NASA’s Nancy Grace Roman Space Telescope is performing even better than scientists anticipated after engineers completed an extensive series of tests and fine-tuning. The work has brought the observatory’s powerful instruments into exceptionally sharp focus, and its main camera system now delivers almost 30% better image quality than anticipated. The achievement marks an important step toward the telescope’s ambitious mission to investigate dark matter, dark energy and other mysteries of the universe. Even small improvements in optical precision could help Roman capture clearer images during his extensive studies of the cosmos. “All of Roman’s extensive studies will be based on the Wide Field Instrument,” said Dave Hickey, Wide Field Instrument systems engineer at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Many of the mission’s main goals, such as studying dark matter and energy, would not be possible without it, so it is both exciting and a huge relief to see that it is working well.” NASA puts Roman’s powerful camera system to the test Following activation of the wide-field instrument in September, engineers began an extensive testing campaign to confirm that each component was functioning properly on its own and as part of the entire system. Their first priority was to examine the instrument’s 18 detectors, each about the size of a sticky note, along with the electronics that operate them. Next, the team evaluated the element wheel, a rotating mechanism equipped with filters and other optical components. These allow selected wavelengths of light to reach the detectors and can separate the incoming light into its individual colors. Next came the instrument’s internal calibration system. Engineers activated their light-emitting diodes, or LEDs, which function as adjustable light sources that can produce different colors and brightness levels at Earth’s command. By measuring how detectors respond to these controlled signals, scientists can identify differences in sensitivity and make the necessary corrections for accurate observations. Engineers also tested the instrument’s alignment mechanism and moved it from the deliberately out-of-focus position used during launch to the position expected to provide the sharpest images. They ended up reviewing the thermal control system, which maintains the temperatures necessary for the correct functioning of the different components of the instrument. “Calibrating an instrument is like tuning a piano or guitar before a concert,” said Maxime Rizzo, wide-field instrument systems engineer at NASA Goddard. “You adjust everything until it’s exactly how you want it and then you leave the settings as is so that it works consistently while you use it.” A space telescope is already close to perfect alignment While one group focused on preparing the Wide Field Instrument, another worked to refine the telescope’s overall optical alignment. The results were encouraging from the beginning. Roman’s mirrors and other optical components were already positioned with remarkable precision, meaning engineers only had to make minor corrections. “The telescope’s performance was excellent even before we began fine-tuning the alignment of all the mirrors and other optics, thanks to the exceptional preparatory work done on the ground before launch,” said Matthew Bolcar, Roman optical systems leader at NASA Goddard. “All the testing and modeling work done at Goddard and by contractors L3Harris and BAE Space and Mission Systems left us with only minor corrections to get everything in near-perfect alignment.” Achieving that level of precision is especially challenging for a spacecraft. The forces experienced during launch can cause small changes to sensitive equipment, and conditions in orbit differ significantly from those on Earth. Roman launched on August 30 and carried optical systems that had been meticulously tested before takeoff. “Our tests on the ground were not able to exactly reproduce the environment Roman would experience in space, such as the absence of gravity,” Bolcar said. “And there are also slight differences in temperature variations and moisture content. Despite those challenges, recent evaluations showed that our ground tests were excellent, with both instruments meeting or exceeding the performance we had predicted.” A mirror setup smaller than a human hair Roman relies on an intricate arrangement of mirrors and optical components to collect light from distant cosmic objects and direct it toward its two scientific instruments. For the resulting images to remain sharp, the light must follow an extremely precise path. Even a microscopic misalignment can affect how precisely the telescope focuses light. Fortunately, Roman’s optical system was already so well aligned that only incredibly small movements were necessary to improve its performance. “We moved the secondary mirror about one-sixteenth the diameter of a human hair,” said Alden Jurling, a senior optical engineer working on Roman at NASA Goddard. “This adjustment pushed the height of the secondary mirror above the primary. If we scale that height to the height of the Empire State Building, the change in height is a little less than what you would get by stacking a single US cent on top of the tower’s tip.” Although almost impossible to appreciate on a human scale, such a small adjustment can make a significant difference when a telescope is designed to study objects across vast cosmic distances. Initially, the team focused on Roman’s coronagraph instrument, which had been producing slightly out-of-focus images. The problem was less serious than the engineers had anticipated, so its solution was relatively simple. The coronagraph now meets the team’s demanding performance expectations. Engineers will continue to refine its camera settings and determine the optimal settings for its deformable mirrors, which can change shape to make extremely precise corrections to incoming light. In the coming weeks, the team plans to test the coronagraph’s ability to block unwanted light, an essential ability for observing faint objects near much brighter ones. With the coronagraph focus optimized, engineers returned to the Wide Field Instrument for another round of adjustments. Roman’s image quality exceeds expectations by almost 30% Final measurements revealed a particularly impressive result. Roman’s wide-field instrument achieved nearly 30% better optical performance than the team had predicted. Before launch, engineers set an image quality requirement of approximately 90 nanometers RMS (Root Mean Square). This measurement describes the amount of optical error in the system, or how far incoming light deviates from an ideal, perfectly focused wavefront. To put that scale into perspective, 90 nanometers is about 1,000 times thinner than a human hair. In these tiny dimensions, even seemingly insignificant imperfections can influence the sharpness of astronomical images. However, after the latest corrections, engineers measured an optical error of only 63 nanometers RMS, substantially better than the original requirement. That improvement means Roman should be able to produce even sharper images than its designers initially expected, strengthening its ability to carry out detailed observations over huge regions of the sky. “All the images from Roman’s cosmic surveys will be even sharper than we expected,” Jurling said. “It’s a great result of years of careful modeling, precision metrology and testing.” With both scientific instruments meeting or exceeding expected performance, Roman has achieved another important milestone in its preparation for routine observations. The results highlight the extraordinary precision of the telescope’s design and offer a promising perspective for its upcoming investigations into the hidden structure and evolution of the universe.
