Large, high-quality mirrors that are significantly thinner than the primary mirrors previously used for telescopes placed in space can now be produced and shaped in innovative ways by researchers. The resulting membrane mirrors may be rolled up and placed neatly within a launch vehicle since they are sufficiently flexible.
“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.”
Rabien reports the successful manufacture of parabolic membrane mirror prototypes up to 30 cm in diameter in the journal Applied Optics. These mirrors were made by growing membrane mirrors atop a rotating liquid inside a vacuum chamber using chemical vapour deposition. These mirrors may be scaled up to the sizes required in space telescopes. Also, he created a technique that employs heat to adaptively fix any flaws that might appear once 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.”
Applying an old process in a new way
The COVID-19 epidemic, according to Rabien, provided him some extra time to reflect and test out novel ideas, which is how the new approach came to be. He explained, “In a long series of tests, we evaluated different liquids to determine their suitability for the process, investigated how the homogenous polymer development may be carried out, and worked to optimise the process.
A precursor substance is evaporated and thermally divided into monomeric molecules for chemical vapour deposition. These molecules unite to create a polymer after depositing on the surfaces in a vacuum chamber. It is normal practise to use this method to apply coatings, such as those that make devices waterproof, but this is the first time it has been utilised to produce parabolic membrane mirrors with the optical qualities necessary for use in telescopes.
The researchers then added a revolving container containing a little amount of liquid to the interior of the vacuum chamber to give the precise shape required for a telescope mirror. On top of the liquid’s flawless parabolic shape, the polymer can develop to form the mirror foundation. After the polymer is sufficiently thick, evaporation is used to apply a reflective metal layer to the top, and the liquid is then removed.
“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 aluminum.”
These mirrors are normally shaped using a high-quality optical mould, while other groups have developed thin membranes for comparable purposes. It is far more inexpensive and easier to scale up to big sizes when the shape is formed using a liquid.
Reshaping a folded mirror
This process results in a small, light mirror that may be readily folded or rolled up for space travel. But, after unpacking, it would be quite difficult to restore it to its ideal parabolic shape. The researchers established a thermal technique to reshape the membrane mirror, which leverages a localised temperature change induced by light to provide adaptive shape management and enable the thin membrane to assume the required optical shape.
By fabricating 30-cm diameter membrane mirrors in a vacuum deposition chamber, the researchers tested their strategy. They were able to create premium mirrors with a surface form ideal for telescopes after considerable trial and error. Using a series of radiators and illumination from a digital light projector, they also demonstrated the effectiveness of their thermal radiative adaptive shaping technique.
In adaptive optics systems, the novel membrane-based mirrors might also be employed. In order to account for incoming light distortion, adaptive optics uses a deformable mirror to enhance the performance of optical devices. The novel membrane mirrors’ malleable surface allows them to be shaped with electrostatic actuators to produce deformable mirrors that are less expensive to produce than those made with traditional membrane mirrors.
In order to learn how well the end surface can be moulded and how much of an initial distortion can be tolerated, the researchers will then use more advanced adaptive control. In order to better understand the surface structure, packaging, and unfolding processes for a large-scale primary mirror, they also intend to build a deposition chamber with a diameter of one metre.
