Engineers and astronomers at the University of Utah have designed a unique new kind of telescope lens: a flat lens with microscopic etchings to refract light. If the concept can be scaled up, these lenses could one day replace the heavier, bulkier lenses and mirrors typically used in telescopes, particularly those of professional observatories on the ground and in space. Down the line, they could also be implemented on amateur telescopes, the team says.
“Our computational techniques suggested we could design multi-level diffractive flat lenses with large apertures that could focus light across the visible spectrum,” Rajesh Menon, a Utah professor of engineering, said in a statement.
There are two basic types of telescope: refractors and reflectors. A refractor uses lenses to refract light and bring it to a focus. Reflectors utilize at least two mirrors to reflect the light to a focal point. Because large lenses are heavy and expensive to make, larger telescopes tend to use mirrors (sometimes in combination with smaller lenses). Lenses can also suffer from a malaise known as chromatic aberration, in which different wavelengths of light are refracted to slightly different degrees so that different colors come to focus at different points, resulting in color fringing around objects. Optical technicians can alleviate this through the complex use of glass coatings and multiple lenses — though that adds cost and expense.
However, the days of bulky, expensive telescope lenses could soon be coming to an end thanks to the team’s new flat lens measuring less than a millimeter thick. that has been developed by a team led by Apratim Majumder, who is a member of Menon’s lab at Utah.
“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,” Apratim Majumder, a member of Menon’s lab at Utah and leader of the crew behind the new lens prototype, said in the statement.
Majumder, Menon and their team designed a 100mm (4-inch) flat lens, where microscopic concentric rings are etched onto a glass substrate using a technique called “grayscale optical lithography,” which is a variation of a method typically used for etching electronics onto a silicon wafer. Most of the half-millimeter thickness of the flat lens is the glass — the ringed grooves are just 2.4 microns deep.
The use of concentric rings on a flat lens isn’t new — a precursor, called a Fresnel zone plate, tries to do the same trick but is unable to eradicate chromatic aberration. However, the Utah team’s multilevel diffractive lens (MDL) is able to bring all the wavelengths of light for which it was designed to detect (400–800 nanometers, covering the range of visible light and into the near-infrared) to a focus at the same point, thanks to how the size of the rings and the spacing between affect how the incoming light is refracted. Because all the colors come to focus at the same point, there is no chromatic aberration.
Back when the telescope was invented by Hans Lippershey in 1608, it was done so by experimenting with putting lenses together. Today, a telescope’s optical design involves complex computer modelling and large amounts of data.
“Simulating the performance of these lenses over a very large bandwidth, from visible to near-infrared, involved solving complex computational problems involving very large datasets,” said Majumder. “Once we optimized the design of the lens’ microstructures, the manufacturing process involved required very stringent process control and environmental stability.”
The resulting 100mm lens, which has a focal length of 200mm, was then tested on both the sun and the moon, successfully showing sunspots and (in an artificially color-enhanced image) accurate geological features on the lunar surface. The 100mm MDL weighs just 25 grams (0.88 oz), compared to the 211 grams (7.44 oz) of a similarly sized, commercially available 100mm lens that is 17mm thick at its curved center.
The Hubble Space Telescope uses a 2.4-meter primary mirror with a total mass of 1,825 pounds (828 kilograms), and the James Webb Space Telescope incorporates 18 segments in its primary 21-foot (6.5-meter) mirror in total weighing (on Earth) 1,555 pounds (705 kilograms). On Earth, individual telescope mirrors have an upper size limit of 26-33 feet (8–10 meters), beyond which gravity starts to cause them to sag. The development of a flat, lightweight lens could therefore transform telescopes, particularly for space launches where mass is a key limiting factor for getting off the Earth.
A description of the flat MDL lens was published on Feb. 3 in the journal Applied Physics Letters.