While waiting for the end of the PR CHRoCC transient, we decided to perform the first measurement on the NI. From the HWS-DET map, we noticed a non-uniform illumination pattern on the sensor with the ITF in CARM NULL 1F, so Suzanne and Andrea went to the EDB to adjust the steering mirror in front of the HWS. Figure 1 shows the illumination image before and after their intervention.
1) Acquisition started at 15:12 UTC, with the folder named 20251028T1612.
At 15:17 UTC, the ITF gently unlocked (WF 10). Immediately after the unlock, the NI was accidentally misaligned, but Diego realigned it promptly.
Starting from WF14 (about 2 minutes after the gentle unlock), a previously unseen structure appeared in the wavefront map.
The picture below shows a comparison between the last measurement on the NI (left – 59448) and today’s measurement (right).
The measurement was interrupted at 16:11 UTC (WF 108).
To investigate whether the structure observed in the map could be an artifact, we checked that the SLED of HWS-INJ was OFF and all mirrors, except for the NI, were misaligned. Everything was Okay.
To confirm the results, we decided to repeat the measurement. The ITF was relocked at CARM NULL 1F at 16:27 UTC.
Before starting the measurement, I slightly increased the voltage applied to the SLED (from 1.05 V to 1.08 V) to raise the counts to approximately 3500 (see the third figure attached).
2) At 17.29 UTC, the HWS-DET acquisition started (20251028T1829).
Acquisition stopped at 18:25 UTC (WF 103). The structure was still present (see Fig. 4).
At this point, Piernicola suggested that this structure could be due to a spurious beam from the new OMC installed on EDB.
We therefore contacted Romain to ask about the possibility of blocking the YAG beam near the viewports.
He gave us the green light, and Andrea, with remote support from Piernicola, went to DET to place an absorbing glass in front of the viewports (see Fig. 5).
3) So, after the ITF had been locked at CARM NULL 1F for one hour, we started the third acquisition at 20:04 UTC (20251028T2104).
The structure was still there. The measurement has been interrupted at 20.40 UTC (WF 67). At this point, Andrea returned to DET to remove the absorbing glass in front of the viewport and we restarted the lock acquisition at 21.00 UTC.
In conclusion, a new structure appears in the HWS-DET wavefronts approximately one minute after the gentle unlock. We repeated the measurements three times, and the results were consistent. Initially, we considered the possibility that it was caused by initial misalignment of the NI mirror. For the second measurement, we repeated the procedure, and for the third, we covered the viewports with absorbing glass to investigate whether the structure could be caused by a spurious beam from the new OMC. But even after blocking the viewports, the structure persisted in the maps.
