  Q1: Why deploy N wavefront sensors on a three mirror anastigmat (TMA) and not
N + 1?
  Q2: Why measure M Zernike coefficients and not M + 1?
  Q3: Why control L rigid body degrees of freedom (total) on the secondary and
tertiary and not L + 1?
  The usual answer: "We did a lot of ray tracing and N,M, and L seemed OK." We
show how straightforward results from aberration theory may be used to address
these questions. We consider, in particular, the case of a three mirror
anastigmat.

Instrumentation and Methods for Astrophysics