Kalman filter, measurements and estimation

[flo24601]

Hello,

I am using a Kalman filter with a PI controller.
The goal is to control a mirror to stabilize the position of a laser spot: I am sending a disturbance on the mirror and the PI sends command to the same mirror so the spot stays fix.
With a camera I am recording the image of the laser and I compute the centroid to get its position.
With the Kalman filter I also get an estimation of this position.

My problem is the following:

Without the KF the PI controller works fine: after an overshoot the error goes to 0 (observing the measurements)
With the KF I thought I would decrease the overshoot but the measurements are the same (slightly worse even) but the overshoot of the estimations is lower

My questions are:

Which values should I consider (being the real position): the measurements or the estimations ?
Why doesn't the KF have an impact on the measurements ?


I can of course clarify things if needed

Thanks