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Hi Petra,
I'm no expert, but for both our sakes I will now go out on a limb and parade my ignorance, so that someone more knowledgable can leap in with corrections.
Refracting telescopes work by bending (i.e. refracting) light so that all light rays come to a focal point. Unfortunately, as you see when light is refracted through a prism, light of different wave lengths (colours) bend different amounts through any given medium (e.g. glass). Hence the rainbow. In telescopes this is called Chromatic Aberration (CA).
ACHROMATIC (Latin: Without colour) lenses are designed to correct for this to a limited extent, and by semi-agreed definition they ought to bring two separate wavelengths (colours) to the same focal point. Most cheap refractors are achromats. CA isn't totally eliminated, and is most obvious on bright objects like the moon (where most typically you can see a rim of blue on one side and yellow on the other).
APOCHROMATIC lenses (Latin: really and truly without colour, and this time we mean it) are semi-defined to mean bringing three separate wavelengths of light to the same focal point. According to some, they are also meant to correct for another sort of aberration called spherochromaticism (I'm going down for the third time now. Please, someone save me...)
True apochromatic lenses, so I believe, require at least three lenses in the light path, and sometimes more (e.g. Petzval design). But, if you use glasses of EXTRA-LOW DISPERSION, you can come close with just two lenses. (And you have the advantage of not losing contrast with all those air-glass interfaces.) Fluorite, which is actually a crystal that can be grown in a laboratory, is one of the best ED materials around, and the new fluorite doublet lens that we are referring to uses it to give apochromatic views (for real world situations, anyway). Or at least, that's what we're all hoping.
Of course, you can avoid all this by using a mirror! (It's right about now that the apochromatic aficionados will leap in and start yet another refractor vs reflector battleground.)
Is that what you were after?
Cheers,
Brian.
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