Generic Misalignment Aberration Patterns and the Subspace of Benign   Misalignment
  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. 
Paul L. Schechter and Rebecca Sobel Levinson
astro-ph.IM