Evaluation of image-shift measurement algorithms for solar Shack-Hartmann wavefront sensors

¹ Institute for Solar Physics of the Royal Swedish Academy of Sciences, AlbaNova University Center, 10691 Stockholm, Sweden
e-mail: mats@astro.su.se
² Stockholm Observatory, AlbaNova University Center, 10691 Stockholm, Sweden

Received: 3 July 2010
Accepted: 13 September 2010

Abstract

Context. Solar Shack–Hartmann wavefront sensors measure differential wavefront tilts as the relative shift between images from different subapertures. There are several methods in use for measuring these shifts.

Aims. We evaluate the inherent accuracy of the methods and the effects of various sources of error, such as noise, bias mismatch, and blurring. We investigate whether Z-tilts or G-tilts are measured.

Methods. We test the algorithms on two kinds of artificial data sets, one corresponding to images with known shifts and one corresponding to seeing with different r₀.

Results. Our results show that the best methods for shift measurements are based on the square difference function and the absolute difference function squared, with subpixel accuracy accomplished by use of two-dimensional quadratic interpolation. These methods measure Z-tilts rather than G-tilts.