Abstract:

I confirmed LSC_LOCK guardin can reach OBSERVATION state if the alignment is good.
The stability is not good due to the oscillation of roll after reaching observation state.
At this moment, the rason of the oscillation is not clear but tuning of notch filters in LSC and ASC loop might solve the problem.

Detail:

Since IFO could not be locked, I performed initial alignment of X arm, Y arm, and PRMI.
Ten, IFO could be locked and LSC_LOCK guaridna could reach OBSERVATION state.

However, soon after reaching the observation state, OMC transmission starts ringing up as shown in fig1, which finnaly causes saturation of OMC PDS and lockloss.
According to the spectrum measurement during the oscillation (fig2), oscillation frequency is 53.78Hz, which is close to the ETMX roll resonances measured before.
So, I implemented 53.78Hz notch filter at FM2 of K1:VIS-ETMX_IM_LOCK_L (fig3) and FM8 of K1:VIS-ETMX_MN_LOCK_L (fig4).

After notch filter implementation, oscillation seems to stop, so I keep this configuration for a while to check the stability.