A modular imaging system includes an antenna panels, a sensor, and at least one data processor. The antenna panels include an array of antenna elements including at least two antenna elements separated by a spacing more than a half wavelength. The plurality of antenna panels are configurable to be spatially arranged and oriented with respect to one another to measure radar returns of an observation domain for a target. The sensor has a field of view overlapping the observation domain and for measuring an image. The at least one data processor forms part of at least one computing system and is adapted to receive data characterizing the optical image and the radar returns, determine a spatial location of the target, and construct a radar return image of the target using a sparsity constraint determined from the spatial location of the target. Related apparatus, systems, techniques, and articles are also described.