Yesterday we modified the notch fitler to face the reintroduction of noise at the WE violin modes frequencies. In order to improve the response we relaxed the Q ratio of the pole/sero structures. (fig.1,2)

The results was to loose an amount of phase at the DARM HF line frequency (491 Hz), see fig.3, and at the UGF of DARM (fig.4).

The results didn't affect the performance of the loop itself, but as a drowback this morning we had to retune the monitor of the DCP, which is based the loop for the alignment of SR.

Once we reached CARM NULL 1f, we noticed that the DCP monitor was measuring 40Hz. We knew that the usual values are between 380 and 400Hz, and even if this probe is not used until LN3, at that stage it is used to "misalign" the SR, in particular we misalign it until the DCP reaches 175Hz. However, with the DCP monitor pointing 40Hz, the SR alignment loop wasn't working properly.

So we took a measurement of the OLTF of DARM while in CARM NULL 1f (09.13.38 UTC, 120s) and we adjusted the calibration factor accordingly in the ini file (from 32 to 275). Then we adjusted also the calibration in LN2 based on the same measurement (since there are no changes in the optical configuration in between), going from 148 to 1468. Finally we also compensated the calibration in LN3, and we made a last measurement to make sure that the misalinged position was indeed around 175Hz as requested (10.28.41 UTC, 120s). The range seemed recovered so we let the interferometer in this configuration.

The change in the DARM line phase due to the change in violin mode notch filters may have changed the demodulation phase for the the SDB1/OMC and SR alignment signals which are based on the demodulation of the 491.3Hz DARM line. For the moment it is not easy to say if it is the case or not, as the signals have a much higher noise due to the high amplitude of the violin modes. 

Figure 1 shows a lock acquisition from a few days ago, when one can see the SDB1 TY misalignment by 10urad as the bench follows changes due to the SR misalignment. One can see a transient in the I quadrature (but not the Q) of SDB1_B1_DARM_TY, as the loop tries to follow. And one can see that ASC_SR_TY_DCP_HF_phi_B1_I acquires an offset, as SR is misaligned.

Figure 2 in the successful lock acquisition of today these signals are much more noisy, by about a factor 10, so these changes are not visible.

It may be the case that there is no issue, as SDB1/OMC looks at the magnitude of the line, which doesn't depend on the phase. And SR looks as the phase of the line, which acquires a static change, which is then removed by demodulation at the SR TX and TY line frequencies. If there is an issue, it is not too large, as both loops seem to work well enough to keep the interferometer aligned.