[Jenne, Robert]

As a result of Keita's alog 28196 regarding the beam position on the BS, we wanted to move the beam splitter around in relation to the beamline, to see if that would change any clipping that we may have on the baffles.  Short answer: nope.

First, we moved ST1 by putting offsets in the isolation loops.  JeffK tells us that these are calibrated in nm, so our 5,000 count offsets correspond to about 0.5mm of motion.  We moved ST1 up and down, as well as laterally along the plane of the beam splitter (+x+y and -x-y).  No effect seen in the power recycling gain. 

Next, we moved HEPI in a similar fashion.  The thought here is that the ITM elliptical baffles are suspended from this ST0, so we weren't moving them earlier.  (By moving both ST1 and ST0 we had hoped to differentiate which set of baffles was causing us trouble.)  We moved up and down, as well as in RZ, rotation about the z-axis.  RZ is calibrated into nrad, and the baffles are order 1m away from the center of the ISI, so they were each moved on the order of 0.5mm also.  Again no effect seen in power recycling gain.

Attached is a snapshot of our striptool, with the first offsets starting at about 0:06:00 UTC, and the last ones ending around 1:00:00 UTC.  Teal is the power recycling gain.  The POP18 seems to be still relaxing from the power up to 40W for the first few minutes of our tests, but doesn't seem to be correlated with our movements.  Red trace is the vertical CPS measure of BS ST1 ISI position, and orange is superimposed with brick red measuring our lateral motion.  Light purple is vertical HEPI motion and light green is RZ HEPI motion.

We felt that if we were really dominated by clipping losses around the beam splitter, moving by 0.5mm in some direction should show us some change in recycling gain.  Since it doesn't, we conclude that the power loss must be somewhere else.