• I moved the vertical micrometer stages to around 6 mm, which is close to their midrange and adjusted the two folding mirrors for the beams reach the QPD centers.
• I set the length sensing QPD at the appropriate position with respect to the lens:
  • I applied white noise actuation to TM-P and observed the amplitude of the 0.843 Hz resonance peak in the amplitude spectral density of the LEN VER  channel.
  • I normalized the channel values at the resonance with TILT VER in order to cancel out variations in TM-P actuation at each ASD measurement.
  • The plot shows the values before and after normalization.

As can be seen in the plot, the position of the length sensing QPD that minimizes the coupling from TM-P is at 8.50 mm. The ASD plot shows that the LEN VER (in blue) is at the noise floor level at such position.