A long time ago I came up with some blend filters to try to improve the 1-10 hz HAM RX/RY motion, to try to reduce SRCL. We ran the filter for about a week last October, but I've been having trouble trying to find good data to compare to the current configuration. Sam Cooper has a document comparing the differences and some of the motivation for finding a better filter at https://dcc.ligo.org/DocDB/0159/T1900107/001/main.pdf. I've switched HAMs 4&5 RX&RY blends to this filter to get some data overnight (or weekend). I made the transition while Corey was locking the IFO, and it's running fine for now. I've accepted the differences in SDF, so this shouldn't prevent us from going to OBSERVE.
The new blends are called "between_350" on the HAM ISI blend screens. If there is some issue where SRCL motion is bad, the old filters are called "notch_rolloff". If there are any questions or anyone needs help reverting over the weekend, give me a call.
Some initial data, looks like the blend filters do improve SRCL. Attached plot shows SRCL err from a lock after switching the blends (blue) and a segment from a NLN segment last night before the switch (red). Microseism hasn't changed much, but it is windier now than it was last night, but still the SRCL rms is about half of what it was. The magnitude units are wrong because I'm not sure of the dewhitening filter gain but the filter is shaped with 2 poles at 1 hz and 2 zeros at 100hz. Also the blue traces where taken around DC readout, while the red are from the IFO in NLN, so the blue is missing some high frequency cut-offs making the >10hz noise worse on the blue trace.
I'm attaching some plots comparing two NLN SRCL asds, first plot is the SRCL ERR, blue is with the new blends, red is the old blends. The new blends are about a factor of 2 better rms between 1 and 10 hz for the SRCL error. I would like to try adding some notches to the blend to see if we can do any better, but this seems like a solid improvement.
The second plot is SRCL suspoint, which uses projections of the ISI GS13s to each relevant suspensions top mass to estimate the SRCL cavity length. This channel combines the suspoint motions of the BSCs and both HAMs, so is not purely deteremined by the improved motion on HAMs 4&5. Again, red is from a lock with the old blends, blue is with the new blends. This also shows a good improvement over the 1-20hz band, with some worse performance below 1hz, but not too bad.
Attached is a comparison of the RX HAM4 blend changes at LHO during May vs the nominal L1 blend filter. The current filter for LHO is the purple one.
We see that LHO has now more isolation than LLO between 1.5-8Hz, but less roll-off above 8Hz. It would be interesting to understand how this difference in the filtering affects the suspoint motion at both sites, as a follow up from 46407.