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!

Thanks for the thoughtful reply.

As you said, I replaced it with an off-axis mirror.
Still could not overcome the problem of astigmatism.

Can you give me some hints to solve the astigmatism problem?