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The Calibration and Alignment Wavefront Sensor (CAWS) is a common-path interferometer for high-order adaptive optics (AO). With a resolution comparable to that of a 51-by-51 sub-apertures Shack-Hartmann sensor (SHS), it can directly measure the phase at a pupil without explicit reconstruction. In the past, interferometers have been deemed unsuitable for AO. Using a separate reference beam meant the interference pattern was susceptible to piston variations. Unlike SH wavefront sensors, interferometers are usually monochromatic, meaning they can only use a small portion of light. This is solved by CAWS’ common-path design in which the reference beam is self-generated using a share of the input light. As a consequence, the reference beam always has the same piston as the measured wavefront and has the same intensity. This holds true for all wavelengths and allows CAWS to do interferometry with narrow-band polychromatic light. Having more flux than monochromatic interferometers, CAWS can run at a higher frame rate. Empirical tests show the instrument’s frequency response is kept in the linear regime at low RMS phase aberrations. Both these features allow CAWS to measure, in real-time, residual phase aberrations in closed-loop AO systems. An application of this is to distinguish and discard speckles from small features in high-order or extreme AO. This work presents the optical design and characterisation of the instrument using laboratory results. The results show the instrument’s frequency response in a monochromatic regime.

Optical design and laboratory characterisation of CAWS: the Calibration and Alignment Wavefront Sensor for high-order adaptive optics