An additional ghost is caused by an internal reflection inside the MgF2 field flattener lens immediately in front of each CCD (Figure 5.11). The field flattener ghost is doughnut shaped (image of OTA pupil) in the WFC, but is smaller and more disk-like on the PC. This ghost contains ~0.15% of the total energy of the star. It is positioned on a line through the CCD center and the bright star; the distance from the ghost to the CCD center is 1.25 to 1.4 times the distance from the bright star to the CCD center. This geometry results from curvature of the field flattener lens.
The large ghost image expected to be caused by reflection off the CCD back to the filter and then back to the CCD is not seen. It was deliberately eliminated in the PC by tilting the CCD slightly.