash, combined with humidity, can be caustic.”

Mueller, a telescope operator at Palomar from 1985 to 2014, called her path to astronomy “nonstandard.” She had a graduate degree in library science and had worked as a librarian for USC for 10 years when she learned about a job opening at a different Southern California observatory: Mt. Wilson, near Pasadena, run by the Carnegie Institution of Washington. The role: collecting data and operating the 60-inch telescope.

Mueller had begun exploring astronomy by taking a four-week evening class at Griffith Observatory. Drawn to know more, she continued taking classes at USC and Rio Hondo College. As Mueller’s astronomy community grew, her friend Howard Lanning, an astronomer and telescope operator, encouraged her to apply for a position at Mt. Wilson Observatory.

“That was probably when my life changed,” Mueller said. “It had never occurred to me to leave my library job and pursue astronomy. I didn’t have an astronomy degree; I had just taken a handful of classes.”

For as long as she can recall, Mueller has loved the stars. She remembers one specific day in 1958, when she was just 8 years old:

“My brother and I were jumping on the bed, and he told me Halley’s Comet would be visible in 1985.”

Mueller was born in an era when major research telescopes throughout the country still excluded women. Since the early 1900s, although the Carnegie Institution employed women as “computers,” with few exceptions, they were not permitted to use its telescopes. Both Mt. Wilson and Palomar had named their astronomers’ quarters “The Monastery,” male retreats where women were barred from scientific conversations. The male-only housing later became a justification to routinely deny women access to these telescopes.

By the 1950s, women were only beginning to overcome gender barriers to gain access to the telescopes at the Mt. Wilson and Palomar observatories. From Margaret Burbidge to Vera Rubin to Nobel Prize winner Andrea Ghez, pioneering women astronomers built an intergenerational legacy of research and discoveries at Palomar that would transform understanding of the universe forever.

As the first female telescope operator on Palomar, Mueller supported their work, and generations of astronomers. With the expertise and technical training she gained on Palomar, she also began to make her own discoveries.

When Mueller was offered the Mt. Wilson job, she initially worried about the financial risk of changing careers and leaving 10 years of previous experience at USC. But during this time, she chanced to attend a lecture by author Ray Bradbury.

Mueller still remembers the words that led her to take the leap into astronomy. “Whatever you do,” Bradbury advised, “be sure it makes you happy.”

After operating the 60-inch telescope on Mt. Wilson for two and a half years and becoming the first woman to operate the observatory’s 100-inch Hooker Telescope, Mueller interviewed for a new job at Palomar. In 1985, she became the operator for the Samuel Oschin 48-inch telescope, making her the first female telescope operator at the Palomar Observatory. She would stay for 29 years.

“During her first year at Palomar, Mueller worked with Caltech staff astronomer Charles Kowal, who had successfully searched for solar system objects and supernovae. An expert in taking images and scanning the fragile 14-by-14-inch plates that captured data from Palomar’s telescopes during those years, Kowal taught Mueller critical techniques in the complex process.

“Charlie Kowal was the first person to tell me that transient objects like comets and asteroids needed to be identified in a timely manner,” Mueller said. She also learned from Alain J. Maury, a French photographic scientist for the Palomar Observatory Second Sky Survey (POSSII), who taught her astrometry techniques to record the location of celestial objects.

With the encouragement of Kowal and Maury, Mueller began scanning POSSII’s plates, looking for comets, asteroids and supernovae. Scanning involved moving the plate by hand beneath a stationary eyepiece.

“There was something unbelievably exciting about discovering a new comet in the sky,” Mueller said. “A real adrenaline rush.

Mueller learned to operate all three large telescopes on Palomar: the 200-inch Hale Telescope, where she was the senior operator for 15 years; the 60-inch telescope, and the 48-inch Samuel Oschin Telescope.

Over the course of her observational career, Mueller made significant discoveries of her own. Using the Samuel Oschin Telescope, she discovered 15 comets, 13 asteroids — seven of which are near-Earth objects — and 107 supernovae.

And when Comet Halley appeared in the skies in December 1985, Mueller was operating the 200-inch telescope on Palomar. At the time, it was the most powerful telescope in the world.