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JWST's primary mirror, consisting of 18 segments. Credit: NASA/ESA/CSA
The invention of the telescopes in the early 17th century was a boon for astronomy. Using a telescope of his own design, Galileo conducted groundbreaking observations of the Moon, Mars, Jupiter, and the Sun. These spurred on the revolution that began with Copernicus' model of the Heliocentric Universe, forever altering humanity's perception of the cosmos and our place in it. From that point onward, the lenses that give telescopes their magnification have remained largely the same - bulky and curved.
However, traditional lenses suffer from one flaw: the more powerful they must be, the larger and heavier they are - similar to the Rocket Equation. While several attempts have been made to reduce the weight of lenses using slimmer alternatives, they have limited capacity and are expensive to manufacture. But now, scientists from the University of Utah have found a promising solution using a large aperture flat lens they developed. This lens focuses light as effectively as traditional curved lenses while preserving accurate color.
The research team was led by Rajesh Menon, a Professor at the Department of Electrical and Computer Engineering (ECE) at the University of Utah. He and his colleagues were assisted by Oblate Optics Inc., a San Diego-based laser optics and meta-lenses manufacturer. The paper that describes their findings was featured on the cover of the latest issue of Applied Physics Letters. Menon lab member and ECE Research Assistant Professor Apratim Majumder was the lead author.
As they indicate, this technology could potentially transform astrophotography imaging systems, especially where aircraft, satellites, and space-based telescopes are concerned. For smaller instruments, like backyard telescopes and cameras, lens thickness is not an issue. But when it comes to astronomy and astrophotography, where telescopes must focus light from objects millions (or billions) of light-years away, the size and weight of the lenses become impractical.
This is why many modern observatories and space-based telescopes rely on massive curved mirrors, which are thinner and lighter, to achieve the same light-bending effect. Examples include the primary mirror on the James Webb Space Telescope (JWST), which comprises 18 segmented mirrors, and the Extremely Large Telescope (ELT), which will rely on a five-mirror optical design. Another solution is to use flat lenses that bend light differently, such as Fresnel Zone Plate (FZP) lenses that use concentric ridges to focus light.
While this method has led to lighter compact lenses, the ridges on FZPs are optimized for a single wavelength of light and diffract others at different angles. As a result, they cannot reproduce true color in their images, leading to what is known as "chromatic aberrations" (aka. color distortions). Conversely, Menon and his team created a Multi-level Diffractive Lens (MDL) using an inverse-design approach and grayscale lithography. This flat lens relies on microscopically small concentric rings that simultaneously bring all wavelengths of light into focus.
As Menon explained in a Utah press release, this allows for accurate true-color images. "Our computational techniques suggested we could design multi-level diffractive flat lenses with large apertures that could focus light across the visible spectrum, and we have the resources in the Utah Nanofab to actually make them," he said. The size and spacing of the indentations are what keep the diffracted wavelengths of light close enough together to produce clear, full-color images. As Majumder explained:
"Simulating the performance of these lenses over a very large bandwidth, from visible to near-IR, involved solving complex computational problems involving very large datasets. Once we optimized the design of the lens’ microstructures, the manufacturing process involved required very stringent process control and environmental stability. Our demonstration is a stepping stone towards creating very large aperture lightweight flat lenses with the capability of capturing full-color images for use in air- and space-based telescopes.
The team conducted imaging experiments to demonstrate the capabilities of their MDL lens. The images they acquired demonstrated the MDL's ability to capture high-quality, full-color images of the Moon, the Sun, and several distant terrestrial scenes. The color-enhanced lunar images revealed key geological features, while solar imaging identified visible sunspots. This proof-of-concept shows that large, flat lenses can capture true-color images, which could have massive implications for many industries.
However, the greatest implications are reserved for astronomy. By integrating their MDL with a refractive achromatic lens, the team was able to develop a hybrid telescope that would significantly reduce the weight of airborne and space-based imaging applications. It may not be long before flat lenses become a regular alternative to conventional refractive systems, providing a lightweight alternative for space-based and ground-based telescopes.
Further Reading: University of Utah, Applied Physics Letters