[Rachel Langgin and Caroline Capuano]

Rachel and I volunteered to check if the alignment flucations were any worse than before IR1 upgrades (see commissoning meeting notes). To compare this, we check during O4c (gps 1444003218, October 9th, 2025) as our reference time and compared this spectrum with our most recent data from Tuesday (gps 1474675218, September 29, 2026) when everything was quiet. The script we used to make these plots can be found here.

Please keep in mind that this is comparing the old beam splitter to the new beam splitter so the change you see here could be related to that. This comparison can be re-run once we are locked for a more accurate comparison.

Optics_ASD_1444003218_vs_1474675218.pdf shows the ReferenceTime and CheckTime on the same figure for each optic, and Optics_ASD_1444003218vs1474675218-2.pdf shows the ratio between the two spectrums; both of these have a 60 second time span. Most of the optics do agree at higher frequencies, but a lot of them show differences in optics motion at low frequencies. Some optics shows some major motion differents below 10 Hz, like PRM, RM1, RM2, OM1 pitch and yaw, OM3, OMC.

Update:

Below are plots with a longer time span (10 minutes) and fft length of 64, and a different check time as well:

• Comparison: Optics_ASD_1444003218_vs_1471046418.pdf, Ratio: Optics_ASD_1444003218vs1471046418.pdf Check time: August 18th, 2026
• Comparison: Optics_ASD_1444003218_vs_1474070418.pdf, Ratio: Optics_ASD_1444003218vs1474070418.pdf