When the interferometer is locked at step 12, it is possible to extract from the correction sent to one mirror actuators an estimation of the free mirror motion. Indeed the locking loop has very high gains at the frequencies of interest (below 10 Hz, where real seismic motion of the payload is present) and therefore the correction is directly related to the free mirror motion, given a calibration factor which is the simply the actuator response.

Using data from the PR (which is the simplest one from the actuation point of view, since there is no re-allocation) from many different locks from February to April 2008, and the calibration of 13.8 microns/V (as given in Mirror and marionette actuation calibration for VSR1) the following estimates are obtained:

Taking the maximum value for each frequency, the following estimate is obtained, plotted together with the integrated RMS:

The total RMS motion of the suspended mirror if left free is about 0.2 microns.