Edward Gill
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Those are beautiful, Alex. What is it about the mirror design that makes it technically APO?
Brad
Mirror lenses, as long as they have no lenses elements - just mirrors, do not refract light and therefore do not creat chromatic error (different colors of light focused at different points). APO lenses are corrected for three wavelengths (colors) of the light spectrum (generally red, green, and blue). Meaning that these three colors will be brought to focus at the same point. Lens designers use glass with differing refractive indicies to bend the light (remember the sciince experiments in school using a prism to split white light into the different components of color?). This is where all those terms like UD, LD, AD, ED come from describing, the types of glass used in the optical design. Wikapedia has a pretty do explanation here: http://en.wikipedia.org/wiki/Apochromat
Back to your question: As long as a mirror lens uses no lenses they do not refract the light and introduce color error (chromatic abberation). Actually most photography mirror optics do include refractive lenses elements to correct other optical problems in the overall design and can actually have chromatic aberations. Its just easier for the optical designers to keep the chromatic errors small and unnoticable because the lenses in mirror optics do not need to do a lot of light bending and can also use expensive glass since they are small.
Sorry for the long winded answer, but optical terms are often poorly understood and erroniously used by folks that should know better like on popular lens testing site that equates chromatic aberations to anti-reflective lens coatings rather than the true source which is the designers choice of glass and the optical formulation to correct aberations.
Ed
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