Optical systems with segmented mirrors require precision coalignment of their segments to attain the desired performance. A novel sensor for measuring piston over a ± 10 wave range by analyzing the image of a white light point source is described. The sensor combines a Young's two-slit aperture pair for coarse sensing with another set of apertures, which include a phase retardation portion for fine sensing. The course sensor has an accuracy of ± 0.25 wave over the full range. The fine sensor works within ± 0.5 wave of zero with ± 0.005 wave accuracy, and a repeatability of the order of ± 0.001 wave. Theoretical derivation and experimental results are provided.