This paper presents a simple slit-mask-based sensing scheme that can be applied to a robust piston error sensor for segmented optical systems, even in space. Utilizing crossed fringes generated from multiple double-slit pairs, the sensor simultaneously detects multiple points in a segmented optical system in one shot. The experiments tested six pistons of a single-ring segmented mirror. The results showed measurement stability better than 15 nm in the range of 0–5 µm and absolute accuracy of λ / 20 . The theoretical perspective of applicability for larger piston measurement and N-ring segmented systems is also discussed, with simulation results for a two-ring system presented.