High-quality membrane mirrors and the power of large space telescopes

Research has revealed a new way of producing and shaping large high-quality mirrors for space telescopes allowing for them to be rolled up and stored compactly inside during launch

A key obstacle in developing large space telescopes or (balloon-borne observatories) is the areal weight of their primary mirrors; traditional large membrane mirrors can be used due to their low areal weight; however, these mirrors are difficult to manufacture when requiring such a high optical quality that is needed for a space telescope.

Balancing weight and optical quality appears to be a continuing problem for space researchers, however, a team from Optica appear to have a solution.

“Launching and deploying space telescopes is a complicated and costly procedure,” said Sebastian Rabien from 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 packaging issues for telescope mirrors, enabling much larger, and thus more sensitive, telescopes to be placed in orbit.”

Published in the Optica Publishing Group journal Applied Optics, the team have reported the successful fabrication of a “parabolic membrane mirror prototype up to 30cm in diameter” which could be scaled up to the sizes needed for space telescopes.

Growing membrane mirrors for future space exploration

Using chemical vapor deposition, the team were able to grow membrane mirrors on a rotating liquid inside a vacuum chamber. Rabien has also developed a method that uses heat to adaptively correct imperfections that might occur after the mirror is unfolded.

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

“Enabling space-based telescopes that are orders of magnitude more sensitive than ones currently deployed”

“In a long series of tests, we researched many liquids to find out their usability for the process, investigated how the polymer growth can be carried out homogeneously, and worked to optimize the process,” Rabien said.

Chemical vapor deposition

For chemical vapor deposition, a precursor material is evaporated and thermally split into monomeric molecules. Those molecules deposit on the surfaces in a vacuum chamber and then combine to form a polymer. This process is commonly used to apply coatings such as the ones that make electronics water-resistant, but this is the first time it has been used to create parabolic membrane mirrors with the optical qualities necessary for use in telescopes.

To create the precise shape necessary for a telescope 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 parabolic shape onto which the polymer can grow, forming the mirror base. When the polymer is thick enough, a reflective metal layer is applied to the top via evaporation and the liquid is washed away.

“It has long been known that rotating liquids that are aligned with the local gravitational axis will naturally form a paraboloid surface shape,” said Rabien. “Utilizing this basic physics phenomenon, we deposited a polymer onto this perfect optical surface, which formed a parabolic thin membrane that can be used as the primary mirror of a telescope once coated with a reflecting surface such as aluminium.”

Although other groups have created thin membranes for similar purposes, these mirrors are typically shaped using a high-quality optical mould. Using a liquid to form the shape is much more affordable and can be more easily scaled up to large sizes.

The potential for reshaping a folded mirror

Using this technique, a thin and lightweight mirror will be created, allowing for easy folding or rolling during the trip to space.

However, it would be nearly impossible to get it back to the perfect parabolic shape after unpacking.

To ensure the reshaping of the membrane mirror, the team developed a thermal method that uses a “localized temperature change created with light to enable adaptive shape control that can bring the thin membrane into the desired optical shape”.

By creating their 30cm membrane mirror within a vacuum deposition chamber as a practice, the team, following much trial and error) were able to create high-quality mirrors with a surface shape suitable for telescopes.

They also showed that their thermal radiative adaptive shaping method worked well, as demonstrated with an array of radiators and illumination from a digital light projector.

What is next for space telescopes and the creation of lightweight and large-scale primary mirrors?

The team hope to apply more sophisticated adaptive control to study how well the final surface can be shaped and how much of an initial distortion can be tolerated. Along with this, they are also planning to create a meter-sized deposition chamber to better study the surface structure and packaging and unfolding processes for a large-scale primary mirror.