I tried to ask this the last time Heliogen was on HN, but still don't understand the solution, so I'll ask again: You have four photodiodes surrounding the tar

I tried to ask this the last time Heliogen was on HN, but still don't understand the solution, so I'll ask again: You have four photodiodes surrounding the target, and 1000 mirrors aligned to reflect the sun toward that target. One of the 1000 mirrors goes out of alignment, and your photodiodes tell you so. How do you know which mirror needs to be adjusted?