Apochromatic refractor

An apochromatic refractor, often shortened to APO, is a refracting telescope built with special low-dispersion glass elements, sometimes including fluorite, arranged so that three or more wavelengths of light are brought into focus at nearly the same point. Ordinary achromatic refractors only correct two wavelengths well, which leaves a visible purple or blue fringe around bright objects that an apochromatic design is specifically engineered to minimize.

The payoff is a noticeably sharper, higher-contrast view with color fringing reduced to a level most observers cannot detect, particularly valuable for detailed planetary observation and for astrophotography, where uncorrected color fringing shows up clearly in photos even when it is barely noticeable to the eye. The special glass and tighter manufacturing tolerances needed to achieve this make apochromatic refractors considerably more expensive per inch of aperture than achromatic models.

Because of the cost curve, apochromatic refractors are usually found in smaller apertures than reflectors or Dobsonians at a similar price point, trading raw light-gathering power for optical precision and portability. When buying, check whether a scope is marketed as a true apochromat or as an improved achromat, since the terms are sometimes used loosely and the actual level of color correction can vary significantly between models described with similar language.