subapertures

Subaperture is a portion of an optical aperture used for sampling the incoming wavefront or pupil in various optical systems. In adaptive optics and wavefront sensing, a common implementation is a Shack-Hartmann sensor, where a microlens array subdivides the telescope pupil into many subapertures. Each microlens forms a focal spot whose position measures the local wavefront slope over that subaperture, enabling reconstruction of the full wavefront when combined across the pupil.

In interferometry and high-resolution imaging, subaperture techniques include pupil masking, where a mask selects discrete portions of the pupil to create a sparse interferometric array. Non-redundant masking uses a set of subapertures arranged to produce unique baselines, improving phase retrieval and dynamic range despite a reduced light throughput.

In large telescopes with segmented mirrors, each primary mirror segment acts as a subaperture. The relative piston, tip, and tilt between subapertures must be controlled to maintain coherent combination of light and to phasing the segments.

In metrology and optical testing, scanning or sampling with subapertures enables localized wavefront measurements and helps characterize aperture irregularities, aberrations, or the quality of converging beams.

Limitations include a trade-off between subaperture size and sampling density: smaller subapertures yield higher spatial sampling but less signal per subaperture; larger subapertures reduce sampling. Accurate calibration and alignment are essential to avoid aliasing and systematic errors.