We repeated the Gouy phase measurement from yesterday (alog 52639) with the SR3 ring heater set to 0W (off).

We measured the (one-way) SRC Gouy phase to be 25.25deg +-0.06deg (statistical) +-1.8deg (systematic).
This is almost 4deg less than yesterday in alog 52639, i.e. very roughly, the ring heater adds about 1deg/W of one-way Gouy phase.

Fitting details:
- We used the s3 quadrant for finding the 00 peaks. It has the least contamination from higher order modes.
- We used 3 data sets to fit the 10 peaks: SUM (blue), SEG2 (red) and SEG1 (black).
- We used all 13 fringes visible in the raw data set.
- Note that compared to yesterday's measurement we have more contamination from the 00 modes, leading to a systematic shift to lower Gouy phase if we use the SUM and SEG2 data set.
- We thus reported the result from the SEG1 data set (black) for the reported value of the Gouy phase.
- But we used the maximum difference between between the 3 data sets as reported systematic error: 1.8deg.
- The statistical error is comparatively negligible: 0.063deg.

Plots:
- Plot 1: SRC scan across 13 fringes. Blue: AS_C_SUM, Red: AS_C_SEG2, Black: AS_C_SEG1 (all derived from IOP channels).
  Magenta: round trip Gouy phase fit from the blue and black results. The black result is used for the reported value.
- Plot 2: Raw data strech of the 13 fringes.

The method is described here: alogs 52639 and 52504, klogs 9241 and 9246.
The code and raw data is found in  /ligo/home/controls/sballmer/20191023.