Maintaining Perpendicularity in a Focusing Circle Setup

[asdf1]

Why is it that for a given plane in a sample to remain perpendicular to a radius of the focusing circle, the detector must rotate around the latter at a rate twice that of the sample?

 

[berkeman]

Beats me. Do you have a web pointer?

 

[DaveC426913]

http://ucxray.berkeley.edu/~jamesh/elves/manual/basics.html"might be the guy to ask.