As a result, the geometrical OPD of the output beam is not equal to zero as expected in according to the pure geometrical optics model (Fig. 5), although it appears to be equal to zero if we account for diffraction propagation. To take into account the above effect, an additional term $PHASE(x,y)$ for the M2 mirror shape should be introduced Display Formula
$M2(x,y)=OAP(x,y)+f2(r)+OAT2(x,y)+PHASE(x,y).$(19)
This term describes the OPD change caused by the diffraction propagation in the system. To calculate the M2 shape in this case, Eq. (16) should be modified in the following way: Display Formula$dM2,k(x2,y2)=\u2212OPLk(x2,y2)\u2212OPL(0,0)\u2212OPDd(x,y)2,$(20)
where $OPDd(x,y)$ is the geometrical OPD in the output beam for the baseline on-axis system. The $PHASE(x,y)$ term is equal to the difference between the M2 shape calculated in according with Eq. (20) and the M2 shape obtained under the assumption of the pure geometrical propagation [Eq. (16)].