To what accuracy can JWST's mirror actuators position the mirror?

JWST’s mirror actuators can position the mirror with an accuracy of 10 nanometers. That is 10 billionths of a meter—far smaller than the width of a human hair—and it lets engineers coordinate the telescope’s 18 primary-mirror segments as one optical surface.

Each segment has seven actuators: six adjust its position and tilt, while another can slightly change its curvature. The actuators move the beryllium segments from their launch configuration into the precisely aligned shape required for observations.

The mirror is not positioned once and forgotten. Webb’s wavefront-sensing system repeatedly measures starlight from test stars, determines how the segments’ reflected images differ, and commands tiny corrections. This process is called wavefront sensing and control, and it is essential because separate segments would otherwise produce separate, blurred images.

A common mix-up is confusing actuator step size or positioning accuracy with the telescope’s overall image quality. The 10-nanometer figure describes the mechanical control capability; the final optical alignment is assessed through wavefront measurements and can be affected by thermal changes and micrometeoroid impacts.