I've seen this done with a four-quadrant photodiode and an aperture plate,
and with four solar cells and a cruciform center baffle.  The idea was to
drive the motors such that the outputs of the X and Y pairs were equal,
within the pair. That is to say that left and right are equal, and up and
down are equal, but up does not necessarily equal right.

But factoring in the cost and complexity, it does seem to bear minimal
return.
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