We saw with achromatic refractors how an optical designer can fold the focus positions of red and blue wavelengths so that they coincide, and in so doing, other colours focus positions move closer to each other. The residual chromatic aberration is termedsecondary spectrum. Violet wavelengths are the most common colours that still appear as an unfocused blur in the image. What this actually means is that the focused image of a star is the familiar sharp bright point, and the violet blur is the unfocused image of the star in violet. If we changed the focus position so that the violet focus is achieved, all the other colours are out of focus and we can't see the star as a point, but an expanded disc of light.
If we apochromatize an objective lens, we would like to achieve full colour correction so that all visible wavelengths coincide their focus positions, and we see a bright and sharp image with no chromatic aberration. What apochromatizing a lens actually does is to bring three colours to the same focus positions, and the remainder so close that secondary spectrum is not visible in the image, even at high magnification on a bright white star.
The resulting image is sharper and brighter, as we are no longer losing light to the focused image from unfocused wavelengths.
Apochromatic refractors or APOs as they have become known, can be designed using normal glass types or exotic glass types. Different glass types have different refractive indices. The refractive index of a glass is a quantitative ratio statement of the speed of light in the glass, compared to unity (1), which is the figure given to the speed of light in "normalised" air. It is the speed of light in a glass lens that determines the lens's refraction of light, and, as white light is polychromatic, it also therefore determines the refraction differences between the separate colours. The vast majority of true apochromats have triplet objective lenses, as three lens components are required to remove all visible secondary spectrum. Some telescope lens designers (often because of economics), prefer to use a simpler doublet, and include what has become known as an exotic glass as one of the elements. Glass types known as exotic in the context of astronomical refractors are termed ED (extra-low dispersion) glasses, or even Fluorite, a lens made from a crystal of Calcium Flouride. In most cases, even though secondary spectrum is reduced to a barely noticeable presence, doublet ED lenses are not true apochromats. Reduced secondary spectrum achromats is a more accurate description. Marketing being what it is, has led to many if not all of these doublet ED telescopes unfairly being labelled ED as a blanket term.
Some APO refractors use an ED glass as an element in a triplet group, and some have four or even five lens elements as part of the objective. The extra lens or group is there usually to remove one or more of the non-chromatic aberrations, so that these telescopes can be used successfully as telephoto lenses with large flat fields for deep-sky imaging.
A benefit to apochromatizing an objective lens is that APOs can be much shorter in focal length and focal ratio compared to traditional achromatic refractors, and are thus much more compact. As a commercial product APOs are normally offered as f/5 or above, as shorter focal ratio APOs have increasing problems with spherochromatism (variation of spherical aberration with wavelength).
APO refractors are the choice of astronomers that enjoy wide-field astrophotography, or who simply wish to use a small compact instrument that offers the best image quality for its aperture. The high street price of APOs is the highest for any given aperture. For this reason the vast majority of owners choose a 150mm or less aperture APO, and also because APOs larger than about 150mm in aperture are very heavy and demand a very large heavy duty mount.