Motivations for the shift :
This logbook presents only the measurement. The analysis and the results will be presented later.
In this shift, we made scans to characterize the arms. We first ran a series of scans with the mirrors well aligned in order to estimate the amplitude of astigmatism. Then, a lot of scans were made with different pairs of orthogonal misalignment axes in order to measure the orientation of astigmatism. The aim was to do the same scans as presented in 66820, but the measurement presented in this logbook concerns only the north arm since it was made during the change of the West end mirror. During the previous measurement, ramps were generated by variation on ALS_NEB_Corr rather than LSC_CARM_INPUT in the usual method used today. When we made those single cavity scan, we observed that the data were noisy such that we were not able to conclude concerning the orientation of astigmatism.
Shift report
Date of the shift : 2th December 2025
To do the scans, we followed the process described in the Optical Characterization wiki page (section : FSR slow scan (with the green laser)):
All the scans were run with the command CARM_SET_scan_FSR(numiteration = 10, f_FSR=110000,timescan = 60.) Each iteration corresponds to one descending scan and one ascending scan. The misalignment angles used were chosen such that the total amplitude is equal to 0.5urad. This high amplitude was chosen to make the misalignment that we chose significant compared to the one usually observed during 'aligned' scans. Then we expect the position of the first higher order mode to be more representative of the orientation of the misalignement axis used.
| Time (UTC) | Action |
| 14:50 | Run 1 aligned scan to test the command and check that everything is working |
| 14:57 | Arm lock IR |
| 15:00 | ARM lock beating |
| 15:01 | 10 scan aligned |
| 15:23 | Arm Lock IR |
| 15:27 | ARM lock beating |
| 15:29 | Add misalignment offsetNE_TX+=0.5urad WE_TX+=0.5urad |
| 15:30 | 10 scans |
| 15:50 | Arm Lock IR |
| 15:53 | ARM lock beating |
| 15:54 | Add misalignment offset NE_TY+=0.5urad WE_TY+=0.5urad |
| 15:55 | 10 scans |
| 16:16 | Arm Lock IR |
| 16:19 | ARM lock beating |
| 16:21 | Add misalignment offset NE_TX/Y+=0.35urad/0.35urad WE_TX/Y+=0.35urad/0.35urad |
| 16:22 | 10 scans |
| 16:43 | Arm Lock IR |
| 16:44 | ARM lock beating |
| 16:45 | Add misalignment offset NE_TX/Y+=0.35urad/-0.35urad WE_TX/Y+=0.35urad/-0.35urad |
| 16:47 | 10 scans |
| 16:55 | Unlcock of the arm |
The arms unlocked during the last scans presented in the table above. After many attempts, we were not able to run the scans again with the misalignment angle we planned. Starting at 18:32 UTC and for 20 minutes, we made scans with no misalignment to check if the problems came from the misalignment applied. All the scans were performed correctly, suggesting that the amplitude of the misalignment was the problems.
In order to continue the measurement, scans were made with smaller misalignment, keeping the same orientation for the misalignment axes but with a total amplitude of 0.3urad rather than 0.5urad.
| Time (UTC) | Action |
| 19:00 | locked arms beating |
| 19:04 | Add misalignment offset NE_TX/Y+=0.21urad/-0.21urad WE_TX/Y+=0.21urad/-0.21urad |
| 19:05 | Run 10 scans |
| 19:28 | Lock arms beating |
| 19:32 | Add misalignment offset NE_TX/Y+=0.26urad/0.15urad WE_TX/Y+=0.26urad/0.15urad |
| 19:34 | Run 10 scans |
| 19:56 | Lock arms beating |
| 19:59 | Add misalignment offset NE_TX/Y+=-0.15urad/0.26urad WE_TX/Y+=-0.15urad/0.26urad |
| 20:00 | Run 10 scans |
| 20:22 | Lock arms beating |
| 20:26 | Add misalignment offset NE_TX/Y+=0.29urad/0.08urad WE_TX/Y+=0.29urad/0.08urad |
| 20:27 | Run 10 scans |
| 20:49 | Lock arms beating |
| 20:52 | Add misalignment offset NE_TX/Y+=-0.08urad/0.29urad WE_TX/Y+=-0.08urad/0.29urad |
| 20:54 | Run 10 scans |
| 21:15 | End of the scans |
The data will be analyzed soon and presented in another logbook