LLO and LHO use somewhat different RZ blends on St1 of our BSCs. Because the T240s have a RZ coupling to Z drives, we use a high CPS blend to avoid injecting this coupling into the control loops. The blends are shown in my first plot. The low pass CPS (red) is common to both, but LLO uses a CPS/L4C blend ( their L4C high pass is dashed green), while LHO is using a blend that has both the T240 (light blue) and L4C (brown). RichMs seismic log 647 shows that the CPS/L4C ends up injecting a bunch of L4C noise  at low frequency. Arnaud and Rich have worked on testing a CPS/L4C blend that rolls the L4C off more at low frequency, see LLO alog 21941. The blend that LHO uses rolls off the L4C more, but replaces that signal with possibly spurious T240 signal. I tested that this morning and it looks like this is not a problem for LHO. Second plot is the CPS and oplev for ITMX, third plot is L4C and T240, references are with the L4C/CPS RZ blend, the live traces are with the T240/L4C/CPS blend. The CPS shows that the L4C only blend does indeed move a lot more at low frequency, though the oplev doesn't see any difference. The T240 and L4C don't really see a difference, but the T240 is suspect and the L4C is a poor low frequency sensor.

There is one reason why LHO maybe okay with T240s in our RZ blend while LLO may not, we are using HEPI sensor correction to offload low frequency Z isolation ( and thus, drive) to HEPI. Arnaud has also tried lower blends on St1 with L4C/CPS RZ blends succesfully, something I want to test, which may not work as well at LHO because we are using T240s in RZ. I would also like to try Rich's new 750mhz blend, with and without T240s, but all of that will probably wait until after 01.