A simple, dual knife-edge test for phasing segmented aperture space telescopes

Over the past twenty-five years, ground based telescopes have made the leap from single, continuous primary mirrors to segmented apertures. This transition was motivated by the cost and complexity of large, monolithic optical surfaces. Space telescopes, driven by the same practicalities, are beginning to make this same transition. The challenge is to accurately phase these apertures such that they behave as a single monolithic mirror. Here we introduce a new method for this co-alignment based upon a classic knife-edge technique. The advantage of this method is: 1) it has a very large dynamic range, 2) it works with broadband light and is therefore very photon efficient, 3) it requires only a single mechanism and single sensor, 4) it works equally well with sparsely filled apertures as well as filled apertures, 5) it phases all the segments simultaneously, 6) requires no additional segment motion diversity for sensing. In this paper, we review the knife-edge method, provide context for the segment phasing problem, perform numerical simulations of the sensor, and provide preliminary laboratory confirmation of this method with a demonstration with a segmented deformable mirror.