( ?? 1 - ?? 2 ) ( ?? i , ?? k ) = ( ?? 1 - ?? 2 ) ( ?? i , ?? i ) ??? ?? k ?? i , with 1 ??? k , i ??? N wherein ??1 and ??2 are the alignments of the first and second focus positions, ??i is the phase between the detector sampling frequency fD and the focus springing frequency fF, ??k is a value proportional to the focus deflection (such as a deflection current), and the alignments ??1 and ??2 ar