The goal of this shift was to test the effects of the drift control on SDB1, in conjunction with the alignment of the SR mirror.
In previous shifts was observed that tilting the SR on TY produced asymmetrical results, with TY+ being beneficial and TY- being almost immediately distruptive, This led us to believe that SDB1 could be misaligned wrt the ITF, and we decided to cooperate with DET people to engage its drift control loop, and confront the results with the misalignment of the SR.
Since the beginning of the shift locking the CITF was very difficult, and the lock acqusition attempts were distrupted by a 13-14 Hz oscillation at 160mW visible on most of the longitudinal DoFs, due to SRCL gain being too high (changed 0.8->0.65, tested and saved in the function). This considerably slowed down the tests in the first half of the shift.
To improve the stability of the PR alignment during the pre alignment, Ruggi swapped the optical lever used for the local control of the PR TY. This made the pre alignment process considerably easier, and we could proceed in the second half of the shift with the scheduled tests.
Closing SDB1 drift control on DC and SPRB drift control on 12MHz signals first, and 112MHz signals later, allowed us to achieve a lock that lasted 3h 40m (Fig.4). During this lock we:
- closed the drift control of SDB1 on DC signal (immedaitely after locking the CITF)
- changed the alignment of the SR from -1 to +1 urad wrt the initial point to investigate its effect
- swapped the drift control of SPRB form 12MHz signal to 112MHz signal
- towards the end, we tilted NE_TY in order to bring to 0 the dithering signals (this produced the power increase visible in the last minutes of the lock, in Fig.4)
As usual per the last few shifts, at carm offset zero we did not close any AA loop other than DIFFp and PR_X/Y, TX/Y, nor engaged SSFS.
In the figure is clearly visible an abrupt improvement of the quality of dark fringe, as well as the power on B7, B8 and B4 DC, or the shapes on the phase cameras, that became more symmetrical (Figs.2-3). This is due to the galvo loops on SDB2_B1p_QD2, that had opened without us realizing it, and closed again at that moment (Fig.1).
The lock was ultimately killed while changing the setpoint of NE_TY. The investigation of the unlock will be carried on in the next shifts, as well as the analysis of the data gathered during this lock.
We leave the ITF in DOWN to allow for the TCS actuators calibration.