Sagittal Ray Confusion.

ThereIam

According to Wikipedia:

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?