We show in Chapter 10 that this image, the one midway between paraxial and
marginal focus, is the one with the minimum root-mean-square (rms) wavefront
error.
From our discussion we see that the diameter of the circle of least confusion is
\r'/AR^\
when the object is at infinity. At the mirror this blur subtends an angle a
where
a = ?/2R^ =
1/128F^
(4.2.9)
As the focal ratio F increases, the subtended angle a decreases. A point is
reached, however, where the image diameter no longer decreases but reaches the
limit set by diffraction according to Eq. (3.7.1). The smallest F for which a
spherica ...
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