NASA Studies Coatings for High-Tech Mirrors
It may be aluminum that lies behind some of NASA’s most powerful mirrors, eyed for use in its most sensitive telescopes—but it’s a coating that will make the whole project work.
A team of researchers at the space agency’s Goddard Space Flight Center is working to develop the perfect coating to protect the highly reflective aluminum mirrors that the agency says are sensitive to three bands of wavelength: infrared, optical and far-ultraviolet.
Aluminum has been the key to developing the mirrors themselves, but the problem the team has encountered is that the aluminum substrate easily oxidizes, leading to a loss of reflectivity.
Enter specialized protective coatings.
“Aluminum is a metal that nature has given us the broadest spectral coverage,” lead researcher Manuel Quijada said. “However, aluminum needs to be protected from naturally occurring oxides with a thin film … of transparent material.”
The challenge for the team is to develop a coating that will protect the aluminum’s reflectivity in the “Lyman Alpha range,” a range NASA describes as 90 to 130 nanometers. With reflectivity in this range, the mirrors have the potential to uncover faraway planets that are invisible by any other means.
“The low reflectivity of coatings in this range is one of the biggest constraints in far-ultraviolet telescope and spectrograph design,” Quijada said.
NASA says one approach involves the physical vapor deposition of a thin layer of xenon difuoride gas onto the aluminum surface. The fluorine ions that are formed then prevent the aluminum from oxidizing.
Another approach is to coat the substrate with aluminum trifluoride, via ion-assisted physical vapor deposition or atomic layer deposition.
The closest the team has come in the past is a proven method of applying a film of magnesium fluoride or lithium fluoride via a physical vapor deposition process, achieving 90 percent reflectance at the 133.6-154.5 nanometer range, NASA says.
As the team works to develop the ideal coating for the mirrors, it hopes to prolong the useful life of observation tools like the Far Ultraviolet Spectroscopic Explorer, a satellite unit that was responsible for more than 6,000 observations during its eight-year orbit. The aluminum substrate of that unit eventually degraded due to oxidation, NASA says.