Last night I measured our RAM offsets and looked at how those affect the PRMI situation. It seems like the RAM is not creating significant offsets that we need to worry about.
Words here about data gathering, calibration and calculations.
Step 1: Lock PRMI on sideband, drive PRM at 675.13Hz with 100 counts (675Hz notches on in both MICH and PRCL). Find peak heights for I-phases in DTT to get calibration number.
Step 2: Same lock, drive ITMs differentially at 675.13Hz with 2,000 counts. find peak heights for Q-phases in DTT to get calibration number.
Step 3: Look up actuator calibrations. PRM = 19.6e-9/f^2 meters/count and ITMs = 4.68e-9/f^2 meters/count. So, I was driving PRM about 4pm, and the ITMs about 20pm.
Step 4: Unlock PRMI, allow flashes, collect time series data of REFL RF siganls.
Step 5: Significantly misalign ITMs, collect RAM offset time series data.
Step 6: Close PSL shutter, collect dark offset time series data.
Step 7: Apply calibration to each PD time series. For each I-phase of PDs, calibration is (PRM actuator / peak height from step 1). For each Q-phase of PDs, calibration is (ITM actuator / peak height from step 2).
Step 8: Look at DC difference between RAM offset and dark offset of each PD. This is the first 4 rows of data in the summary table below.
Step 9: Look at what peak-to-peak values of signals mean. For PRCL, I used the largest pk-pk values in the plots below. For MICH I used a calculation of what a half of a fringe is - bright to dark. (Whole fringe distance) = (lambda/2), so I estimate that a half fringe is (lambda/4), which is 266nm for IR. This is the next 4 rows of data in the table.
Step 10: Divide. This ratio (RAM offset / pk-pk value) is my estimate of how important the RAM offset is to each length degree of freedom.
Plots (Left side is several PRMI flashes, right side is a zoom to see the RAM offset more clearly):