Apochromatic Refractor Telescopes

Apochromatic refractors have a significantly better correction of chromatic aberration than achromatic refractors due to an extra-low dispersion glass element, traditionally composed of calcium fluoride, known as “fluorite”. The substantial color correction is done by reducing the difference in refractive index of blue wavelength spectrum related to red and green wavelength spectrums focusing on the same focal plane. Blue or purple halos on bright objects such as the moon or planets are not noticeable even in telescopes with medium or fast focal ratio (f/5 to f/7). They can have 2 elements (doublet) or 3 elements (triplet). The best apochromatic telescopes typically have three elements with one extra-low dispersion glass element sandwiched between the other two glass elements. Apochromatic refractors deliver the best quality views with the highest contrast than any other telescope design.