An instrument for a solar energy system including a receiver and a heliostat to reflect solar energy on to the receiver. The instrument includes a first element adapted to form an image of the sun, a second element adapted to form an image of the receiver, and a detector. The detector is positioned to receive each of the images and a comparator is adapted to detect a distance between the images. The comparator generates an error correction signal based on the distance between the images. The error correction signal is received by a controller that controls the operation of a postioner that adjusts the position of the heliostat accordingly.