Soon after the Hubble Space Telescope was launched in 1990, astronomers were faced with a significant issue: the images it transmitted back to Earth were disappointingly blurry. Teams of engineers and scientists from NASA, industry, and academia worked to diagnose the problem, discovering that the primary mirror was improperly shaped, causing incorrect light focusing.
A rescue mission was launched, and in 1993, astronauts aboard the space shuttle Endeavor replaced two of Hubble’s cameras with new optical systems designed to correct the mirror’s flaw. The result was a dramatic improvement in image clarity, with one of the new cameras becoming a highly productive astronomical instrument for decades.
What began as a major setback for NASA spurred a significant wave of research in optics. To avoid similar costly and time-consuming fixes for future telescopes, researchers are now focusing on real-time blur measurement and analysis using image data itself.
At the Jet Propulsion Laboratory (JPL) at Caltech, under NASA contract, a team has developed software designed to correct various types of image blur, including the kind that plagued early Hubble images. The software leverages flexible mirrors in future telescopes, enabling remote adjustments to compensate for image deficiencies. This eliminates the need for physical intervention by astronauts. The innovative approach uses the telescope’s existing camera as its own error sensor, avoiding the need for separate devices.
This software has enhanced the capabilities of telescopes for deep space observation and has potential applications in vision science. It’s believed this technology could improve human vision beyond the standard 20/20 acuity, potentially enabling “superhuman” vision.
Imperfect vision in humans is caused by wavefront aberrations, distortions in the light waves entering the eye. These distortions prevent light from converging perfectly on the retina, resulting in blurry images. Telescopes face similar issues.
Just as corrective optics were created for Hubble and laser surgery corrects vision, flexible lenses and mirrors in modern telescopes can be adjusted in real-time. Current and next-generation telescopes utilize deformable mirrors – thin, adjustable surfaces controlled by actuators. These mirrors morph to counteract wavefront errors, sharpening images and even removing atmospheric “twinkle” for ground-based telescopes.
The foundation of wavefront sensing dates back to 1904 with astrophysicist Johannes Hartmann’s screen technique. Later, in the 1960s, Roland Shack developed the Shack-Hartmann sensor, replacing Hartmann’s screen with lenslets for more efficient light capture, which became crucial for satellite imaging.
Versions of the Shack-Hartmann sensor are used in adaptive optics systems of many advanced telescopes. In 2007, astronomers at Palomar Observatory used an adaptive optics system, incorporating a Shack-Hartmann sensor and deformable mirror to achieve images of the M13 globular cluster with twice the resolution of Hubble, showcasing the power of ground-based observatories when equipped with such technology.
However, the Shack-Hartmann sensor has hardware limitations. A new approach focuses on measuring error directly at the camera, eliminating extra hardware. This software-based method, based on “phase retrieval,” processes images to decode wavefront errors, adapting algorithms initially developed for electron microscopy in the 1970s.
The software analyzes images taken at different focus settings, refining its estimate of wavefront error through iterative loops. A key innovation at JPL has been improving the optical system model within the software itself, overcoming limitations when wavefront errors are large. This adaptive model also accounts for factors like telescope movement or focus changes during image capture, leading to more accurate error correction. The software, a modified Gerchberg-Saxton algorithm, has demonstrated accuracy to within 2 billionths of a human hair in wavefront sensing.
This software can be used with various optical hardware, adjusting movable lenses or deformable mirrors to optimize image focus. It is currently used at Palomar Observatory and is being explored for applications such as exoplanet detection, where altering wavefronts can redistribute starlight to reveal faint orbiting companions.
Beyond real-time correction, the software can also enhance existing blurry images. By estimating the “point-spread function,” it can apply advanced image-processing techniques to achieve “superresolution,” recovering image data lost due to optical imperfections. This was demonstrated successfully with images from NASA’s Deep Impact mission, bringing an out-of-focus image of Earth’s moon into sharp clarity.
The next generation of space telescopes, including the James Webb Space Telescope, will benefit significantly from this software for on-orbit wavefront error correction, especially for telescopes with segmented, deployable mirrors. The software has already been recognized with NASA’s Software of the Year Award in 2007 for its contribution to space programs.
Beyond visible light, the algorithm’s principles are applicable across the electromagnetic spectrum, potentially benefiting systems like the Deep Space Network. Unexpectedly, heat can also play a role in blur correction. By strategically using heaters on spacecraft instruments, the optics can be adjusted to counteract wavefront errors induced by temperature changes or mechanical strain.
Looking ahead, the software could revolutionize vision correction. While current eyeglasses and LASIK address basic vision errors, the technology could correct for more subtle wavefront distortions, potentially unlocking “superhuman vision.” Researchers are working to accelerate the software’s processing speed, with advancements in graphics processing allowing for near real-time wavefront error correction. The ultimate goal is real-time astronomical image correction and unprecedented visual clarity, continuing the legacy of Hubble and transforming initial setbacks into breakthroughs across the electromagnetic spectrum.
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