I scanned Y arm for 5FSR (below).
I could done this after I put a whitening filter.
Currently, whitening filter between the beatbox and AA filter is made of
Ponoma blue box(passive filter with zero at 1 Hz, pole at 10 Hz) + SR560(flat gain 100)
I couldn't do more than 5FSR because SR560 overloads. I checked it by staring at the indicator during the scan.
Reason why we kept loosing lock last night was the overload of SR560. Mystery solved!
Anyway, 5FSR is enough.
Our next step is to reduce residual arm length fluctuation.
Also, I increased the alingnment of IR. So, the higher order modes are less than the last scan.