Why are all non-principal sagittal rays considered skew?

[ThereIam]

According to Wikipedia:

A sagittal ray or transverse ray from an off-axis object point is a ray that propagates in the plane that is perpendicular to the meridional plane and contains the principal ray.[3] Saggital rays intersect the pupil along a line that is perpendicular to the meridional plane for the ray's object point and passes through the optical axis. If the axis direction is defined to be the z axis, and the meridional plane is the y-z plane, saggital rays intersect the pupil at yp=0. The principal ray is both sagittal and meridional.[3] All other sagittal rays are skew rays.

A skew ray is a ray that does not propagate in a plane that contains both the object point and the optical axis. Such rays do not cross the optical axis anywhere, and are not parallel to it.[3]

The part I don't understand is why all rays that are sagittal rays that are not the chief/principal rays must be skew. Do they just mean all other rays originating from the same object point must be skew? Because that makes sense. Are these things always discussed with respect to a particular object point...? Now that I've written that, it seems obvious, but can someone please confirm?

 

[Andy Resnick]

I also struggle to remember all this correctly- Kingslake's book is essential for this information.

First, the meridional plane: this one is straightforward to define, and remains constant for a rotationally symmetric optical system: if the axis of symmetry is 'z', the y-z plane is the conventional choice. Meridional rays always lie within this plane. Skew rays leave the meridional plane and never intersect the 'z' axis. Most optical diagrams are of the meridional plane, with meridional rays.

Even symmetric optical systems have asymmetric aberrations- coma, for example. Understanding the origin of comatic/asymmetric aberrations require use of skew rays, and conventionally, the skew rays located in a plane perpendicular to the meridional plane that also contains the principal ray are referred to as 'sagittal rays'. The complication is that the sagittal plane is not a constant throughout the optical system, but changes its tilt at each optical surface.

To some degree, this is somewhat more clear when discussing astigmatism- meridional rays come to focus on a sagittal plane, while sagittal rays come to focus on a meridional plane.

Does this help?