In order that the PVs point in the correct direction the controller must be able to tell which direction the sun is in. This sun direction sensor does just that by detecting which of the three photo transistors is in shadow and which is not. The angular difference between Fully exposed to light and shadow is about +/-8 Degrees according to the math and as best as I can measure seems to be about +/- 10 degrees. The PVs give >90% of their output providing they are within +/- 20 degrees that's good enough...