Introduction

Telescopes are conventionally designed for viewing distant objects, but are often used for viewing closer objects. Equation (17.6a) shows that for telescopes, as an object comes closer, the image also comes closer, thus requiring the user to accommodate but to a greater level than the vergence of the object. In practice, this accommodation demand can be eased, because most telescopes can be refocussed by adjusting their length. These aspects have been discussed in Chapter 17. The closest working distance will then depend upon the length of adjustment and the maximum possible accommodation that the user can exert.

On the other hand, some specialized “telescopes” are designed for working distances that are relatively short, for example 20 cm to several metres. These have sometimes been called “near point telescopes”, “intermediate distance telescopes” or “tele-microscopes”. Perhaps a better term would be “macroscope,” and this term would cover what are basically two lens magnifiers with working distances intermediate between microscopes and telescopes. However instruments of this type are often not readily available and when working distances are short, it is often more convenient to adapt a conventional (afocal) telescope (see Section 18.5).

The use of terms containing the words “telescope” and “microscope” to describe these instruments leads to confusion in the interpretation of magnification. Telescopes and microscopes use different definitions of magnifications and these in turn will be different from that defined for macroscopes. These points are discussed in greater detail in Section 18.3 and a numerical example will be given showing how the differences in definition of magnification can lead to very different outcomes.