At the request of Stefan, here are the expected Gouy phases accumulated between mirrors in the SRC. I added the results for PRC for good measure. As we expected, almost all the Gouy phase is accumulated between S/PRM and S/PR2. The model these values were calculated from included 34.5km "thermal" lenses in the ITMs, and no ITM substrate non-thermal lenses. The no ITM substrate lenses is probably close to reality, since the CO2 laser was used to match ITMX to ITMY. Probably the +34.5km thermal lens isn't very true right now though. In general if this lens is not present all Gouy phases get smaller, but the relatve fractions accumulated between optics remain pretty similar.