The goal of the shift was to finalize the centering of the NI CH beam on the CP, starting from the point reached on January 23rd (50552).

A considerable fraction of the shift has been spent in finding a position of the CITF optics (NITM, BS and SR) and detection benches (SDB1 and SDB2) that would allow the probe beam to get back to the HWS in the correct position. The final HWS illumination pattern is shown in figure 1.

Thus, only at 18:59  UTC it was possible to start the HWS data acquisition . To quickly check the correct position of the HWS beam on the NI mirror, we decided to switch on the NI-CH (instead of the RH), since it reacts more promptly and it was well visible in the HWS maps at the end of the last shift (50552). At  19:08 UTC, we switched on CH with (CH_Pickoff power  5.3 W*0.15) 0.8 W. The lens induced by the CH became clearly visible on the HWS maps after a few minutes (see figure 2).

We then switched off the CH (19:15 UTC) and turned on the NI-RH (19:19 UTC) with  45 W, in order to check if the position of the center of the mirror had moved wrt January 21st (50519). Indeed, after about one hour, the RH curvature became pretty evident (but not as centered as it was on 50519), see figure 3. After having acquired the new coordinates for the center of the mirror (y = 724+-5,  x = 710+-7), we switched off the NI-RH at 20:54 UTC and turned on the CH and, after some movements with the hot mirror, at 22:03 UTC we reached the situation shown in figure 4.

We then moved to the last step i.e. we put back in place L_CH to have the design size of the beam on the CP and proceed with the fine tuning of the alignement. The CH has been switched on again at 23:10 UTC with 1.3 W shined on the CP. We increased the power to partially compensate for the lower power density of the beam and the higher thermal time constant. In fact, the CH beam appeared on the HWS at 23:30 UTC (after about 20 minutes, against 3 minutes with the reduced size), as shown in figure 5. Not surprisingly (the focal length of the L_CH is 140 mm), the beam moved a bit on the CP. We acted again on the hot mirror to re-center the beam. The effect of the displacement stabilized around 00:05 UTC (see figure 6). The HWS image appeared polluted by a high level of tilt (the RH was off since 20:54 UTC). In view of the Friday morning shift, there was no time to switch on the RH again and it was therefore necessary some more data processing to estimate the position of the CH center corresponding to this last step. We removed the piston and tilt from the map and obtained the result shown in figure 7. The coordinates for the CH center were found by eye to be around (y = 706,  x = 793). This result will have to be confirmed with an additional measurement with both CH and RH on, to check the piston/tilt removal does not affect the determination of the CH center. 

At 00:16 UTC, we switched off the NI-CH and closed the shift.

Final remarks: from the shifts done till now, we have learnt that these measurements are more difficult and lenghty to carry on wrt the same ones done in preparation for O3. The present activities are in fact suffering from a not-yet-perfect stability of the ITF optics along the HWS beam path and a considerable fraction of the shift time is required to recover the proper reference position of the HWS beams.