Efficient method for the reduction of large piston errors in segmented-mirror telescopes

Abstract

ABSTRACT Phase Discontinuity Sensing (PDS) is one of two successful approaches to segment phasing currently in use atthe Keck telescopes, but it has only very limited capture range. In this work, we describe and present numericalsimulations of a broadband version of the current (narrowband) PDS algorithm which can extend the capture range from 0.4 m to 40 m. Like the original algorithm, the new broadband PDS algorithm (BPDS) does not require any special-purpose hardware, only a high-resolution area detector operating in the 2 - 3 m range. The potential application of this algorithm to Extremely Large Telescopes is also discussed.Keywords: phasing, segmented mirrors, telescopes, wavefront sensing 1. INTRODUCTION In a typical generic process in which a physical parameter is adjusted or tuned, the coarse adjustment tends tobe relatively easy, with the difficulty increasing as one proceeds to finer and finer adjustments. However, in thecase of phasing mirror segments in a segmented-mirror telescope, the situation is reversed. Because of aliasing,it is the coarse adjustments (i.e., those corresponding to phase errors of many wavelengths) that are the mostdifficult to make. Once the phase errors are reduced below a half-wave (a quarter-wave of surface error), theproblem is enormously simplified.At the Keck Observatory, the coarse phasing problem has been solved by means of the so-called broadbandPCS (for Phasing Camera System) algorithm,' which has a capture range of