Varun, Stefan

We measured the (one-way) SRC Gouy phase to be 28.97deg +-0.11deg (statistical) +-0.43deg (systematic).
We used a method pioneered in KAGRA (klog 9241 and 9246). A similar method was used for the PRC Gouy phase measurement(alog 52504).

Plots:
 - Plot 1: SRC scan across 6 fringes. Blue: AS_C_SUM, Red: AS_C_SEG2 (both derived from IOP channels). Magenta: round trip Gouy phase fit from the two results.
 - Plot 2: Raw data strech of 6 best fringes.

Method:
- Align BS and SRM (in SRY) carefully (especially BS)
- Misalign SRM by 70 urad (we did pos pitch)
- Let SRMI freely swing (i.e. SRM and BS aligned, PRC misaligned)
- SRCL1 and MICH1 gains 0 (we don't need the control loops)
- Put an offset in SRCL2_OFFSET (we used -1e6cts)
- Turn off all whitening on ASC-AS_C
- We want the LSC trigger to fire and stay latched (and apply the offset) when ASC-AS_C reaches the darkest possible spot.
  This corresponds to the BS in the correct position, increasing the finesse of the SRC to its maximum.
- We achieved that with a negative trigger matrix element, a slightly positive enable threshold (since AS_C swing slightly negative), and a very negative disable threshold
  We used: H1:LSC-TRIG_MTRX_4_14 =-100, H1:LSC-SRCL_TRIG_THRESH_ON = 0.015, H1:LSC-SRCL_TRIG_THRESH_OFF = -100
- To read the data we need the highest possible data rate. So in addition to H1:ASC-AS_C_NSUM_OUT we recorded the IOP channels
  H1:IOP-ASC0_MADC6_TP_CH9, H1:IOP-ASC0_MADC6_TP_CH10, H1:IOP-ASC0_MADC6_TP_CH11, H1:IOP-ASC0_MADC6_TP_CH12
- Those IOP channels are before the fixed anti-whitening filter. So we did the anti-whitening filter in diaggui ('zpk([40],[0.4],1,"n")').
- The 4 quadrants of ASC-AS_C saw different combinations of the higher order modes.
- We used SEG4 (CH9) for finding the 00 modes.
- We used SEG2 (CH11) and the sum of all 4 segments for 10 modes (red and blue traces respecively).
- We chose a bank of 6 fringes with very low AS_DC values - those have the highest finesse.
- A spline fit was used to interpolate the time to degrees in-between 00 peaks.
- We calculated the mean from the red and blue traces separately. Their mean is reported as final value.
- We calculated the standard deviation of the mean from the red and blue traces separately. The maximum of the two errors is reported as statistical  error.
- The difference between the red and blue result is reported as systematic error.

The code and raw data is found in /ligo/home/controls/sballmer/20191022.