Page 318 - Modern Optical Engineering The Design of Optical Systems

Optical System Layout 297

Figure 13.5 The vignetting action of the eyelens determines the
field of view in an astronomical telescope.

apparent that the “effective” position of the exit pupil shifts inward as
the amount of vignetting increases.
Let us now determine the minimum power for a field lens which will
completely eliminate the vignetting at a field angle of 0.0625 rad.
From Fig. 13.6, it can be seen that the field lens must bend the rays
from the objective so that ray B strikes no higher than the upper rim
of the eyelens. The slope of ray B is equal to 1 in (the difference in the
heights at which it strikes the objective and the field lens) divided by
8 in (the distance from field lens to objective), or 0.125. After passing
through the field lens, we desire the slope to be zero (in this case)
as indicated by the dashed ray B′. Using Eq. 4.1, we can solve for the
power of the field lens as follows:

u′ u y
f
0.0 0.125 (0.5)
f
0.25
f
1
f 4 in
f

Figure 13.6 Ray diagram used to determine field lens
power in Example 13.1.