Approximate formulas are derived for the axial coma resulting from til
t and decenter of a surface and for the spherical aberration resulting
from a change in its axial position. These expressions include terms
that represent aberrations induced by the subsystem preceding the surf
ace in addition to other terms that are intrinsic contributions from t
he misaligned surface itself. This separation of the terms gives a sim
ple method of designing a system that is insensitive to a misalignment
at a given surface. The method is illustrated by applying it to a two
-mirror astronomical telescope with corrector. Two examples are given-
one for tilt and the other for despace. In both examples an appreciabl
e reduction in the sensitivity is obtained. The limitations of these s
olutions and the problem of simultaneous correction for two types of m
isalignment are examined.