At the summit, one can find several of the world's finest telescopes, including the W. M. Keck Observatory, a partnership between Caltech and the University of California. Situated below Keck's twin telescopes is an area dubbed Submillimeter Valley, where the silver geodesic dome of the Caltech Submillimeter Observatory (CSO) has resided since the observatory's inception in 1985, along with two other facilities that observe the cosmos using similar wavelengths of light.

This past summer, astronomers, engineers, and technicians dismantled CSO's telescope, including its 10.4-meter primary mirror, or reflector, which is made of 84 hexagonal aluminum honeycomb panels and weighs 10,000 pounds. They carefully reverse engineered the mirror frame, or truss, separating it into eight pieces and then loaded these and other telescope components into shipping containers that wound down the mountain on trucks.

Ultimately, astronomers plan to ship the telescope to the Atacama Desert in Chile, where it will be reassembled and renamed the Leighton Chajnantor Telescope. The name honors both the inventor of the telescope, the late Caltech professor Robert B. Leighton (BS '41, MS '44, PhD '47), and the planned site for the observatory on the high Chajnantor Plateau.

"The submillimeter light we want to observe from the cosmos can be absorbed by water vapor in the air, and the air is even drier at Chajnantor, so being there will enhance our observations," says Sunil Golwala, director of CSO since 2013 and a professor of physics at Caltech.

With its rebirth in Chile, the observatory will focus in part on the fast-growing field of time-domain astronomy. It will observe, in real time, a wide range of stellar eruptions and explosions, for which millimeter-submillimeter observations are in their early days. The facility will also expand on its original work, observing objects near and far, from planetary and stellar nurseries in our own galaxies to the most distant galaxies that date back to the universe's first billion years.

Piece by Piece
Caltech announced that CSO would be decommissioned in 2009. In 2015, Caltech suspended CSO operations and initiated the decommissioning process, which is outlined by the State of Hawai'i's 2010 Decommissioning Plan for Maunakea Observatories. The physical decommissioning began in 2022 and is being done in accordance with the Conservation District Use Permit issued by the State of Hawaii Department of Land and Natural Resources.

The first step in dismantling CSO's mirror was to remove the 84 panels that make up the mirror surface. Screws attaching the panels to the truss were removed one by one, and the panels were lifted off using a crane. The truss itself was then lifted off the base of the telescope in one piece by a large crane and placed down on the ground, where it was then further dismantled into eight pieces. The crane extracted the heavy base of the telescope in seven pieces.

"A large crane was needed because of the weight of the individual pieces, up to 10,000 to 20,000 pounds, and because the crane had to reach into the dome to get at the telescope," Golwala says.

The mirror panels, truss, and other telescope parts have been carefully packed into standard shipping containers and are stored at the Kawaihae Harbor on the west coast of Hawai'i island awaiting shipment to Chile in 2024.

Still remaining on the mountain is the helmet-like dome of the observatory. Its removal, paused due to winter weather, will continue in the spring of 2024.

Prior to the telescope disassembly, a blessing was performed to commemorate the beginning of efforts to restore the CSO site. Representatives from Caltech and the Center for Maunakea Stewardship were in attendance. Maunakea is also known as Mauna a Wakea, which refers to the first-born mountain son of the mythical creators of the Hawaiian Islands: Papahanaumoku (Earth Mother) and Wakea (Sky Father). "We seek to thank and honor Maunakea and those in its genealogy by respectfully restoring the site," Golwala says.

New Window to the Cosmos
Robert Leighton designed and led the construction of a set of seven telescopes in the 1970s and 1980s (the Raman Research Institute, in Bengaluru, India, built an eighth telescope using Leighton's design). Six of Leighton's telescopes have been used at Caltech's Owens Valley Radio Observatory (OVRO) near Bishop, California, for decades. "Leighton made the CSO's mirror surface more precise than the others, so it would work well at the submillimeter wavelengths accessible from Maunakea," Golwala says.

The telescopes were made at a time when the field of submillimeter and millimeter astronomy was just beginning to take off. Submillimeter light, which has a wavelength about 10 times the width of a human hair, falls between infrared and radio on the electromagnetic spectrum. It can be used to peer into regions of the cosmos that are thick with dust, and it can carry signatures of dust and other molecules that play crucial roles in the formation of planets, stars, and galaxies.

To enable the cost-effective construction of such large mirrors at these wavelengths, Leighton came up with a clever design that included lightweight aluminum honeycomb panels tiled together in a hexagonal pattern and supported by a truss made of steel-tube struts and posts. Leighton wrote a computer program to figure out the dimensions of the roughly 80 different kinds of struts needed to make the desired truss shape.

The observatory dome itself housed the control room, laboratories, and other support systems. It was first assembled on Caltech's then-football field beginning in 1984 and was later dismantled and reassembled on Maunakea (the telescope was constructed on Maunakea for the first time). CSO achieved "first light" with a spectrum of Messier 82, a starburst galaxy, in March of 1987.

"When the CSO first began observations, you had to be standing right there to tune the receivers," recalls Golwala, who was a postdoc at Caltech in the early 2000s when the observatory was in full swing. "Telescopes and instruments are controlled remotely by computer now, so one no longer needs to be in the dome to use the observatory."

Receiver Revolution
The submillimeter light receivers employed at CSO were a first-of-their-kind innovation from the late Caltech professor Thomas Phillips, who served as director of CSO from 1986 to 2013. The devices, called superconductor-insulator-superconductor (SIS) receivers, opened a window to the previously unexplored territory of submillimeter light, reshaping astronomy. They were first used at OVRO and then further developed for CSO and other observatories. In fact, they are still in use today at the world's most productive ground-based observatory in operation, the Atacama Large Millimeter/submillimeter Array (ALMA), in Chile.

In addition to the SIS receivers, Caltech professor of physics Jamie Bock and late Caltech professor Andrew Lange developed bolometers for CSO using JPL's Microdevices Laboratory (JPL is managed by Caltech for NASA). Bolometers are simpler in concept than SIS receivers and easier to fabricate. They detect light by measuring the heating of a microscopic light absorber using a sensitive thermometer (the word stems in part from the Greek word "bole," which can translate to ray of light).

CSO was one of the first observatories where spiderweb bolometers-bolometers in which the absorbing material is supported by a web etched out of silicon nitride-were demonstrated. Such bolometers were later used on Lange and Bock's BOOMERAnG balloon telescope, which showed that the universe's geometry is not curved but flat. These bolometers were also used in the successful Herschel Space Observatory and Planck missions.

"CSO opened a window to the electromagnetic spectrum at submillimeter wavelengths, which was a new frontier to explore, so there were many discoveries made," says Jonas Zmuidzinas (BS '81), the Merle Kinglsey Professor of Physics at Caltech. "It was also a place where instrument builders could experiment with new technologies, bringing them from proof of concept to working receivers."

Among its many discoveries, CSO determined the role of atomic carbon in the space between stars; discovered a new phase of stellar evolution for red giant stars; made the first ground-based detection of heavy, or deuterium-based, water in a comet; identified a rare type of ammonia in space that includes three atoms of deuterium; imaged planet-forming disks around stars; and, in 2011, uncovered what was then the largest and farthest reservoir of water in the universe: a quasar, or actively accreting supermassive black hole, surrounded by water molecules.

The observatory also pioneered studies of the Sunyaev-Zel'dovich effect, which occurs when the light from the early universe (the cosmic microwave background) becomes skewed as it passes through massive galaxy clusters on its way to Earth. These observations are important for, among other pursuits, studying the incredibly energetic collisions that occur when clusters of galaxies merge.

A Clever Design
One of the challenges faced by Golwala and his collaborators was to figure out how to take the aging observatory apart. "It's 40 years old, and we don't have a lot of documentation on how things were put together," he says. For advice, the team spoke with OVRO assistant director David Woody, an expert on the telescope's design and performance, who worked with Leighton when the telescopes were built.

"It's a rare situation," Golwala says. "Most astronomers do not get the opportunity to figure out how to take apart and rebuild a telescope. Luckily, Bob Leighton's telescope design was so clever that it has been relatively easy to figure it out. We were also lucky to find independent contractor Bill Johnson, who has experience building telescopes around the world, to lead the disassembly process."

Simon Radford, former CSO technical manager who serves as project manager for the move, says the possibility of overstressing and damaging the truss struts was a major concern during disassembly. "As was done during the original assembly, we floated the truss on springs so the connections between sections were almost stress free," Radford says. The struts came out with only modest effort."

Now that the telescope has been removed from Maunakea, general contractor Goodfellow Bros. Inc. (GBI) will remove the CSO dome and related structures, including underground plumbing, electrical and communications conduits, and other components. GBI will restore the site, grading and contouring the land and adding ash and small rocks to restore its natural appearance. The Leighton Chajnantor Telescope, CSO's next incarnation, is expected to begin operations in 2026.

"What was once the site of one of the world's premier submillimeter telescopes will be home to lichens and insects and be indistinguishable from the mauna around it," Golwala says. "We are grateful for the time we spent there."

The decommissioning effort is funded in large part by the Gordon and Betty Moore Foundation, while the careful preparation of the telescope for shipment and reuse in Chile is funded by the Heising-Simons Foundation. The move to Chile will be partially supported by the municipality of Shanghai through a collaboration with Shanghai Normal University. In Chile, the project's lead institution is the Universidad de Concepcion.

Related Links
Caltech Submillimeter Observatory
Stellar Chemistry, The Universe And All Within It