Stefan and Varun measured the SRC gouy phase at LHO to be higher than design.
LHO single pass gouy phase measurement is 25.3 +- 1.8 degrees.
LLO single pass gouy phase measurement is 21.3 +- 0.3 degrees.

I modeled the nominal SRC cavity, as has been done many times before, and got 23.5 degrees of single pass gouy phase.
optics_move_vs_single_pass_SRC_gouy_phase.pdf shows that moving either SR2 and SR3 back by 8 mm could explain the discrepancy between measurement and nominal model
optics_change_RoC_vs_single_pass_SRC_gouy_phase.pdf shows Aiden's calibration of the SR3 disk heater of -3 mm/W to be good.  Also, -16 mm on the RoC of SR3 or -22 mm on the RoC of SR2 could explain the discrepancy.

From the Nominal_SRC_gouy_phase_beamtrace.pdf, we see from the beam trace most of the gouy phase accumulates between SR2 and SRM.


Numbers I used for the nominal model were taken from a mixture of Paul Fulda's previous SRC work in DCC T1500230 and galaxy, reproduced below:

nn = 1.45 # index of refraction of silica

# Radii of Curvature
RoC_SRM  = -5.6780 # m, taken from galaxy
RoC_SR2  = -6.4240 # m 
RoC_SR3  = 36.0130 # m 
RoC_ITMY = -1939.3900 # m 

# ITM lens
f_ITMY = 34500.0000 # m 
f_ITMX = 34500.0000 # m 

# Common lengths
ll_SRM_SR2   = 15.7400 + 5e-3 # m, plus 5 mm from SR2 move
ll_SR2_SR3   = 15.4601 + 5e-3 # m 
ll_SR3_BSAR  = 19.3661 # m
ll_BSsub     = 68.5e-3 # m

# SRC Y only
ll_BSHR_CPY   = 4.847  # m
ll_CPYsub     = 100e-3 # m
ll_CPY_ITMYAR = 20e-3  # m
ll_ITMYsub    = 200e-3 # m 

# Useful lengths
Length SRC common path = 50.666 m
Length SRC Y = 55.968 m
Length SRC X = 56.049 m
Schnupp Asymmetry = 8.132 cm
Average SRC length = 56.008 m