Researchers have invented a new method for producing and shaping big, high-quality mirrors that are much smaller than the main mirrors previously used for space observatories. The resulting mirrors are compact enough to be folded up and stowed inside a launch car.
“Launching and deploying space telescopes is a difficult and expensive procedure,” said Sebastian Rabien of Germany’s Max Planck Institute for Extraterrestrial Physics. “This new approach, which is very different from traditional mirror production and polishing procedures,” according to the researchers, “could help solve weight and packaging issues for telescope mirrors, allowing much larger, and thus more sensitive, telescopes to be placed in orbit.”
Rabien describes the effective fabrication of parabolic membrane mirror prototypes up to 30 centimeters in diameter in the journal Applied Optics. These mirrors were made by growing membrane mirrors on a rotating liquid inside a vacuum container using chemical vapor deposition. These mirrors could be expanded up to the proportions required in space observatories. Additionally, he created a technique that employs heat to adaptively fix any flaws that might appear after the mirror is unfurled.
“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.”
Using an old technique in a novel manner: The new technique was created during the COVID-19 pandemic, which Rabien claims allowed him more time to ponder and experiment with new ideas. “We researched many liquids to determine their suitability for the process, investigated how polymer growth can be carried out homogeneously, and worked to optimize the process,” he explained.
A precursor substance is evaporated and chemically divided into monomeric molecules for chemical vapor deposition. In a vacuum container, these molecules deposit on the surfaces and then join to create a polymer. This method is frequently used to add coatings, such as those that make devices waterproof, but it has never been used to produce parabolic membrane reflectors with the optical properties required for use in binoculars.
The researchers then added a rotating container containing a tiny quantity of liquid to the interior of the vacuum compartment to give the exact form required for a telescope mirror. On top of the liquid’s flawless parabolic shape, the polymer can develop to create the mirror base. When the polymer is sufficiently thick, evaporation is used to apply a shiny metal coating to the top, and the liquid is then removed.
“It has long been known that rotating liquids aligned with the local gravitational axis form a paraboloid surface shape,” Rabien explained. “We deposited a polymer onto this perfect optical surface, forming a parabolic thin membrane that can be used as the primary mirror of a telescope once coated with a reflecting surface such as aluminum.”
Despite the fact that other groups have developed thin membranes for comparable reasons, these mirrors are usually formed using a high-quality optical mold. Using a liquid to create the structure is much less expensive and can be expanded up to larger proportions much more easily.
Changing the shape of a foldable mirror: This method produces a thin and lightweight mirror that can be easily folded or coiled up during the journey to space. However, restoring it to its original parabolic form after packaging would be virtually impossible. The researchers devised a thermal technique that employs a localized temperature change caused by light to allow adaptive shape control, which can move the thin membrane into the desired optical shape.
The researchers put their method to the test by fabricating 30-cm-diameter membrane screens in a vacuum deposition room. They were able to make good quality mirrors with a surface form appropriate for telescopes after much trial and error. Using a series of heaters and lighting from a digital light projector, they also displayed the effectiveness of their thermal radiative adaptive shaping technique.
In adaptive optics devices, novel membrane-based mirrors might also be employed. In order to account for oncoming light distortion, adaptive optics uses a deformable reflector to enhance the efficacy of optical devices. The novel membrane mirrors’ deformable surface allows for the creation of deformable mirrors at a lower cost than those made using more traditional techniques. These deformable mirrors can be molded using electrostatic actuators.
The researchers intend to use more complex adaptive control in the future to investigate how well the finished surface can be shaped and how much early distortion can be endured. They also intend to build a meter-sized deposition room in order to better understand the surface structure as well as the packing and unfolding processes for a large-scale main mirror.
