| ||When light is focussed by passing it through a lens made from ordinary glass, such as crown glass, each wavelength of light bends a different amount. This is the reason, we are able to see light separated into its spectrum when it passes through a glass prism. This different bending leads to a problem, because each wavelength focusses at a different point. The result is a focal zone rather than a focal point. When a bright object is viewed through such a lens, it is blurry and has a fringe of false colour. Technically, this is referred to as chromatic aberration. Reflectors don't suffer from this effect because their light rays don't pass through any glass. A second problem, called spherical aberration, occurs when optical surfaces of lenses or mirrors are not properly figured or shaped. As with chromatic aberration, the focal point becomes a focal zone. |
The first way telescope makers tried to correct these problems was to make telescopes longer. This results in a higher focal ratio and the aberrations become less pronounced. The focal ratio is the focal length, (the distance from the primary lens or mirror to the focal point), divided by the aperture (the diameter of the primary). Small focal ratio telescopes, often referred to as fast telescopes, are more subject to chromatic aberration. Making telescopes longer is fine for small apertures, but with large apertures they quickly become unwieldy.
A second approach is to add another matching lens of a glass having a different refractive index. For example, when positive, low-index, BK7 crown glass is matched with negative, high-index, F2 flint glass, the light rays are bent again so that all wavelengths focus near the same point. The result is called an achromatic refractor and the matched lenses may either be cemented together, or air-spaced by mounting them in a cell which holds them in their correct positions. The two-element lenses used in today's achromats greatly reduce the chromatic aberration. For example, it has been brought to low levels in Sky-Watcher 1201EQ5 and 15012EQ6.
In the ongoing search for the perfect telescope, lens makers produced other lens element combinations and special types of glass, in order to remove all of the false colour. These developments have resulted in semi-apochromatic (almost without colour) and apochromatic (corrected in three colours) refractors but these are very expensive compared to achromatic refractors.