[Anchal, Paco]
I redid the measurement of Schnupp asymmetry today and found it to be 3.8 cm  0.9 cm.
Method
- One of the arms is misalgined both at ITM and ETM.
- The other arm is locked and aligned using ASS.
- The SRCL oscillator's output is changed to the ETM of the chosen arm.
- The AS55_Q channel in demodulation of SRCL oscillator is configured (phase corrected) so that all signal comes in C1:CAL-SENSMAT_SRCL_AS55_Q_DEMOD_I_OUT.
- The rotation angle of AS55 RFPD is scanned and the C1:CAL-SENSMAT_SRCL_AS55_Q_DEMOD_I_OUT is averaged over 10s after waiting for 5s to let the transients pass.
- This data is used to find the zero crossing of AS55_Q signal when light is coming from one particular arm only.
- The same is repeated for the other arm.
- The difference in the zero crossing phase angles is twice the phase accumulated by a 55 MHz signal in travelling the length difference between the arm cavities i.e. the Schnupp Asymmetry.
I measured a phase difference of 5 1 degrees between the two paths.
The uncertainty in this measurement is much more than gautam's 15956 measurement. I'm not sure yet why, but would look into it.
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I used the Valera technique to measure the Schnupp asymmetry to be  , see Attachment #1. The data points are points, and the zero crossing is estimated using a linear fit. I repeated the measurement 3 times for each arm to see if I get consistent results - seems like I do. Subtle effects like possible differential detuning of each arm cavity (since the measurement is done one arm at a time) are not included in the error analysis, but I think it's not controversial to say that our Schnupp asymmetry has not changed by a huge amount from past measurements. Jamie set a pretty high bar with his plot which I've tried to live up to.
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