Model-based large-dynamic iterative piston correction using extended objects.

Cophasing is crucial for segmented or sparse aperture telescopes to achieve high resolution. In this Letter, we propose a novel, to the best of our knowledge, model-based piston correction method that can remove large-scale piston errors within a few iterations using extended objects. The relation between the piston error and a metric function is derived theoretically under broadband illumination. The metric function is based on the image's power spectral density at the spatial frequency where the sidelobe peak of the modulation transfer function (MTF) appears. The piston error is iteratively estimated and corrected by introducing positive and negative piston biases. The dynamic range of piston correction can be as large as the coherence length of light. The correction accuracy in experiments is affected by the image noises and the accuracy of the introduced piston biases.