In some case, our modeling requirements may not be met with normal circular or rectangular apertures, we can use User Defined Aperture instead. For more information, you can navigate to "The Setup Tab > Editors Group (Setup Tab) > Lens Data Editor > Surface Properties > Aperture (surface properties) > User Defined Apertures and Obscurations" section of Help Manual.
How about if we want to create a aperture shaped by two linked circle for example?
If we created above aperture by using two overlapped circle by using CIR commands in UDA file, the overlapped region will became obstruction rather than aperture:
Instead, we have to use ARC or LIN commend to create every single curve separately in UDA file.
