Combining beams from separated telescopes.

To use separated telescopes to achieve diffraction-limited performance over a finite field of view, the following rule must be obeyed: as viewed from a point in the focal plane, beams from separated telescopes must be recombined so that they appear to be coming directly from a single large telescope which has been masked so as to reproduce exactly the ensemble of collecting telescopes. This condition leads to constraints and tolerances which are of central interest in multiple-beam telescope systems.