This shift was focused on understanding the differences in HWS-DET illumination observed when the ITF goes from a locked to an unlocked state using a gentle unlock procedure. In this unlock configuration, all mirrors are misaligned except for the NI, BS, and SR (see entry 68056).

This state corresponds to the NI single bounce configuration with SR aligned. So, we started by performing the following measurements:

1. Lock ITF in NI SINGLE BOUNCE + SR aligned

Folder name: 20251125T1455_NI_Single_Bounce

2. Lock ITF in SINGLE BOUNCE + SR aligned + NE aligned

Folder name: 20251125T1501_NI_Single_Bounce_NE_aligned

The CCD illumination patterns acquired in the two configurations, as well as their difference, are shown in Figure 1. From the difference map, it is clear that the NE contributes to the changes observed in the HWS-DET illumination.

Then, two independent sets of measurements were performed, scanning the TX and TY of the NE mirror one at a time.

The reference position for the NE mirror is: TX = –113 µrad, TY = 38 µrad.

The CCD illumination patterns acquired at each step, as well as their differences with respect to the aligned configuration, are shown in Figure 2- 3 for the TY scan and in Figure 3-4 for the TX scan.

After the measurement, we decided to attempt the absorption measurement by performing a gentle unlock, this time without misaligning the NE mirror.  Diego implemented this procedure in the automation.

After the ITF was locked in CARM NULL 1f, Diego noticed that in all previous measurements, the SR position had been mistakenly parked at the misaligned position used for LN3, meaning it was misaligned by –2.3 µrad in TY.

I performed an acquisition in the SR-aligned configuration (ITF in carm null 1F) and compared it with the one acquired with the ITF in SINGLE BOUNCE + NE aligned (see fig.6).

To remain consistent with all the measurement sets performed during the shift, I asked Diego to misalign the SR again by –2.3 µrad in TY. Again, I performed an acquisition in the SR-misaligned configuration and compared it with the one acquired with the ITF in NI SINGLE BOUNCE + NE aligned (see fig.7). 

With the ITF in CARM NULL for 1 hour and 2 minutes (and SR misaligned by -2.3 urad in TY), we started the HWS-DET acquisition at 18:22 UTC ( 20251125T1922). 

At 18:27 UTC, the ITF was manually unlocked (WF 9) using this special gentle unlock configuration, in which the NE remained aligned.

One of the wavefront acquired after the unlock is shown in figure 8.

The structure observed in the measurements described in entries 68058 or in 68051 is no longer visible. It was caused by the HWS beam reflected by the NE, which disappears after the unlock. The final check would be to repeat the measurement with the SR aligned.
At 19 UTC, I interrupted the measurement.