I've not followed this thread in detail so sorry if I'm repeating
anything...

About 8 years ago I was using a position measuring device made by Melles

Griot.  It used 4 photocells and calculated the centroid of the incident

beam.  It had an accuaracy of a few microns and cost a few thousand $.

The point being that it only used a few cells and centroided to
calculate
the position of the beam

Technology moves on, but it looks like they still make a similar
product.
See http://www.mellesgriot.com/pdf/0048.9-48.10.pdf

Ian Smith


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