The original viewers used simple two-element plastic lenses, cheap to mold and good enough for a toy. Figuring out what to replace them with meant working through a series of different lens configurations, comparing sharpness, color fringing, and how each one actually felt to look through, not just how it measured on paper.
We landed on bonded achromatic doublet pairs: glass, coated, and built to correct the chromatic fringing the original plastic lenses couldn’t avoid. Getting there took a fair amount of trial and error, and there’s a lot more to say about what didn’t work before we settled on the final path.