The first telescope will give you a good feel for lens aberrations (fig. 3). In this simple instrument, chromatic aberration is the most conspicuous. The aberrations can be greatly reduced by means of careful lens design. As it is not possible to limit all kinds of aberrations using only a single lens, objectives and eyepieces are created using multiple lenses. By selecting different types of glass for the various lenses and using appropriate surface curvatures and distances between lenses, it is possible to control in a satisfactory manner the aberration of the system. In general, the success of an objective or an eyepiece in correcting aberrations depends on the number of lenses used to make it. For the second telescope, shown in figure 10, we use an achromatic objective, made up of two lenses of different shapes, one converging and the other diverging. Sometimes they are glued together by means of Canada Balsam or a synthetic resin (cemented doublet), other times they are kept separated (air-spaced doublet). These two lenses have different indices of refraction, one high (Flint glass), and the other low (Crown glass). Hence, the chromatic aberrations of the two lenses act in opposite senses, and tend to cancel each other out, thus producing a much more distinct image than a single lens could achieve.
Usually, these objectives are constructed to reduce other types of aberration as well. Obviously, achromatic objectives vary in quality. In some of them, it is still possible to perceive a residual chromatic aberration, or the images they produce are well focused in the center only, or they produce a pincushion or barrel distortion. Figure 3 describes the main optical aberrations.
