I have noticed that ALS noise has been at 1KHz rms since LSC arm lock servos have been used to lock arms using ALS error signals. May be this has not been given much attention.

But looking more closely at the ALS noise (better dtt resolution for noise power spectrum) , there seems to be too much noise suppression at <1Hz and not much happening at around 10Hz.

Attachment 1 (data files at /users/manasa/data/140421/)

So I made a bunch of transfer function measurements for ALS and phase tracker servo. Koji will be using these and redesigning the servo filters so that we can get more suppression at 10Hz.

Other than this I also found that the Y arm showed more high frequency noise as compared to the X arm. (Edit by manasa: Thinking back now, this could be related to the onset of 60Hz noise at the Y end elog 9838. But still has to be looked at after fixing TRY)

Attachment 2

Tip: Once the arms are ALS locked, enabling the SLOW_SERVO helps hold the lock stably. 

P.S. I realigned the Y green to the arm and brought GTRY to 0.93

To do:

Find out what makes Y arm in-loop noise at high frequency higher than X arm.