Hi Saif,

There are two way that can be used to make Off axis mirrors!

1. You can define the aperture (rectangular, circular etc.) and off axis distance from surface setting tab under aperture section. Off axis distance will decide the offset from center of parent mirror and defined aperture will decide the size (geometry) of OAP.
2. If you have complex geometry (Hexagon etc.), you can define a "User defined aperture" based on your coordinate points and apply the defined aperture to parent mirror.

Hope this helps!

Thanks.