Researchers create new membrane mirrors for large space telescopes

Researchers have developed a new method for producing and shaping large, high-quality mirrors that are much thinner than the primary mirrors previously used for space telescopes. The resulting mirrors are flexible enough to be rolled up and stored compactly inside the launch vehicle.

“Launching and deploying space telescopes is a complex and expensive procedure,” said Sebastien Rabian of the Max Planck Institute for Extraterrestrial Physics in Germany. “This new approach—which is very different from typical mirror production and polishing procedures—could help solve weight and packing issues for telescope mirrors, enabling much larger, and therefore more sensitive, telescopes to be put into orbit.”

in the journal applied opticsRabih reported the successful manufacture of prototypes of parabolic film mirrors up to 30 cm in diameter. These mirrors, which can be increased to required sizes in space telescopes, were created by using chemical vapor deposition to grow membrane mirrors on a rotating fluid inside a vacuum chamber. He also developed a method that uses heat to adaptively correct defects that may occur after exposure of the mirror.

“Although this work only demonstrated the feasibility of the methods, it lays the groundwork for less costly packable large mirror systems,” Rabian said. “It could make lightweight mirrors 15 or 20 meters in diameter a reality, enabling space telescopes that are orders of magnitude more sensitive than those currently deployed or planned.”

Applying an old process in a new way

The new method was developed during the COVID-19 pandemic, which Rabian says gave him some extra time to think and try new concepts. “In a long series of tests, we investigated various liquids for their usability in the process, investigated how to implement polymer growth homogeneously, and worked on process optimization,” he said.

For chemical vapor deposition, the starting material is evaporated and thermally divided into monomeric particles. These particles are deposited on surfaces in a vacuum chamber and then combine to form a polymer. This process is commonly used to apply coatings such as those that make electronics waterproof, but this is the first time it has been used to create equivalent film mirrors with the optical qualities needed for use in telescopes.

To create the exact shape needed for the telescope’s mirror, the researchers added a rotating container filled with a small amount of liquid to the inside of the vacuum chamber. The liquid forms a perfect parabola on which the polymer can grow, forming the base of the mirror. When the polymer is thick enough, a metallic reflective layer is applied over the top by evaporation and the liquid is washed away.

“It has been known for a long time that rotating fluids aligned with the local gravitational axis will naturally form a parabolic surface shape,” Rabian said. “Using this underlying physical phenomenon, we placed a polymer on this perfect optical surface, which formed a parabolic thin film that could be used as a telescope’s primary mirror once covered with a reflective surface like aluminum.”

Although other groups have made thin films for similar purposes, these mirrors are usually machined using a high-quality optical mold. The use of liquid to form the shape is more affordable and can be more easily scaled to large sizes.

Reconfiguration of a folded mirror

The thin, lightweight mirror created using this technology can easily be folded or rolled up during the flight into space. However, it would be nearly impossible to return it to the perfect parabolic shape after it was deflated. To reshape the membrane mirror, the researchers developed a thermal method that uses a localized temperature change created by light to enable adaptive shape control that can bring the thin film to the desired optical shape.

The researchers tested their approach by creating membrane mirrors 30 cm in diameter in a vacuum sedimentation chamber. After much trial and error, they were able to produce high quality mirrors with a surface shape suitable for telescopes. They also showed that the radiative thermo-adaptive modulation method works well, as shown with a combination of radiators and illumination from a digital projector.

New film-based mirrors can also be used in adaptive optics systems. Adaptive optics can improve the performance of optical systems by using a deformable mirror to compensate for distortion in incoming light. Because the surface of new membrane mirrors is deformable, these mirrors can be shaped with electrostatic actuators to create deformable mirrors that are less expensive to make than those created by conventional methods.

Next, the researchers plan to apply more sophisticated adaptive control to study how well the final surface is formed and how much initial distortion can be tolerated. They also plan to set up a meter-sized deposition chamber in order to better study the surface structure, packaging and opening processes of a large-scale primary mirror.