Sagittal and Tangential

The term "tangential" refers to data computed in the tangential plane, which is the plane defined by a line and one point: the line is the axis of symmetry, and the point is the field point in object space. The sagittal plane is the plane orthogonal to the tangential plane, which also intersects the axis of symmetry at the entrance pupil position.

For typical rotationally symmetric systems with field points lying along the Y axis, the tangential plane is the YZ plane and the sagittal plane is the plane orthogonal to the YZ plane which intersects the center of the entrance pupil.

The problem with this definition is that it is not readily extended to non-rotationally symmetric systems. For this reason, OpticStudio instead defines the tangential plane to be the YZ plane regardless of where the field point is, and tangential data is always computed along the local y axis in object space. The sagittal plane is the orthogonal to the YZ plane, and intersects the center of the entrance pupil in the usual way, and sagittal data is always computed along the x axis in object space.

The pupil coordinates are rotated with respect to the tangential and sagittal planes according to the Tangential Angle (TAN) parameter in the Fields section of the System Explorer about the z axis. See Vignetting Factors . However, this does not redefine the tangential/sagittal planes for the MTF calculation. This means that when calculating the MTF from PSF, the choice of cutoff frequency won't rotate with the setting of tangential angle. Therefore, using the TAN parameter does not provide the same effect on MTF as rotating the field point in the object space, not even in rotationally symmetric systems. There will always be some discrepancy in MTF value even if the tangential angle and field point are carefully selected.

The philosophy behind this convention is as follows. If the system is rotationally symmetric, then field points along the Y axis alone define the system imaging properties, and these points should be used. In this case, the two different definitions of the reference planes are redundant and identical. If the system is not rotationally symmetric, then there is no axis of symmetry, and the choice of reference plane is arbitrary.

One feature, the computation of Fast Semi-Diameters (see the Advanced Options (system explorer) of the System Explorer), does use the "true" tangential plane, which OpticStudio defines as the plane that contains the actual field point and the z axis in object space.