Hi @Nanday, 

Looking at your file, I see the rays are striking your optic at the center of the optic, directly upon the local axis of the object:

Consider the sag profile of the Standard Lens type, as shown below:

If we plot this equation for k = 0 and k = -1, then we will see that for your aperture, the sag profiles are essentially the same:

It is only at large values of “r” that we start to see some deviation, as shown below here:

It is for this reason that an off-axis mirror is typically used at some point offset from the local axis origin.

In Sequential Mode, we can achieve this through the use of an aperture decenter and/or Coordinate Break surface, as is shown in the sample file C:\...\Zemax\Samples\Short course\Advanced Optical System Design Using OpticStudio\OAP_OSDUZ.zmx.

In Non-Sequential Mode, I would suggest switching to the Off-axis Mirror object type. This object type lets us define the surface sag, as well as the surface offset as discussed in the following forum post:

Try out that alternative object type and see if you can get the performance you’re after. Let us know if you have any other questions!