Researchers have developed a new way to make and shape large, high-quality mirrors that are much thinner than the primary mirrors previously used for telescopes deployed in space. The resulting mirrors are flexible enough to fold and compactly store inside a launch vehicle.

“Launching and placing space telescopes is a complex and expensive procedure,” said Sebastian Rabien of the Max Planck Institute for Extraterrestrial Physics in Germany. “Substantially different from typical mirror fabrication and polishing procedures, this new approach could help solve the weight and packaging challenges of telescope mirrors, allowing much larger and therefore more sensitive telescopes to be placed in orbit.”

in the magazine Applied Optics Rabien reported the successful fabrication of prototype parabolic membrane mirrors up to 30 cm in diameter. These mirrors, which can be scaled down to the dimensions required for space telescopes, were created using chemical vapor deposition to magnify the membrane mirrors on a rotating liquid. The vacuum chamber has also developed a method that uses heat to adaptively correct imperfections that may occur after the mirror is placed.

“While this study only demonstrated the feasibility of the methods, it lays the groundwork for larger compact mirror systems that are less expensive,” Rabien said. “This could make lightweight mirrors 15 or 20 meters in diameter a reality and enable space telescopes that are much more sensitive than currently deployed or planned.”

Implementing an old process in a new way

The new method was developed during the COVID-19 pandemic, which Rabien says gave him extra time to think and test new concepts. “Across a long series of tests, we examined many fluids to find their suitability for the process, investigated how homogeneous the polymer growth could be, and worked to optimize the process,” he said.

For chemical vapor deposition, the starting material is evaporated and thermally broken down into monomeric molecules. These molecules settle on surfaces in a vacuum chamber and then combine to form a polymer. The 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 parabolic membrane mirrors with the necessary optical qualities for use in telescopes.

To create the exact shape of the telescope mirror, the researchers added a rotating container filled with a small amount of liquid inside the vacuum chamber. The liquid creates a perfect parabolic shape on which the polymer can grow to form a mirror. When the polymer has thickened enough, a reflective metal layer is deposited on it by evaporation and the liquid is washed away.

“It has long been known that fluids rotating around a local gravitational axis naturally form a paraboloidal surface shape,” Rabien said. Said. “Using this fundamental physical phenomenon, we applied a polymer to this ideal optical surface, which, after being coated with a reflective surface such as aluminum, forms a parabolic thin membrane that can be used as the primary mirror of a telescope.”

These mirrors are typically created using a high-quality optical die, although other groups have created membranes for similar purposes. It is much more economical to use a molding fluid and can be more easily scaled up to large sizes.

Changing the shape of a folded mirror

A thin and light mirror created using this technique can be easily folded and folded during space travel. However, once you open the package, it will be nearly impossible to return it to its perfect parabolic shape. Researchers have developed a thermal method to change the shape of the membrane mirror, using a localized temperature change generated by light to provide adaptive shape control that can bend the membrane into a desired optical shape.

The researchers tested their approach by constructing 30 cm diameter membrane mirrors in a vacuum deposition chamber. After much trial and error, they were able to produce high-quality mirrors with a suitable surface shape for telescopes. They also demonstrated that their thermal radiation adaptive molding method works well, as demonstrated by illumination from a heat sink array and a digital light projector.

The new membrane 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 distortions in incident light. Because the surface of new membrane mirrors can be deformed, these mirrors can be shaped using electrostatic actuators to create deformable mirrors that are less expensive to manufacture than those created by conventional methods.

Next, the researchers plan to apply more sophisticated adaptive control to study how well the final surface can be formed and how much initial degradation can be tolerated. They also plan to build a meter-scale deposition chamber to better study the surface structure and packaging and unpacking processes of the large primary mirror.