The new H1 ITMs ROC (ITM03 and ITM11) are similar to the ones in L1, but they are swapped (the wavefront error is larger from X than from Y). Based on T1300954 (table 3) and Hiro's wisdom, the effective ROCs of the H1 optics, as measured in reflection, going through the bulk, are: R_ITMX (ITM03) = 1939.3 + (-10.92*2*1.457); R_ITMY (ITM11)= 1939.2 + (1.56*2*1.457); By looking at the L1 data in single bounce without TCS (below), one should expect something like ~20% mode mismatch for X and something somehow better for Y. L1 Mode mis-match: NO TCS: ITMX 14.5% ITMY 22% Even with an input beam perfectly matched to the PRM, I would expect something like: modematching asX with OMC = 0.8408 modematching asY with OMC = 0.91229