Edge sensor concept for segment stabilization

To meet ambitious science goals and leverage NASA investments for the James Webb Space Telescope, some proposed mission concepts include large aperture telescopes with segmented primary mirrors. Aberration control at the segment level becomes critical for these architectures because rigid body motion of the individual mirrors overtakes full aperture aberrations as the driver for wavefront stability. Perturbations at the segment level cannot be effectively sensed by existing full pupil low-order wavefront sensors because the errors are discontinuous and low-order techniques applied to individual segments would have insufficient photon flux. Thus, an additional "mid-order" control loop is required. We propose the use of capacitive edge sensors to locally sense the relative motion of segments in piston, tip and tilt, which then provide input to a compensation arm. Ball Aerospace has developed capacitive sensor technology with proven measurement precision of <12 pm RMS that can be adapted for measuring primary mirror segment motion. This performance approaches the 10 pm stability often stated as a requirement for direct imaging of earth-like exoplanets with a coronagraph. Using the geometry of the existing hardware as a baseline, the sensor gap and plate area can be scaled to accommodate mounting to mirror segments while maintaining or increasing sensitivity and multiple plates in different orientations can be used to sense individual degrees of freedom. This paper will present measured results from the Ball capacitive sensor and use those results to develop expected sensitives and a notional sensor head geometry for stabilizing a large, segmented primary mirror with edge sensors.