Nudging a mirror by a sixteenth of a hair’s width made Roman’s images 30% sharper than predicted

NASA’s Nancy Grace Roman Space Telescope is delivering sharper optics than its builders predicted, and part of the gain came from moving the secondary mirror by about one-sixteenth of the width of a human hair. Engineers at NASA’s Goddard Space Flight Center measured the Wide Field Instrument’s optical error at 63 nanometers RMS after the adjustment, and the agency describes the instrument’s performance as nearly 30% better than the team expected.

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Roman launched on August 30, 2026, and its first weeks in space have been spent on commissioning: switching instruments on, checking focus and tuning the optics before the sky surveys begin.

A secondary mirror moved by a fraction of a hair

Alden Jurling, a senior optical engineer at Goddard, said in NASA’s commissioning update that the team shifted the secondary mirror by about one-sixteenth of the diameter of a human hair. The secondary is the smaller mirror that sits in front of the large primary and bounces collected light back toward the instruments, so even a tiny change in its position alters where the light comes to a focus. Jurling added that images from Roman’s cosmic surveys should end up sharper than the team had anticipated.

NASA gave the move only as a fraction of a hair and published no distance in microns. Human hair typically measures roughly 70 to 100 microns across, so the arithmetic puts the shift at about 4 to 6 microns. That conversion is an estimate built on a typical hair width, not a figure from the agency or the mission team.

The mirror in question matters because Roman is built for wide, deep surveys of the sky. Its Wide Field Instrument is meant to image enormous patches of space in a single pointing, and that only pays off if every star and galaxy in those frames stays crisp out to the edges. A slightly misplaced secondary smears the focus across the field, so alignment at the scale of a few microns is the difference between a survey image that can be measured and one that must be corrected after the fact.

What 63 nanometers against a 90-nanometer requirement means

Two different comparisons sit behind the 30% number, and they should not be blended. NASA says the Wide Field Instrument now performs nearly 30% better than the Roman team predicted, but the agency does not publish the predicted value itself. The measured optical error of 63 nanometers RMS is set against a requirement of about 90 nanometers, which is the bar the mission had to clear. Sixty-three is roughly 30% below ninety, so the two comparisons land in the same place, yet only the second is anchored to numbers a reader can check.

The wording in NASA’s account is careful, and the same care applies here. A 63-nanometer RMS figure describes the shape of the light wave reaching the detectors, which is a measure of optical quality, not a direct count of how many pixels a star spans. The “sharper than predicted” language is the team’s own summary of that measurement relative to its pre-launch modeling, and it was reached after the in-orbit alignment work rather than before launch.

Optical error in RMS terms describes how far the wavefront of incoming light departs from a perfect one. Lower is better, and a smaller error means a tighter, cleaner image at the detectors.

Matthew Bolcar, Roman’s optical systems lead at Goddard, said the telescope’s performance was already excellent before the alignment work began, and he credited pre-launch modeling and ground testing by contractors L3Harris and BAE Space and Mission Systems. The adjustment, in other words, refined an optical system that had been built and verified to come in under its error budget, rather than rescuing one that had missed it. Details of NASA’s account are also republished by ScienceDaily.

Coronagraph focus and the Wide Field Instrument’s 18 detectors

The focus work began with the Coronagraph Instrument, whose first images came back slightly out of focus. Once the team had optimized that focus, it adjusted the telescope alignment for the Wide Field Instrument, Roman’s main camera. NASA had already reported the September activation of the primary instrument and the first checkout of the coronagraph, and the latest update is the follow-up on how well the optics hold up.

The Wide Field Instrument records light with 18 sticky-note-sized detectors, and Dave Hickey, a systems engineer on the instrument at Goddard, said in NASA’s update that “All of Roman’s sweeping surveys will rely on the Wide Field Instrument,” which is why its image quality carries so much weight for the mission. Colleague Maxime Rizzo compared calibration to “tuning a piano or guitar before a concert,” a job done so the settings can stay fixed while the instrument performs consistently.

The coronagraph team still has work ahead. It will keep calibrating its camera settings and determine the best settings for its deformable mirrors, then try to demonstrate its light-blocking capabilities in the coming weeks, according to NASA. The agency tracks the full sequence of checkouts on its Roman commissioning page, and the telescope’s August 30 launch remains the starting point for that timeline.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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