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DOD’s largest telescope atop Haleakalā on Maui gets mirror recoat, preserves space domain awareness

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VIDEO | 06:45 | The Advanced Electro-Optical System (AEOS) telescope receives a recoat. (Courtesy photo / Boeing)

By Jeanne Dailey, Air Force Research Laboratory

The 3.6-meter, 75-ton Advanced Electro-Optical System, or AEOS. telescope, shown with the mirror recoat team, is the largest optical telescope in the Department of Defense. The mirror received its second recoat since the installation of AEOS at the Air Force Maui Optical and Supercomputing site in 1997. AMOS is part of the Air Force Research Laboratory and keeping the mirror in prime condition is key to the U.S. Space Force’s space domain awareness mission. (Courtesy photo / Boeing)

The Air Force Maui Optical and Supercomputing site’s Advanced Electro-Optical System, or AEOS, the Department of Defense’s largest telescope, measuring 3.6-meters or 11.9 feet, has received a face-lift.

Located on the summit of the 10,023-foot volcano Haleakalā, the telescope is part of a series of telescopes called the Maui Space Surveillance System, which the US Space Force uses for space domain awareness, or SDA, recognizing space as a priority domain for advancing national security.

The site combines a research and development mission under the Air Force Research Laboratory, one lab supporting two services, and an operational mission under the US Space Force’s 15th Space Surveillance Squadron, a USSF Space Operations Command unit activated in May 2022.

Following a year of planning and four months of execution, the site completed the recoating of the AEOS, the telescope’s primary mirror. AEOS is a reflector telescope, indicating it has a small secondary mirror placed near the prime mirror’s focus to reflect light through a central hole, increasing the magnification and sharpness of objects in the sky.

Workers strip and wash the Advanced Electro-Optical System’s primary mirror at the Air Force Maui Optical and Supercomputing site, Maui, Hawaii, in preparation for its mirror recoat. AMOS is part of the Air Force Research Laboratory, and the AEOS telescope supports the U.S. Space Force with the nation’s space domain mission to operate freely in space. (Courtesy photo / Boeing)

Keeping the AEOS telescope’s primary mirror in quality condition is paramount to the site’s SDA mission, said Lt. Col. Phillip Wagenbach, who is both the squadron commander and branch chief of the AFRL Directed Energy Directorate’s research and development mission.

The Advanced Electro-Optical System’s primary mirror cell, which contains the mirror substrate, moves from its telescope location at the Air Force Maui Optical and Supercomputing (AMOS) site, Maui, Hawaii, to the unit’s mirror coating facility, where it will undergo a mirror recoating. AMOS is part of the Air Force Research Laboratory, and the AEOS telescope supports the U.S. Space Force with the nation’s space domain mission to operate freely in space. (Courtesy photo / Boeing)

“I am honored to lead both of these critical functions that preserve our access to and freedom to operate in space,” Wagenbach said. “Periodic recoating of the AEOS’s primary mirror ensures readiness of the telescope to support the SDA mission for the warfighter. There is never really a good time to take the telescope out of service, but it is better to plan the recoat as a periodic maintenance effort than having to shut down the telescope due to catastrophic mission degradation.”

The first mirror recoat occurred in late 2008, roughly 12 years after application of the original coating in 1997. The long duration between the original coating and the first recoat was mainly driven by the construction of the mirror recoat facility, completed in 2008.

“Large mirrors like the AEOS 3.6-meter should be recoated every 4 to 6 years, but performance requirements are highly dependent on the mission of the telescope,” said Scott Hunt, the site’s technical director. “For our SDA mission, we have the challenge of detecting dim objects during the nighttime and performing imaging of satellites during the daytime. Typically, our SDA objects are brighter than astronomical objects, and therefore we can push the coatings longer than the astronomers can.”

Hunt said scientists and engineers track the degradation of reflectivity and scatter of the mirror over time. They weigh this against the risk of recoating, the telescope being out of service and mission performance for daytime imaging and dim object detection.


“The bare aluminum coating of the AEOS primary mirror degrades overtime,” Hunt said. “When the coating is first applied, it is approximately 1,000 Angstroms thick, or about 1/7th the width of a human hair. Imperfections in the original coating increase scatter and decrease reflectivity and can accelerate degradation. These imperfections include stains, pinholes and splatter spots created by dust and contaminant on the mirror substrate or drips of aluminum at the time of coating.”

Removing the primary mirror cell from the telescope and relocating it to the mirror recoating facility is a delicate and time-consuming process that ends in a rapid recoating of the mirror.

“Once the mirror cell is transferred from the telescope on the fourth floor of the AEOS building to the mirror coating facility on the first floor, it takes about two weeks to remove the mirror substrate from the cell, strip off the old coating, and prepare the coating chamber,” Hunt said. “After the mirror is in the chamber, the reflective coating is applied by vacuum deposition with aluminum-coated tungsten filaments over a period of 15 to 20 minutes. When the aluminum on the filaments begins to vaporize, the actual coating process takes less than a minute.”

The Boeing team poses with the Advanced Electro-Optical System, or AEOS, primary mirror following a recoat at the Air Force Maui Optical and Supercomputing (AMOS) site, Maui, Hawaii. This was the second recoat of the mirror since AEOS’s initial installation in 1997. AMOS is part of the Air Force Research Laboratory, and the 3.6- meter AEOS telescope supports the U.S. Space Force’s space domain awareness mission. (Courtesy photo / Boeing)

On-site Boeing staff performed the recoating along with support from government leadership, the facilities contractor and external experts. During the process, the team encountered a few challenges and even a surprise.

“Probably the biggest challenge was the cleanliness of the mirror coating facility and ensuring little or no contamination on the substrate before sealing in the vacuum chamber,” Hunt said. “Stripping and cleaning the substrate was a critical process, particularly the final wipe-down to remove any residue from chemicals used during the stripping and cleaning process.”


Hunt said through use of a hepa-filter and a cleanroom plastic shroud around the vacuum chamber, they were able to maintain a much lower particle count within the bell of the chamber.

“We also fabricated a ‘drumhead’ cover that was placed over the mirror substrate immediately following the cleaning process,” Hunt said. “The drumhead cover proved to be an effective innovation to mitigate particulate accumulation on the substrate while we made final preparations in the chamber.”

During the process, insects surprised the team several times.

“An excited moth fluttering around on our clean substrate inside the chamber would be catastrophic to the coating process,” Hunt explained. “We were able to perform an extraction with the cleanroom vacuum without suffering any adverse effects to the new coating.”

To validate the recoating process, the Maui team sent results to private industry coating experts in Albuquerque, New Mexico. and Tucson, Arizona.

“The report we received was the coating achieved during this recoat was that ‘the results were excellent and among the best they had ever seen on a large mirror of this type’,” Wagenbach said.


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