This paper presents the first experimental results obtained with the four cameras backward gazing method for measuring the opto-mechanical 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 slopes 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 plant THEMIS with slopes error measurement accuracy estimated as 0.2 mrad. The capacity of characterizing shape distortions due to wind or gravity loads is also investigated. The main experimental error sources are analysed and possible ways of mitigation are proposed, giving reasonable hope in reaching an ultimate measurement accuracy of 0.1 mrad.