Marc and Yuhang

Recently, we found a new spot of filter cavity elog2573, which makes the IR locking accuracy much better (the spectrum below~3Hz reduce by up to a factor of 10). At the beginning, we thought we found a more stable optical axis. However, we did a test of GR length correction signal when using old and new spot, which shows pretty similar spectrum at frequency region below ~3Hz (attached figure 1). Since the GR length correction signal below ~3Hz tells us mirror motion information, this means the mirror motion is similar for the old and new spot.

Meanwhile, the correction measured in this time is different from elog2312. Especially, it seems more high frequency signal is sent to end mirror.