I took a high resolution spectrum of DARM (attachment 1) and looked around 27-29 Hz because this is a region where we have seen large peaks we think come from the HSTS and HLTS mirrors. An assumption I made in the past is that they must come from mirrors in HAM2 and HAM3 due to less isolation in those chambers relative to HAM4 and HAM5. However, I think that assumption may be incorrect.

Attached is the high resolution spectrum. I identified 5 peaks evident. I used the table of modes in 49643 to identify each peak.

27.41 Hz - PR2 bounce mode

27.44 - SRM bounce mode

27.5 Hz - SR2 bounce mode

27.71 Hz - SR3 roll mode

28.2 Hz - PR3 roll mode

Note: according to this document: LIGO-T1700111, the HLTS roll modes are around 28 Hz while the bounce modes are at 40 Hz (how fun and confusing!).

Notably missing here is the PRM bounce mode. I am also not convinced that any of these modes are from MC1-3 due to the fact that none of these peaks are visible in the IMC WFS spectra (see second attachment).

The best way to confirm that these identifications are correct is to try to excite each of these modes in lock, and check the high resolution spectrum. I think there is good motivation to try this measurement for these reasons:

• if we assume we see these peaks because of poorer isolation in HAM2/HAM3, the presence of strong bounce and roll modes from mirrors in HAM4 and HAM5 would disprove that assumption
• if we see these peaks from most of the HSTS/HLTS in the recycling cavities, that suggests that something like BRDs or blade springs would be the best method to reduce or remove them from the spectrum
• why doesn't the PRM mode appear? Maybe there is something to learn there.

These are narrow peaks, but we have seen times in the past when the peaks are rung up enough to cause broadband excess noise. I think it is worth trying to reduce these peaks in some way.