[Jenne, JeffK, Sheila, MatthewB, Adrian]

We're having a look at what the effect is of moving the beam spot position on the optic that we use for Pcal (ETMY).  This is similar to the first attempt we made at the end of May in alog 49541, although there we were moving the spot on ETMX (our DARM actuator).  As we move the spot, we are taking the standard calibration sensing function measurements to see how the spot position change affects the measured sensing function.

• Nominal ETMY spot: P2L=+4.3, which means -15.7mm on the optic (low of center).
  • Have sensing function measurement (plot attached, ends in _spots_m15p7PIT_p13p2YAW.pdf)
• First set of exploratory measurements: P2L=+2.2, which means -6.1mm on the optic.
  • Have sensing function measurement (plot attached, ends in _spots_m6p1PIT_p13p2YAW.pdf)
• Tried to go closer to center of optic, got to P2L of 1.2, but had ~0.5Hz instability ringing up pretty severely.  Moved spot back to P2L=2.2 and the oscillation died out.
• Moved yaw closer to center.  Nominal ETMY is Y2L=3.6, which means 13.2mm from center.  New Yaw position is at the center of the optic, with pitch still at the halfway to center position. 
  • Have sensing function measurement (plot attached, ends in _spots_m6p1PIT_p0p0YAW.pdf)
  • Have PUM actuation measurement (data to be processed later)
  • Have ESD actuation measurement (data to be processed later)
• Moved pit and yaw spots back to nominal.
  • ESD and PUM actuator measurements are currently underway, then we'll go back to observing.  SDFs are already clear and ready.

Our latest thought with these data is that in fact the sensing function is changing as a function of spot position.  The 1/1+G parts of the measurement seem to change at low frequency, because they include C the sensing function.  The C/(1+G) part of the measurement doesn't seem to change at low frequency with different spot positions, but this is because with large G the Cs cancel on top and bottom of the fraction, leaving the measurement looking like 1/(A*D).  So, we're taking PUM and ESD measurements at the extrememely non-nominal spots, as well as the nominial spots, to confirm that those aren't changing.  The attached plots below show that the sensing function is changing at low frequency as we change the spot positions via the A2L gains.