Chapter 6Telescope Objectives
6.1 The Thin Airspaced Doublet
The telescope objective can be considered in many ways to be a classic example of a thin lens. A thin aplanatic (i.e., free of both coma and spherical aberration) lens cannot be corrected for astigmatism. A thin lens has astigmatism as given by Eq. (24.83), transverse astigmatism contribution (TAC) = h2′uk′/2. For an aplanat this astigmatism cannot be changed by moving the stop. The stop shift equation (Eq. (24.95)) indicates that, if spherical and coma are zero, the astigmatism remains the same regardless of the location of the stop.
Therefore, since the astigmatism must be large and ...