NASA’s Future Ultraviolet Telescope Needs Dependable ‘Mirrors

A large ultraviolet telescope orbiting outside Earth’s UV-protective atmosphere will give scientists new ways to understand how stars, planets, and even galaxies form and interact in space.

To do so, said Goddard Astrophysicist Paul Scowen, this NASA observatory will require optics that can measure the polarization of UV light without changing it.

“The space between stars and galaxies is not exactly empty,” he said. “Measuring the polarization of that interstellar medium will show how particles line up there based on the magnetic fields of stars around them. We’re going to be able to measure magnetic fields of another star with better than 0.1% accuracy.”

The Astrophysics Decadal Survey released in early 2022 recommended observations ranging from ultraviolet, through optical and infrared light to study questions such as how planets form and look for signs of habitability. The Habitable Worlds Observatory (HWO) mission concept will provide high-resolution, wide-field imaging surveys over time. Teaming up with NASA’s wide-field Roman Space Telescope, these missions will answer questions about the formation, development, and survival of stars and planets, map the motions and evolution of stellar jets and accretion flows, and identify sources of ionizing radiation. Scowen received an Internal Research and Development, or IRAD, grant to study how UV mirror coatings developed by Manuel Quijada (CuttingEdge, Spring 2017) and microshutter arrays like those developed for the James Webb Space Telescope can rise to the challenges of these explorations.

Toward the high-energy end of the spectrum, UV light does not reflect efficiently off traditional mirrors. Quijada pioneered new materials and coatings to bounce high-energy light at very shallow angles towards a detector. Their mirrors stack almost parallel to the incoming light in concentric rings funneling high-energy light towards the sensor.

Scowen builds on Quijada’s work, literally, adding layers of metal flourides to his mirrors that allow reflectivity down to wavelengths of 100 nanometers. Over the past year, his team acquired an ion gun and hardware to test a vapor deposition process to improve the coating’s precision. Next, Scowen plans to develop techniques to assess and validate those coatings.

“If your instrument changes the polarization,” he said, “you’re going to miss a lot. We need to be able to measure the polarization of our coatings for sensing in the far ultraviolet.”

The team also created six different microshutter array wafers to test different designs, building on a decade of IRAD-funded innovations. Each tiny shutter in this type of array can individually open or close to block out bright foreground stars that might overwhelm distant worlds the telescope is trying to capture. Next, they will test the arrays to identify their reliability.

Scowen worked with optical engineer Mateo Batkis, from St. Michaels College in Vermont, who de- scribed the HWO missions as the future of spacebased astrophysics.

The polarization of UV light will allow scientists to study how planets form, how stars’ magnetic fields compare to our Sun’s, and what’s happening in the interstellar medium as well as in between galaxies, Batkis said.

“If we’re trying to build a true Hubble successor,” he said, “we need to do polarization spectroscopy.”