Accounting for focal shift in the Shack–Hartmann wavefront sensor

Abstract

The Shack–Hartmann wavefront sensor samples a beam of light using an array oflenslets, each ofwhich creates an image onto a pixelated sensor. These images translate from their nominal position by a distance proportional to the average wavefront slope over the corresponding lenslet. This princi- ple fails in partially and/or non-uniformly illuminated lens- lets when the lenslet array is focused to maximize peak intensity, leading to image centroid bias. Here, we show that this bias is due to the low Fresnel number of the lenslets, which shifts the diffraction focus away from the geometrical focus. We then demonstrate how the geometrical focus can be empirically found by minimizing the bias in partially illuminated lenslets.