By Jeanne Dailey, Air Force Research Laboratory Public Affairs
The advanced electro-optical system at the Air Force Maui Optical and Supercomputing site, or AEOS, the Department of Defense’s largest telescope, measuring 3.6 meters or 11.9 feet, has had a facelift.
Located atop the 10,023-foot Haleakalā volcano, 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 area for advancing national security.
The site combines a research and development mission under the Air Force Research Laboratory, a laboratory supporting two services, and an operational mission under the U.S. Space Force’s 15th Space Surveillance Squadron, a unit USSF Space Operations Command Activated May 2022.
After a year of planning and four months of execution, the site has completed the coating of the AEOS, the telescope’s main mirror. AEOS is a reflector telescope, indicating that it has a small secondary mirror placed near the focus of the primary mirror to reflect light through a central hole, increasing the magnification and sharpness of objects in the sky.
Keeping the AEOS telescope’s main mirror in good condition is paramount to the site’s SDA mission, said Lt. Col. Phillip Wagenbach, who is both squadron commander and branch chief of the research and development mission of the AFRL Directed Energy Directorate.
“I am honored to lead these two critical functions that preserve our access and freedom to operate in space,” Wagenbach said. “Periodic coating of the AEOS main mirror ensures the telescope is ready to support the warfighter’s SDA mission. There’s never really a good time to decommission the telescope, but it’s best to plan for recovery as a periodic maintenance effort rather than having to shut down the telescope due to catastrophic mission degradation.
The first mirror coating took place in late 2008, approximately 12 years after the original coating was applied in 1997. The long duration between the original coating and the first coating was mainly due to the construction of the coating facility mirror, completed in 2008.
“Large mirrors like the 3.6-meter AEOS need to be recoated every 4-6 years, but performance requirements are highly dependent on the telescope’s mission,” said Scott Hunt, site technical director. “For our SDA mission, we are challenged to detect dark objects at night and to image satellites during the day. Typically, our SDA objects are brighter than astronomical objects, so we can push coatings longer than astronomers.”
Hunt said scientists and engineers track reflectivity degradation and mirror scattering over time. They weigh that against the risk of overlap, with the telescope out of service, and the mission’s performance for daytime imaging and dark object detection.
“The bare aluminum coating of the AEOS primary mirror degrades over time,” Hunt said. “When the coating is first applied, it is about 1,000 angstroms thick, or about 1/7 the width of a human hair. Imperfections in the original coating increase scatter and decrease reflectivity and may accelerate degradation.These imperfections include smudges, pinholes, and spatter created by dust and contaminants on the mirror substrate or drips of aluminum at the time of coating.
Removing the primary mirror cell from the telescope and moving it to the mirror capping facility is a delicate and time-consuming process that ends in a quick mirror capping.
“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 approximately two weeks to remove the mirror substrate from the cell, remove the old coating and prepare the coating chamber,” Hunt said. “Once 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.
Boeing personnel on site carried out the recovery with support from government management, the facilities contractor and outside experts. During the process, the team encountered a few challenges and even a surprise.
“Probably the biggest challenge was keeping the mirror coating facility clean and ensuring little or no contamination on the substrate before sealing in the vacuum chamber,” Hunt said. “The stripping and cleaning of the substrate was a critical process, especially the final wipe to remove any residue of chemicals used during the stripping and cleaning process.”
Hunt said that through the use of a hepa filter and a clean room plastic shroud around the vacuum chamber, they were able to maintain a much lower particle count in the chamber bell.
“We also fabricated a ‘drumhead’ cover that was placed over the mirror substrate immediately after the cleaning process,” Hunt said. “The drumhead lid proved to be an effective innovation in mitigating particle buildup on the substrate while we were making final preparations in the chamber.”
During the process, insects startled the team several times.
“An excited butterfly floating on our clean substrate inside the chamber would be catastrophic for the coating process,” Hunt explained. “We were able to extract with the cleanroom vacuum without experiencing any adverse effects on the new coating.”
To validate the coating process, the Maui team sent the results to private industry coating experts in Albuquerque, New Mexico. and Tucson, Arizona.
“The report we got was that the coating we got on this overlay was that ‘the results were excellent and among the best they’ve ever seen on a large mirror of this type,'” Wagenbach said.