Hello,

 Recently, I have a problem bothered me alot, anybody please help me out.

 I am using Dimension 3100 AFM. It is a scanned tip SPM. There are two screws to adjust mirrow so that the laser can be reflected to the photodiode. My problem is:

 After the laser was reflected to the center of the photodiode, I do locate tip/focus surface. The red spot will shift to the edge of the photodiode. According to my understanding, the red spot should stay center when I do locate tip/focus surface since it doesn't alter laser optical path. Anybody has same experience before?

Thanks very much for your attention!