[D. Kapasi, S. Dwyer]

We can calculate the SR3 hot radius of curvature (sr3_roc_hot) from the HWS measurements [1] and readback heater power for SR3 [2].

• The average wavefront change, delta_W = (5.9 µD/W + 5.4µD/W)/2 * 1.9W. This is also called as the spherical power (delta_S since it is a differential measurement). delta_W = delta_S = 1.0735e-05 D.
• The spherical power is the inverse of ROC. Therefore, delta_S = 1/delta_ROC where delta_ROC = sr3_roc_hot - sr3_roc_cold (this is a positive quantity since we know that heating an optic (assuming uniformity) will increase the ROC).
• In diopter units, for a curved mirror we can write : D = 2/ROC. Therefore, delta_D = D_cold - D_hot (since ROC_hot > ROC_cold).
• Hence, 2/delta_ROC = 2/sr3_roc_cold - 2/sr3_roc_hot -> 2(delta_S) = 2/sr3_roc_cold - 2/sr3_roc_hot. Solving this gives sr3_roc_hot. 
• The factor of 2 highlighted above is independent of the factor of 2 used for double pass on SR3.

In our case - 
sr3_roc_cold = 36.013; % radius of curvature in m [3].
Therefore, 2*(1.0735e-05) = 2/36.013 - 2/sr3_roc_hot -> sr3_roc_hot = 36.027 m.

Sources
[1] alog 88413 - this give HWS values in µD/W.
[2] alog 88155 - gives the reported readback power and requested power for SR3 heaters.
[3] git issue #33.