After using alamode to calculate the roundtrip mode of the beam at the Faraday exit after retroreflection form the PRM, I'm not able to blame the MMT and TT curvature for the beam ellipticity.
I assume an input waist at the mode cleaner of [0.00159, 0.00151] (in [T, S]). Propagating this through the MMT to PRM, then retroreflecting back with flat TTs I get
w_t/w_s = 0.9955, e = 0.0045
If I give the TTs a 600 m curvature, I get:
w_t/w_s = 1.0419, e = 0.0402
That's just a 4% ellipticity, which is certainly less than we see. I would have to crank up the TT curvature to 100m or so to see an ellipticity of 20%. We're seeing something that looks bigger than 50% to me.
Below are beam size through MMT + PRM retroreflection, TT RoC = 600m:
