Modern-age telescopes have offered us a tantalizing view of what lies in the vast expanse of space, while also giving us glimpses of the universe’s past. However, making telescopes that can peek deep into the cosmos is an incredibly technical and expensive task—but thanks to new research, that could change.
Here’s how an innovative flat lens could solve these problems for satellites and space-based telescopes.
Understanding Telescopes and the Limitations of Lenses
Lenses, the fundamental element of telescopes, are particularly challenging from an engineering standpoint. Glass lenses come with their inherent set of limitations such as chromatic aberrations, resulting in color fringing issues and blurry images due to focus adjustment complexities. Scientists have explored achromatic lenses, but they don’t solve the problems entirely.
The biggest concern, however, is the bulk and size of the lenses, as well as the accompanying kit they are fitted inside. As the lens gets bigger, it needs more space aboard a telescope and stronger support structures. As a result, the weight and size of such telescopes reach a level where the payload capacity becomes yet another issue. The cost of launching these telescopes skyrockets, and so is the challenge of maintaining their structural integrity in space.
The solution to these fundamental problems could be quite simple, according to experts at the University of Utah. Their proposal is to replace curved lenses with a large aperture flat lens. Notably, as per their tests, these flat lenses are not only as effective at focusing light data, but also solve the inherent color correction problems of curved lenses.
How Does a Flat Lens Work?
To evaluate the performance of their unique lens, the team created a specialized optical test platform in a controlled environment where they could accurately assess its optical properties. Next, the researchers fitted the lens into a functional telescope configuration and captured actual images of the Sun and the Moon. The results were encouraging, as the telescope delivered enough sharpness and resolution to clearly distinguish sunspots.
Just to clear any doubt here, these are not the spots you see on the Sun during an eclipse. Instead, sunspots are the dark areas on the Sun’s surface caused by magnetic activity affecting the flow of hot gases. Sunspots are not easy to capture because the immense brightness requires careful exposure calibration and filtering to distinguish the dark spots. Moreover, due to the distance between telescopes and the Sun, an imaging kit with high magnification and strong resolution is required.
In addition to the sunspots, the flat-lens telescope was also able to capture the unique surface details of the Moon. This high level of detail backed the lens’s ability to gather and focus light with high precision, an encouraging sign that it can be deployed for astronomical observations.
“If successful, these flat lenses could lead to simpler, cheaper airborne and space-based imaging systems for astronomy and Earth observation,” says Rajesh Menon, lead author of the paper published in the Applied Physics Letters journal. Menon further highlighted the immense potential of their flat lens at lowering the cost of telescopes, thanks to the significant reduction in weight and size it helps achieve.
Curved Lens vs. Flat Mirrors
There are predominantly two types of telescopes: those that rely on lenses, and the second and more advanced class that depend on mirrors. Both types of telescopes follow a different approach to how they interact with light.
For example, the iconic Hubble Space Telescope falls into the latter category. The primary mirror on this floating space telescope is a disk of specialized glass with a diameter of 7.9 feet and a weight worth 1,825 pounds.
A lens-based telescope relies on a process called refraction, which is essentially the bending of light when it passes through a different medium. When a lens is fitted inside a telescope, refraction makes faraway objects appear closer. The further you want to see, the thicker is the lens you need. As a result, for long-range ground-based telescopes, the lenses quickly become huge and heavy. It also needs to be practically free of any surface or material flaws, or it will produce blurry images with color inaccuracies.
Mirror-based telescopes employ the concept of light reflection. Compared to lenses, mirrors can be thin and light, despite their large footprint. Owing to these fundamental advantages, mirror-based variants such as the Hubble and James Webb Space telescopes have revolutionized astronomy. But they are not without limitations. The mirrors require consistent cleaning and maintenance to avoid degradation of surface coating and dust accumulation due to atmospheric pollution factors, which also affect ground-based stargazing.
Their alignment also needs to be fixed periodically so that they don’t yield blurry images. They are also more sensitive to environmental factors, such as high temperatures changing the shape of their surface—like the Hubble telescope, which was developed for roughly $2 billion, but has cost the equivalent of $16 billion so far in total operation and maintenance.
Ready for Long Range Color Capture
The flat lens developed by the experts at the University of Utah aims to strike a middle ground between the traditional curved lens and mirror system for building telescopes. The flat lens, which is technically referred to as a multi-level diffractive lens (MDL), is 100 millimeters in diameter and offers a 200mm native focal length.
Unlike previous attempts at creating a flat lens with concentric rings, which failed at producing accurate colors, the new MDL flat lens can focus light signals covering the range of colors visible to human eyes and deliver clearer images. The surface rings were created using a technique called grayscale lithography. The thickness of this flat lens is just 2.4 microns, but its biggest achievement is that it can cover wavelengths in the range of 400 to 800 nanometers range.
“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,” says Apratim Majumder, a faculty member at the university’s department of Electrical and Computer Engineering.
The team also experimented with integrating the MDL with another lens to create a hybrid telescope design. As per the research paper, the device was able to capture and resolve details on the Sun, the Moon, and long-range terrestrial scenes. The team is now pushing their concept of flat lenses as a lightweight substitute for conventional refractive systems in long-range astrophotography systems.
This flat lens breakthrough has the potential to not only speed up the deployment of low-cost space missions, but also holds immense promise for ground-based analysis. It would be interesting to see how soon this DARPA-backed project makes its presence felt in the science landscape.
