Ultrafast Time-of-Flight Imaging Combats Telescopes' Stray Light Problem

Researchers at Centre Spatial de Liege (CSL) of the University of Liege have advanced the field of space engineering through a newly developed, laser-based method for the identification of the contributors and origins of stray light on space telescopes. As the power (and, as a result, imaging capabilities) of space telescopes continues to increase, the developed method is poised to help in the acquisition of even finer space images and the development of increasingly efficient space instruments.

Stray light, which results in light reflections that damage the quality of images obtained via telescope and often leads to blurred images, has proved difficult to check and characterize through existing methods in the development stage of telescopes. These methods are unable to provide information on the origins of stray light — a problem magnified by the diversity of light interaction processes with surfaces, which results in various contributors of stray light. When quantities of stray light are higher than expected, it can be difficult to determine how to improve the system. So-called ghost reflections between lenses, light scattering, and other qualities caused by stray light can force engineers to revise calculations. This can cause considerable delays in the commissioning of equipment.

The CSL researchers collaborated with the University of Strasbourg on the method. A femtosecond laser pulse sends light beams to illuminate the telescope, and stray light rays contained in the telescope take different optical paths from the rays that form the image. Using an ultrafast detector operating at one-thousandth of one-millionth of a second, the method measures the image and different stray light effects at different times. The process allowed the researchers to successfully identify decomposition and each individual contributor of stray light based on their arrival times, which are directly related to their distinct optical paths, said Lionel Clermont, a CSL expert and author on a study introducing the approach.

In addition to a paper, the researchers published film of the ghost reflections — partial reflections between the telescope lens interfaces — arriving at different times. “We have also been able to use these measurements to reverse engineer theoretical models, which will make it possible, for example, to build better image processing models in the future,” Clermont said. By correlating these measurements with numerical models, the scientists will be able to determine the precise origin of the stray light. With that information, they will be able to refine the system through hardware improvements and the development of correction algorithms.

Stray light decomposition by ultrafast time-of-flight imaging. Courtesy of Lionel Clermont / Centre Spatial de Liege / University of Liege.

“We have already received a great deal of interest from the [European Space Agency] and from industrialists in the space sector,” said Marc Georges, a CSL expert and co-author of the study. “This method responds to an urgent problem that has been unresolved until now."

Further planned improvements aim to make the method applicable to scientific instruments, to increase the method’s technology readiness level (TRL), and to bring it to an industrial level. One industrial application is already planned — for the Fluorescent Explore (FLEX) project, an Earth observation telescope that is part of ESA’s Living Planet Program.

The research was published in Scientific Reports (www.doi.org/10.1038/s41598-021-89324-y).