In Zemax, an OAP is really a full parabola with a decentered aperture placed over it.  That means Zemax thinks the center of the OAP is at the center of the parabola.  This is a very different location from that of the mechanical center of the OAP component in the real world.

Looking at the Lens Data Editor, it might seem that the total distance from the legacy output plane (Surface 9) to the OAP (Surface 13) is exactly 1435.000mm. 

However, for the reason given above, this is not the same as the distance between the actual surfaces on the optical table.

We can use the Merit Function Editor to calculate the center-to-center distance between these objects very accurately.

What we’ll do is use the RAGX, RAGY, and RAGZ operands to measure the Global X, Y, and Z coordinates, respectively, of the gut ray (H=0,0 and P=0,0) at the legacy output (Surface 9) and then measure the Global X, Y, and Z coordinates of the gut ray at the OAP (Surface 13). 

Using the Difference (DIFF) operand, we’ll calculate the difference between the two X coordinates, the difference between the two Y coordinates, and the difference between the two Z coordinates.

And then, using the Quadratic Sum (QSUM) operand, we’ll take the square root of the sum of the squares of these differences.  The following merit function operands perform these exact functions:

With this calculation, we see that the actual distance between these two optical elements is 1449.288mm.  Recall that the distance we measured from the Lens Data Editor was 1435.000mm to the center of the parent parabola.  This seemingly small difference would actually result in about 15 waves RMS defocus being introduced into the system. 

When working with an OAP, it’s always a good idea to determine its location using the gut ray location at the parabola surface, and NOT the global coordinates of the parabola’s center.