Sheila, Jenne, Hang, Rob, Lisa, Matt, Stefan

Comparing the AS port WFS DC centering vs RF90 centering we came across the following observation, that in retrospect makes perfect sense, put certainly was initially surprising to me:

Observation:

We are in DRMI-locked, no arms, i.e. the light at the AS port is completely dominated by the 45MHz fields. Moving the OMs in pitch (e.g. up) we see both DC- and RF90-centering signals move up. But moving the BS in pitch sends the two centering signals in opposite direction.

Why?

Since we only have the two 45MHz fields contributing, the only way to get the DC- and RF90-centering signals to disagree is to have BOTH a spatial mismatch AND amplitude difference between the upper and lower sidebands. A BS misalignment will generate upper and lower sideband with opposite sign (because they are passed on opposite sides of the MICH fringe), thus taking care of the spatial mismatch. The SRC cavity passes the upper and lower  at a different location, thus creating the amplitude mismatch.

Below is a plot of the centering loops response to a BS move (at the -1 min mark) and a OM move (at the 0min mark).