Here is an executive summary of the ASC changes that have been made, why they were made, and the potential impact on the noise:

• DHARD P boost is currently disengaged
  • as a part of investigating a still-mysterious 1 Hz instability, I disengaged FM8, which is usually engaged in LOWNOISE_ASC.
  • This boost was designed by Gabriele to reduce the DHARD P RMS, which was beneficial to reducing the DARM RMS
  • This may overall impact the DARM RMS
• MICH P and Y have been reverted to an older loop design
  • during locking, some 0.5 Hz ring ups in the MICH ASC impacted locking, even causing a lockloss
  • I changed the loop design back to a design that was in place pre-O4, which uses FM7 instead of FM10 in MICH and FM5 instead of FM10 for MICH Y
  • This may result in more MICH ASC noise coupling to DARM
  • Changing back to this old design has reduced the RMS in MICH P and Y
• Disengaged BR filter in MICH P and Y
  • I disengaged a bounce/roll filter in the MICH ASC that had been added during O4. We disengaged this same filter in the MICH LSC
  • We may see the beamsplitter B/R modes in DARM, but unlikely because we have dampers on the BS for this
• A new DC centering loop, DC6
  • centers the beam on POPX using PM1
  • likely very low coupling to DARM
• PRC2 ASC is now on POPX
  • this would change the noise in the loop, however PRC2 has very low coupling to DARM, unlikely to impact noise

I ran a coherence in NLN with all the arm loops, MICH and PRC2 ASC. There is some coherence with CHARD P, and with MICH P and Y that would be worth investigating with a noise budget injection.