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Nathaniel Phillips Carleton (1929–2020)

A long-term employee of the Smithsonian Astrophysical Observatory, Nat Carleton's proudest achievement was his involvement in the design and construction of the MMT telescope at Mount Hopkins, Arizona.

Published onJul 20, 2020
Nathaniel Phillips Carleton (1929–2020)

Photo courtesy of the Carleton family.

Nathaniel Phillips Carleton died on Monday the 25th of February, 2020.

Nathaniel (Nat) Phillips Carleton was born in Burlington, Vermont, on March 16, 1929. He died from complications of Parkinson’s disease in Cambridge, Massachusetts, less than a month shy of his 91st birthday.

Carleton attended Phillips Academy Andover, earned his bachelor’s degree from Harvard College in 1951, and earned a doctorate in physics from Harvard University in 1956. In his graduate work he theorized about the causes of the northern lights. While in graduate school he co-founded a company (Granville-Phillips) with Daniel Bills to manufacture a vacuum pump they had patented.

He stayed on at Harvard immediately after graduating to teach physics as an instructor and subsequently Assistant Professor. He later recalled days when he would teach a course at Harvard College and then walk across the Cambridge campus and give the same lecture at Radcliffe College. Years later he expressed satisfaction that dividing the genders was no longer necessary, as the colleges were united.

He was hired as a physicist by the Smithsonian Astrophysical Observatory in 1962, which would later become part of the Harvard-Smithsonian Center for Astrophysics. He was briefly an associate director of the Division of Optical and Infrared Astronomy in the mid-1970s.

After his PhD work he turned from studying the Earth to studying Venus and Mars, using spectroscopy to determine which gases were present in each planet’s atmosphere. He found that Venus was much hotter than previously understood, and discovered oxygen in the atmosphere of Mars.

Nat Carleton’s proudest accomplishment was his involvement in the design and construction of the MMT at Mount Hopkins, Arizona. The initials originally stood for “Multiple Mirror Telescope,” as the light gathering surface was a combination of six mirrors, each of 1.8-m diameter. This gave the equivalent light gathering power of an unobstructed 4.5-m diameter telescope.

Carleton was initially taken aback by what he described as a “paralysis” that seemed to afflict many astronomers who were in awe of the Palomar Observatory 200-inch telescope — a telescope that had revolutionized our view of the universe after its dedication in 1948. “I am almost serious in saying,” he recalled, “that the belief was that the 200-inch had been designed by supernatural beings and that mere human effort to explore improved designs would be futile.” He teamed up with a group of colleagues from the University of Arizona and built a large telescope that included many new features that would aide astronomers in their quest to get the best possible images from Mount Hopkins. Light in design and incorporating six primary mirrors in a common mount, the MMT was designed such that air would flow through to allow the telescope to adapt to temperature changes over the course of the night so that images were not blurred by warm air rising around the structure — a frequent problem with telescopes. Another key feature of the MMT is that it has an altitude-azimuth mounting. All large optical telescopes built since the MMT have alt-az mountings. Carlton recalled: “Our purpose in creating the MMT was twofold: to provide ourselves with a useful large telescope and to jolt the astronomical community into an awareness of new possibilities. We have certainly achieved both goals.”

In 1976 Carleton became the Multiple Mirror Telescope Project Scientist and Resident Director at Mt. Hopkins Observatory, a position he held until 1979. The MMT operated with the six primary mirrors from 1979 to 1998. In 2000 it was rededicated with a single primary mirror of diameter 6.5 meters.

Carleton went on to lead an initiative as Chief Scientist for a collaborative effort among a group of five institutions to build an Infrared Optical Telescope Array (the IOTA project). “I had the privilege of supervising the design, construction, and operation of a prototype array of small optical-infrared telescopes,” he later recalled. “Bringing together the beams from widely separated telescopes has the potential of making images that can show sharper detail than a single telescope can manage.” He was able to bridge the gap between engineers, physicists, and astrophysicists to get the job done, colleagues said. While the quality of the image was of utmost importance, he also kept in mind the cost of building each telescope. “I led a determined effort to demystify the business of telescope construction, so that we might obtain a useful instrument for a very small cost,” he later recalled in written reports regarding his career. Noting that it cost one-third of the Palomar 200-inch telescope, he said, “It has its limitations, but in its designed role as a spectrometer feeder I believe that it must hold a record for productivity per unit cost.”

Carleton officially retired in 1996, but continued to consult on projects for another decade.

Retirement provided opportunities for fun and adventure, such as motorcycle riding, hiking, rock climbing, and sailing all over Lake Champlain. He built a houseboat and lived on it for a time in Boston Harbor. He and his wife built a 42-foot sailing catamaran and sailed from Florida to Boston in the early 2000s.

His love of the stars was nearly eclipsed by his love for singing. He conducted local choirs in Carlisle and later in Winthrop, delighting particularly in the nuances of Bach’s Passion According to St. John. More recently, he sang with the Church of the Covenant choir. When performing, he often wore a tuxedo he bought to sing with when he was in the Harvard Glee Club during his undergraduate studies. It continued to fit him for the rest of his life.

Dr. Carleton is survived by his wife, Kay Lapsley, his five children, Sarah, Phillips, Mary, Jane, and Glen; his stepchildren David Gibbs, Sarah Hoagland, Julia Gibbs, James Gibbs, David Beldock, Jennifer Fogarty, Hannah Beldock, and Benjamin Beldock; his brother Peter Carleton; his grandchildren Emma and Eli Stickgold, Kira, Madeline, and Emily Carleton, Sage Carleton-Ferris, Chantal Peters, Steven Hutchinson, Zakwani and Ozma Gibbs, Isabelle Hoagland, and Amelia Gibbs; and many relatives in Vermont.

Direct and efficient with his words, his family regarded him as thoughtful, wise, and passionate about his opinions. Many noted his overall humility about the work he did over the years.

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