Optical Filters for NASA Imagers Focus on Cutting Edge
By the time the Hubble Space Telescope launched, NASA engineers had already set their sights on its next upgrade. With camera and detector technologies improving at a rapid pace, service missions to replace many of the optical components were already scheduled.
Optical engineers at Goddard Space Flight Center needed specially designed filters for the Wide-Field Planetary Camera 2 (WFPC2) and, later, Wide Field Planetary Camera 3 (WFC3) to get high-quality images of stars, galaxies, and other celestial bodies from the Agency’s flagship imager. Each of the filters had to block all but a specific range of wavelengths of light to capture the best scientific data possible.
“In some cases, the requirements were for very, very narrow-band filters, because we wanted to look at some specific chemical constituent of stars that can be isolated in that narrow band of the light spectrum,” explains Ray Boucarut, an optical engineer at Goddard who oversaw work on WFC3 . Our eyes are sensitive to light wavelengths between about 400 and 700 nanometers, but the cameras on Hubble and many other imaging telescopes look well beyond either end of the visible spectrum.
NASA also required filters on Hubble to be applied all the way to the outer edges of the optic—something that had not been attempted before—and to let in virtually every photon at the wavelengths they weren’t made to block out.
“As you get into longer wavelengths or very short, the materials you can use become challenging,” Boucarut says. “They won’t function properly, they won’t transmit the light very efficiently.” The optics also had to be nearly defect-free, he adds. “The glass had to be polished to very high level to get the best wave-front optical quality.”
Another challenge, particularly for WFC3 , was that the filters for infrared imaging had to function at very low temperatures, around -20 to -40 °C, as warmer temperatures “would swamp your detector” and wash out any images, Boucarut says.
The mission to replace some of Hubble’s original optics meant NASA needed to find a company capable of tackling all those challenges. Following a competitive process, Barr Associates, now part of Materion Precision Optics in Westford, Massachusetts, was selected.
The Hubble contracts presented new challenges for Barr Associates, a company with a long track record of providing precise, custom optical devices for aerospace applications and a range of other industries.
For example, the company had never attempted to combine four filters on a single optic prior to the Hubble work, says David Harrison, a business development manager with Materion who worked at Barr Associates in the days of its Hubble work.
“We had done simple coatings on one substrate before but certainly had never taken anything to the precision level that NASA needed as far as spectral wavelength control, uniformity control, the precision of where each filter needed to be in relation to the substrate and in relation to each other,” he says. “We took some known things in a few areas and put them all together to come up with something that really had never been done and was tighter than we could’ve imagined we could do.”
At the time, the company showed great flexibility and willingness to work with NASA to create the filters, something Boucarut recalls with appreciation. On a few occasions, scientists from Goddard traveled to the company’s headquarters in Massachusetts to talk about the work with engineers. “The scientists would discuss what led to the requirements we put on them,” he says. “Sometimes we would negotiate something that would be easier to make but still satisfy the science. That flexibility on their part was a great benefit.”
Boucarut also points out that the contract from NASA for the Hubble filters wasn’t exceedingly large, roughly $2 million or less. The company’s biggest business at the time was in telecommunications, but it had a small division of experts working on astronomy projects for NASA, universities, and other science-related entities.
In all, Barr Associates made nearly 100 filters for WFPC2 and WFC3 , but the relationship with NASA didn’t end there. The company later provided filtered lenses for the Mast Camera on the Jet Propulsion Laboratory’s Curiosity Rover and, now as part of Materion, has provided filters for the Near Infrared Camera on Goddard’s James Webb Space Telescope, scheduled to launch as Hubble’s successor in 2018.
“A lot of the lessons we learned from the Hubble work, back in the late ’80s and early ’90s, we still employ a lot of that today, certainly in improved processes and procedures,” Harrison says. In addition to its aerospace work, which includes filters for many of the world’s major telescope programs, Materion applies these advances to high-end, precision optics for consumer goods, such as cell phones, laptops, tablets, and other electronics.
In one example of a major commercial success, the company took the technical know-how it developed to put multiple filters on a single Hubble optic and used it to create devices for matching paint colors in hardware stores. “It’s the same idea, in that you now have multiple color coatings on one substrate, which allows a really small instrument to do evaluation of a number of different wavelengths,” Harrison says, noting that Materion now sells thousands of the devices each year.
But advances made to meet NASA’s needs have paid off across the board. Work on optics for the Curiosity Mars rover pushed the company’s ability to eliminate even the slightest defects from its lenses, Harrison says, noting that Curiosity’s lenses could not have imperfections larger than 1 or 2 microns, whereas most lenses can tolerate defects up to 100 or 200 microns.
“All of the protocols we’ve done to solve that one job are now followed in nearly every coating chamber we run nearly every single day now,” he says.
Later, the company had to develop a whole new lens-coating process to supply optics for the James Webb Space Telescope. For that project NASA wanted unprecedented, “ridiculous” uniformity and wavelength positioning in filtering for mid- and long-wavelength light frequencies, Harrison says. While coatings to filter for mid-wavelength frequencies are usually applied through evaporation, those that filter for longer wavelengths are normally applied with an ion gun, he explains.
“Some of the properties they were asking for from their mid-wavelength coatings needed some of the extra energy from the ion-gun, but it wasn’t something that was done up at that wavelength region,” he says. With changes to deposition parameters, ion gun settings, and other points of process, Materion managed to come up with a successful recipe. “Literally, that one development chamber that we used for James Webb, there are now three of them that run standard products for us that way every day,” Harrison says. “That was certainly a case where something NASA-related absolutely stretched our limits and capabilities and made us open our eyes to something completely different.”
He says this is the biggest reason the company has always embraced NASA work. Meeting the Space Agency’s demands for cutting-edge technology “pushes you to go beyond the safe zone, beyond what you can do and come up with ways to accomplish things you didn’t know you could. It always keeps you pushing forward, striving forward, learning better ways to do stuff, which helps the company in so many ways overall,” Harrison says.
“As we solve new problems, it makes us think of new ways and things that are outside the box that cascade down into our everyday commercial-type optics.”