I've seen an optical tool intended for centering the spindle of a mill over a crosshair or mark on a workpiece, one that doesn't require such good vision as many do not have.
Enco has it as item 240-0404 for $170: .
MSC has what appears to be the same thing, with more description and a higher price: . The long description is in the printed catalog, not the website.
Travers also has it as item 57-101-650, for $237! Well, they used to - it no longer appears on their website. In the 2007 paper catalog, it appears on page 805 as a TTC Fine Centering Scope. But it probably didn't sell well.
I did find this thread from 2006: .
And this one .
And this user's website (about halfway down): .
In any event, even at $170 it's kind of expensive for what it does, and I wonder if anybody has any experience with it or anything like it. Also, does anyone know the actual make and model? It seems to start with SKO.