George R. Carruthers, an astrophysicist and engineer who was the principal designer of a telescope that went to the moon as part of NASA’s Apollo 16 mission in 1972 in an effort to examine Earth’s atmosphere and the composition of interstellar space, died Dec. 26 at a Washington hospital. He was 81.

His brother Gerald Carruthers confirmed the death, saying Dr. Carruthers had dementia and other ailments.

Dr. Carruthers, who built his first telescope when he was 10, had a singular focus on space science from an early age and spent virtually his entire career at the Naval Research Laboratory in Washington. He was one of the country’s leading African American astrophysicists and among the few working in the space program.

He began working on his Apollo telescope in 1969, when NASA posted what was called an “announcement of opportunity” to design experiments for Apollo space flights. In November 1969 — four months after the first astronauts walked on the moon — Dr. Carruthers received a patent for an “Image Converter for Detecting Electromagnetic Radiation Especially in Short Wave Lengths.”

In other words, it was a specialized kind of ultraviolet telescope, or spectrograph, that could observe radiation and other properties in space. (Another scientist, Thornton Page, proposed a similar idea, and the two joined forces for the NASA project, with Dr. Carruthers as the principal investigator.)

Assuming the dual roles of conceptual scientist and practical engineer, Dr. Carruthers led a team that designed a telescope that could electronically amplify images from space through a series of lenses, prism and mirror, just three inches in diameter. Then, by converting photons to electrons, the images could be recorded on film. In 1970, an early model of his telescope was included in an unmanned rocket flight that found the first evidence of molecular hydrogen in interstellar space.

The instrument — sometimes called an electronographic camera — had to be small enough to fit aboard a spacecraft, strong enough to withstand the rigors of being on the lunar surface and precise enough to measure materials that could be observed only in ultraviolet light. Plus, it had to be manipulated by an astronaut wearing a spacesuit and thick gloves.

“There was still a dichotomy between engineers and scientists” in his early years at the Naval Research Laboratory, Dr. Carruthers said in a 1992 oral history interview with the American Institute of Physics. “When I talked to engineers, especially when I wanted to get parts made in the machine shop, they had sort of a negative attitude towards scientists because they felt that scientists didn’t know how to design things, they weren’t skilled at putting things together, they were all thumbs. They sort of found it strange that I was claiming to be a scientist, yet I was doing all my own drawings and doing a lot of my own assembly of parts.”

When mounted on a tripod, Dr. Carruthers’s lightweight magnesium telescope stood about four feet high. It was covered in gold plate to protect it from the moon’s extreme temperatures. Dr. Carruthers gave instructions to astronaut John W. Young, the Apollo 16 commander, on how to operate the device.

On April 21, 1972, the lunar module from Apollo 16 touched down on the moon. For the next 71 hours, Young and fellow astronaut Charles Duke used Dr. Carruthers’s telescope to peer deep into space, capturing more than 200 images of Earth’s atmosphere, hundreds of stars and distant galaxies.

In essence, it was a planetary observatory on the moon, the first time such a sophisticated telescope had been used by astronauts in space. The observations had far-reaching implications for astronomy, astrophysics and the understanding of how stars are formed.

“It was spectacularly successful, imaging the earth’s outermost atmosphere in its entirety in the far ultraviolet range of the spectrum,” David DeVorkin, senior curator of the history of astronomy at the National Air and Space Museum, told The Washington Post in an email. “It also surveyed myriad clouds of gas, stars and galaxies in deep space.”

Dr. Carruthers continued to refine his telescopes and develop experiments at the Naval Research Laboratory for decades. In 1986, one of his instruments captured an ultraviolet image of Halley’s comet.

He also designed instruments used aboard Skylab and space shuttle flights and for satellites measuring polar auroras and luminescence in the upper atmosphere. Dr. Carruthers’s original Apollo 16 telescope is still on the moon where the astronauts left it in 1972. A replica has been displayed at the Air and Space Museum.

“George Carruthers was one of the most amazingly focused scientists I have ever met,” DeVorkin said. “He lived to innovate and was endlessly improving his design for a telescope that could electronically amplify light by orders of magnitude and yet was robust enough to survive a rocket flight. His telescopes were physically small, yet extremely powerful.”

George Robert Carruthers was born Oct. 1, 1939, in Cincinnati. His father, an engineer who worked at Ohio’s Wright-Patterson Air Force Base, died when Dr. Carruthers was 12.

The family, which included four children, resettled in Chicago, where his mother worked for the U.S. Postal Service.

Dr. Carruthers built his first telescope out of glass lenses and a cardboard tube. He won science prizes throughout his youth, read science fiction and was fascinated by space exploration, an idea then in its infancy.

At the University of Illinois at Urbana-Champaign, he received a bachelor’s degree in aeronautical engineering in 1961, a master’s degree in nuclear engineering in 1962 and a doctorate in aeronautical and astronautical engineering in 1964.

He then became a research physicist at the Naval Research Laboratory. In a rare 1971 interview with The Post, Dr. Carruthers said he worked 14-hour days, seven days a week. In the seven years he had been at the naval lab, he had not yet taken a vacation.

He was described as “painfully shy,” but “something happens to George when he’s addressing his peers on astrophysics,” a colleague told The Post. “He gives beautiful lectures.”

Beginning in the 1980s, Dr. Carruthers worked extensively with science outreach programs, particularly in schools with large numbers of Black students. He developed an apprentice program for high school students at the Naval Research Laboratory and taught summer courses for science teachers in D.C. public schools.

After retiring from the research laboratory in 2002, he taught Earth and space science for several years at Howard University. Dr. Carruthers, who lived in the District, received the NASA Exceptional Scientific Achievement Medal and was named to the National Inventors Hall of Fame. He also received the National Medal of Technology and Innovation, presented by President Barack Obama at a White House ceremony in 2013.

In 1973, he married Sandra Redhead, who died in 2009. Survivors include his wife of nine years, Debra Thomas, and two brothers.

Dr. Carruthers seemed surprised when a Post reporter asked him in 1971 whether he had any hobbies.

“Hobbies?” he said. “The projects we have here are so varied that it’s hardly necessary to have a hobby.”