Updated: October 4, 2022
By Jeanne Dailey, Air Force Research Laboratory
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.
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.
“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.”
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.