Sheila, Matt, Camilla

As the interferometer stayed locked during the early part of maintence day, we did some injections into the filter cavity length to try to measure the noise caused by backscatter.  (Previous measurements 78579 and 78262, recent range drops that we think are filter cavity related: 86596)

The attached screenshot from Camilla shows the excitations and their appearance in DARM. (Compare to Naoki's measurement here, and to the time with no injection but elevated noise here).   I made a simple script to make the projection based on this injection, which is available here.  It does show about a factor of 3 a higher level of noise in DARM than Naoki's projection 78579, this is large enough that we should add it to our noise budget but far too small to explain our low range times last week.

• Dtt file is userapps/sqz/h1/Templates/dtt/filter_cavity/backscatter_20250825.xml
• reference 14-16 no excitation
• 17-18 amplitude 1000
• ref 20-22 amplitude 3000
• ref 23-25 amplitude 10000 start to see
• ref 26-28 amplitude 20000 pretty linear, maybe I unlocked FC