Marc, Yuhang

Today we got the FC flashes back.

We acted on all picomotors to realign the beam and remove the OpLev offset.

We then took spectra of the OpLev signals and saw that the End Mirror one showed a broadband increase of the noise floor.

This was due to one of the steering mirror screw being almost loose. Therefore we put it to a better situation and compensated it by acting on the steering mirror on the OpLev laser bench.

We also changed the optical density before the End Mirror PSD as it was previously not so well fixed (fig 1 : current situation)

The current Oplev spectra are represented in Fig2 with green/brown the references and blue/red the current ones :

  -  PR and BS show the high frequency noise increase that we hope to solve by using the KAGRA QPDs

  - End pitch exhibits a new peak on pitch around 6 Hz

  - BS pitch exhibits a new peak on pitch around 9 Hz

Before locking the FC we would like to further tune the OpLev. Indeed, we fixed all their optics but then did not retune the PSD positions nor check the diagonalizations.

These may explain the new peaks on BS and End Mirror as their optics were moved quite a lot

PS : the second target is on remote mode and End mirror picomotors have been disconnected