Open loop means that they’re just using a mathematical prediction of where to point the mirror at a specific time, given the relative position between the heliostat, the target and the sun. adjustments to the mirrors are typically done every 10 to 15 seconds, which is sub optimal. Closed loop means that they’re actually using Computer vision to dynamically sense the specific angles between the sun and the target and then adjust the mirrors accordingly, no mathematical model required, and the adjustments can be done tens of times per second.