EXPP, OPD, Ray Aiming and X-direction

Question

Hi All,

I have a question regarding EXPP. It seems from the comments here that Zemax calculates EXPP by tracing two parabasal rays +y and -y which are 1e-3 deviated from the on-axis ray (not sure if deviation is in normalized field coordinates, ray direction at stop, or something else) and then find their intersection. I have a few questions regarding that:

Is EXPP (which is just a Z-offset from image plane) the plane used to draw the reference sphere for OPD and PSF calculations?
Why do we not also trace +x and -x rays and how this affects the accuracy of the reference sphere?
That value of EXPP seems to differ if Ray aiming is OFF vs Real. How does the calculation differ in each case?
Since EXPP uses parabasal rays, it should be accurate for decentered and tilted systems?

Thanks a lot for your insight.

Best,

Kyrollos · Answer

I did some investigation and I can answer the third question. Zemax will aim +y and -y rays Hx=0, Hy=1e-3/-1e-3 and Px=0, Py=0. If ray aiming is off, Zemax will use paraxial entrance pupil to aim the rays (usually not accurate for decentered systems). When ray aiming is on, Zemax will change the ray direction to ensure that it hits the stop correctly which results in a different but more accurate answer.

If anyone has insights for the other questions, please let me know.

EXPP, OPD, Ray Aiming and X-direction

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