Eleonora (remotely) and Yuhang

As suggested by Matteo, we should check oplev singal of INPUT/END mirrors. Besides, as suggested by Raffaele, the control bandwidth of AA can be evaluated by checking how long time loop takes to go back to be aligned from misalignment. Therefore, I did the following check:

1. I lock FC and close AA loop. The locking strategy for AA is: 1.lock with low pass filter 2.switch off low pass filter 3.switch on integrator.

2. By adjusting BS, beam moves up on camera. Then I get oplev error signal as attached figure 1.

By looking at INPUT pitch oplev signal, we could see that AA loop takes almost 4 min to recover the pitch misalignment. Therefore, the control bandwidth should be very small for AA loop pitch. Therefore, it maybe better to increase the gain.

3. I monitor PR/BS/INPUT/END mirrors oplev signal and sum/diff signal of FC_tra_GR PSD for about 40 min. They are shown in attached figure 2, 3 and 4.

According to elog1874, PR/BS oplev signals have similiar calibration factors. Therefore, the attached figure 2 shows each oplev signal in a similar scale, and we could see that PR pitch moved more than others during half an hour. From attached figure 3, we could see that INPUT/END pitch moved accordingly. For yaw direction, there is also correspondence between PR/BS and INPUT/END. But END yaw is always osillating with a frequency around 0.05Hz (time scale around 20 sec). From the attached figure 4, we could see the effect of beam motion in pitch direction. The diff singal from PSD kept chaning in pitch direction (which is consistent with the motion seen from camera). Although the AA loop kept the alignment between beam axis and cavity axis, the green transmission power is not kept.

4. I increase gain of AA loop by 100 times for both INPUT and END mirror.

5. I move BS in pitch direction again. Then the 4 mirrors oplev signals and transmission PSD signals are shown in the attached figure 5, 6 and 7.

From attached figure 5, we could easily see the movement I did for BS pitch. From attached figure 6, we could see that INPU/END pitch follows the change of BS. Especially, after increasing the gain of AA loop in pitch, the recovery of alignment becomes quite rapid. This means the control bandwidth should be larger now. From attached figure 7, we see the beam motion on PSD. But it is a bit strange that there is also signal in yaw direction.