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Phasing the Primary Mirror Segments of the Keck Telescopes:
A Comparison of Different Techniques

Gary Chanan^a , Mitchell Troy^b , and Catherine Ohara^a

^a University of California, Irvine
Department of Physics and Astronomy
Irvine, CA 92697

^b Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91109

Abstract:

We have developed and tested extensively three different methods for phasing the primary mirror segments of the Keck telescopes. Two of these, referred to as the broadband and narrowband algorithms respectively, are physical optics generalizations of the Shack-Hartmann technique. The third, Phase Discontinuity Sensing (PDS), is a physical optics generalization of curvature sensing. We evaluate and compare experimental results with these techniques with regard to capture range (as large as 30 microns), run-to-run variation (as small as 6 nm), execution time (as short as twenty minutes), systematic errors, ease of implementation, and other factors, in the context of the Keck telescopes and also of future very large ground-based telescopes.

Keywords: phasing, segmented mirrors, telescopes