2445A self-test, how does the scope sense the spot on the graticule

J Hunt
 

I have been curious about the self-test sequence for my 2465A scope, particularly the spot dance sequence.
Recent discussion on 2445A calibration (see snippet below) implies that the scope senses the spot position vs. the graticule lines.
Is that the case?

If so, how does the scope detect the spot position without a camera or photodetector?

John Hunt
Portland, OR

Re: 2445A calibration
Oct 16, 2018 , Chuck Harris wrote:
You can see how well you have done when the scope does its
spot dance in steps 111 and 112... and in the other little
automatic sections. The beam does a successive approximation
sequence where it starts some distance away from the graticule,
and converges onto the graticule intersections.
If you haven't properly related the graticule to the cursor,
it will miss the graticule intersection.

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