After this analysis, the PIL optical design team felt the dual wedged spherical lens system ( Figure 9) was very close to meeting all of the required performance parameters.
Figure 9: PIL dual wedged spherical lens design.
Unfortunately, the team found there was yet another performance aspect that needed to be addressed. Analysis of the PIL optics showed that while there was only 1% distortion of the image, there was approximately 10% anamorphism (i.e. differential magnification in perpendicular directions). The design team believed this to be acceptable since we expected such anamorphism could easily be removed in software image processing. However, after reviewing the distortion requirements with the JWST science team, it was agreed that the PIL optics distortion of the image should be less than 2%, and this should include all distortion types. Figure 10 shows the anamorphic distortion of the circumference of the primary mirror from the dual wedged spherical lens design.
Figure 10: Anamorphic image produced by dual wedged spherical lens design.
Modification of the dual wedged spherical lens system failed to meet this requirement. Addition of a third element was the only choice. It is with the addition of the third lens, with one planar and one cylindrical surface, that the PIL optics achieved the required performance.