Designing and Testing the Optics for NASA’s Nancy Grace Roman Space Telescope

An optical technician lays on a diving board suspended between NASA’s Nancy Grace Roman Space Telescope’s primary and secondary mirrors. The photo is a projected reflection through the telescope’s optical path. The technician shines a beam of light through the optical system toward the future location of the Wide Field Instrument, showing how light from cosmic sources will travel through the telescope once the mission launches. (Image: NASA/Chris Gunn)

When it launches no later than May 2027, the National Aeronautics and Space Administration (NASA) Nancy Grace Roman Space Telescope will serve as a powerful eye on deep space, capturing images of billions of distant galaxies and exploring the mysteries of dark matter, supernovae and other cosmic phenomena.

The primary objective of the Nancy Grace Roman Space Telescope is to survey large areas of the sky rapidly and repeatedly in high precision in order to map the distribution of normal (baryonic) matter and dark matter and map the rate of cosmic expansion in various epochs to probe dark energy. This information is critical to our understanding of the origins of the universe, and to help scientists understand what will happen in the distant future of the rapidly expanding cosmos. It will also use large surveys to study planetary systems around other stars to learn whether solar systems like our own are common, rare, or perhaps unique.

This photo shows the Optical Telescope Assembly for NASA’s Nancy Grace Roman Space Telescope, which was recently delivered to the largest clean room at the agency’s Goddard Space Flight Center in Greenbelt, Md. (Image: NASA/Chris Gunn)

A major program milestone was achieved in November 2024 with delivery of the fully completed and tested Optical Telescope Assembly (OTA) from L3Harris to NASA. This hardware acts as the “eyes” of the observatory, collecting and conditioning light from the cosmos for use by the mission’s two instruments.

As a trusted partner of NASA, L3Harris was tasked with the design, fabrication, integration and testing of the OTA. This includes a 2.4-meter (8 feet) diameter primary mirror as well as nine other smaller mirrors, robust structures to align the mirrors to each other, and numerous support systems necessary to enable the telescope to perform in the harsh environment of space.

From the beginning of the program, the OTA has been developed to meet the challenging and unique needs set forth by NASA and the science community for this mission. One of the primary areas of focus of the telescope team was developing the technologies necessary to provide a system that could meet the extreme optical stability needs of the mission. This included development of a new proprietary carbon composite material with coefficients of thermal expansion (CTEs) lower than previously achieved — so low that new techniques needed to be developed to measure its properties. Due to the extremely low CTE, a piece of this material as long as a football field would only change length by 100 microns (the width of a human hair) when its temperature is changed by 100 degrees Fahrenheit (55 degrees Celsius).

This photo shows the entire optics system for NASA’s Nancy Grace Roman Space Telescope. It consists of 10 mirrors, including the 7.9-foot (2.4-meter) primary mirrors seen at the base in this image, that is called the imaging optical assembly (IOA). (Image: NASA/Chris Gunn)

Even with such stable materials, the temperature of the telescope must remain consistent to achieve the mission objectives. L3Harris developed a new temperature sensing and control architecture able to keep key areas of the telescope stable to a few thousandths of a degree Celsius even while different parts of the observatory are exposed to the blistering heat of the sun or facing the nearly absolute zero temperatures of outer space. This state-of-the-art thermal control system ensures that the structures and optics within the telescope will remain ultra-stable (sub nanometer changes in wavefront error) and continue to deliver precise science measurements even while experiencing different thermal extremes.

The OTA is designed such that once it reaches its final operational destination a million miles from Earth it will have optimal optical performance. That means the design had to account for even the tiny effects from gravity on Earth and the telescope cooling down to operating temperatures. Engineers at L3Harris performed extensive simulations to predict the changes that will happen in the telescope as it goes from Earth’s gravity at room-temperature to its cold, zero gravity environment in space. These anticipated changes are accounted for during the design, fabrication, and alignment of the telescope optics. In addition, several key optics can be moved to provide corrections for any unknowns in the predictions.

A computer generated rendering of the completed Nancy Grace Roman Space Telescope, named after NASA’s first Director of Astronomy and Heliophysics. (Image: NASA)

The OTA entered a critical phase early in 2024 as the final optical alignment of its various mirrors was performed. This necessitated the 10 optics to be aligned and positioned relative to each other to microscopic precision and then permanently locked in place. Misalignment errors as small as one tenth the width of the human hair would degrade the imaging performance of the telescope. To achieve such extreme alignment precision, a special camera system called an interferometer was used to monitor the mirrors with nanometer-level accuracy and provide feedback during this crucial alignment process.

After final alignment, the telescope underwent rigorous dynamic testing that encompasses the extreme environment it will experience as it is launched into space perched at the top of a rocket. This included subjecting the telescope to acoustic sound levels louder than what would be experienced standing next to a jet engine as well as acceleration forces several times higher than what the pilot of a fighter jet experiences during high-g maneuvers.

The final test the OTA needed to pass was a thermal-vacuum test where the performance of the system was evaluated while subjected to conditions that simulate the harsh environment the OTA will experience while in space. This test took place in a large vacuum chamber at an L3Harris facility in Rochester, New York. The inner walls of the vacuum chamber were cooled with liquid nitrogen to provide a very cold environment, and the telescope was cooled to temperatures as low as -120 degrees Fahrenheit (-85 degrees Celsius). The OTA demonstrated its abilities to maintain its desired temperatures while providing exquisite optical performance that met all requirements with margin to spare. Following the successful completion of this test, the OTA was delivered to NASA Goddard Space Flight Center to be integrated together with the science instruments and the spacecraft vehicle.

When the Roman Space Telescope is launched it will join NASA’s James Webb Space Telescope orbiting the L2 Lagrange point — 1.5 million kilometers (1 million miles) directly “behind” the Earth as viewed from the Sun. Roman has been designed to work in conjunction with the Webb Telescope to perform complimentary scientific observations that will provide greater insights into cosmological phenomena than either mission could accomplish on its own. The Roman Space Telescope will be able to image large areas of the sky with similar resolution as the Hubble Space Telescope, however, will do so 1000x faster than Hubble. This enables surveys of large areas of the sky with extreme precision to identify targets of interest for the Webb Space Telescope.

Roman will also be the most stable large space telescope ever built, at least 10x more stable than the Webb, and 100x more stable than Hubble. This optical stability is a critical feature of the system that will allow scientists to test fundamental theories of cosmology in ways never before possible. And when the ultra-stable telescope is combined with the coronagraph, it demonstrates key capabilities on the path to NASA’s next flagship astrophysics mission, the Habitable Worlds Observatory, and its goal of finding planets that could support life.

The delivery of the Roman Space Telescope OTA is the latest milestone in L3Harris’ long standing partnership with NASA. For more than 60 years, L3Harris has provided cutting-edge imaging systems and other solutions that advance universe exploration. From the Hubble, Chandra and James Webb telescopes to the International Space Station and Mars Rover, L3Harris has been with NASA every step of the way, pushing the boundaries of human discovery.

This article was written by Peter Miller, Chief Systems Engineer, L3Harris Technologies (Rochester, NY). For more information, visit here .