Abstract:

I measured OMC geophone, OMC error, and DARM spctra and compared them with those before OMC work.

Summary:

OMC geophone spectrum (fig1):

Some peaks in OMC geophone spectrum was shifted acording to the shift of resonant frequency of OMC stack.
Also, high frequency vibration increased due to the reduction of number of stacks.

OMC error signal spectrum (fig2):

In the OMC error signals, 92 Hz peak seemed reduced thanks to the OMC stack connection but 126 Hz peak seems to be appeared instead.
Since the peak height of 126 Hz is smaller than that of 92 Hz peak in the spectrum before OMC stack connection, it should not be problematic.
Somehow, 82 Hz peak increased while geophone signals becomes smaller than before at 82 .

DARM spectrum (fig3):

Several peaks at low frequency seems increased, which might be due to cooling of the mirrors.
Noise floor around 25-35 Hz seems limited by MICH noise, so adjustment of FF seems necessary.
Small peak around 100Hz seems disappeared while the noise floor from 100-250 got slightly worse.
Several structure between 300-400 Hz seems disappeared.

Discussion:

At this moment, I could not say the worse noise floor is due to OMC stack work because ARM ASC is oscillating (fig4).
So, it might be due to bad alignment of PRFPMI, so ASC optimization is the highest priority now.
We started to optimize ASC (hopefully reported later) but still it seems unstable at OBSERVATION state, so further tuning is necessary.