This paper describes a sun backward gazing method with multiple cameras optimized for fast and accurate control of solar concentrators in operating conditions. For Concentrated Solar Power (CSP), such methods should enable easy commissioning of the optical segment of the facility, survey of damaged mirror facets and regular control of wind loads or gravity effects without disturbing the daily electricity production process. The proposed method makes use of a matrix of cameras located at the vicinity of the solar receiver and observing sunrays reflected by the concentrator. The theoretical principle is explained and interpreted in terms of a fictitious enlarged sunshape function. The main data processing steps are detailed and a set of numerical simulations demonstrate the potential of the method to measure large mirror slopes errors. The achieved measurement accuracy is currently estimated to 0.05 mrad RMS and 5 mu m RMS in terms of surface slopes and shape errors respectively. Potential improvement of the method is also evoked.