5. A 3 mm thick plate of glass with a refractive index of 1.5 is placed in front of the image of an ffz diffraction-limited lens. To third order, what is the blur diameter at the paraxial focus of the spherical aberration introduced? Answer: 6. A 5 inch focal length lens with 12 micron detectors is tested for ensquared energy using a 60 inch focal length collimator. What might be a reasonable size for the collimator pinhole