Update, beat measurement after several optics replacement. Peaks around 10 Hz, 35 Hz show up this time.

    Optics that we replaced are:

•  Beam splitter that divides the beam to ACAV and RCAV path. Now the new BS is 1" cube for large beam
• Fixed the orientation of mirrors on the periscope and the turning mirror for ACAV RFPD. A few of them were flipped back causing stray light in the beam path.
• Two of the mirrors on periscope had transmission of 2% or so, we replaced with a high reflective ones (0.1% transmission).

The problem with the curve mirror from last entry has not been fixed yet. It turns out that the mirror we borrow from 40m is worse than the one we have (surface is more milky), so we leave the original mirror as it is.

Note: The beat measurement was done when the air springs were inactive. Noise at high frequency goes down a bit.

The power input to each cavity is 1mW, setup on PLL is 1kHz input range, with gain = 200.