This paper presents the first experimental results obtained with the four cameras backward gazing method for measuring the optomechanical errors of solar concentrating surfaces in operation. It consists in combining the images simultaneously acquired by four cameras placed near the solar receiver in order to reconstruct the slope errors of the mirrors digitally. New algorithms adapted to real acquired images are described. The experiment has been conducted on a sun-tracking heliostat of the experimental solar tower THEMIS with canting error measurement accuracy estimated as 0.2 mrad. The main experimental error sources are analyzed, and possible ways of mitigation are proposed, giving reasonable hope of reaching an ultimate measurement accuracy of 0.1 mrad.