The fact that the primary mirror image is to be placed on a single chip array (SCA), which is one quadrant of the FPA and contains 2k x 2k pixels, implies a magnification of 0.0041. The fourth design parameter was the lens material. Fused silica was chosen for the PIL optics because the system operates over a narrow waveband (Filter F187N) which implies that no chromatic correction is required. Some additional design parameters were that the field of view of the PIL optics is the primary mirror, and that the aperture stop for pupil imagery is the PIL lens group.
Engineering the PIL optics solution
In order to design an optical system that meets the list of requirements, the first choice was a single spherical lens design (Figure 4). The single spherical lens was located between the last two fold mirrors before the FPA in the short wavelength beam.
NIRCam optical section ─ pupil wheel to FPA
Pupil imaging lens (PIL) simplest implementation single spherical lens
PIL spherical lens in correct magnification location
NIRCam pupil wheel
NIRCam Mono OTE 03/02/04 Figure 4: NIRCam optics section with single spherical PIL lens.
A detailed analysis of using a single spherical lens as the PIL optics showed completely unacceptable performance. The transverse ray aberrations of the single spherical lens design had gross astigmatism as shown in Figure 5.