The narrowband segment phasing algorithm that was originally developed at Keck was replaced many years ago by a broadband algorithm that, although slower and less accurate than the former, has proved to be much more robust. A thorough investigation into the lack of robustness of the narrowband algorithm has now shown that this results from systematic errors (∼ 20 nm on average) that are wavelength-dependent. We show that the seemingly continuous distribution of these chromatic errors in fact results from (at least) two independent causes. The largest and most problematic eﬀects are due to “plateaus” of unremoved material that were covered by supports during the ion beam ﬁguring of three of the segments, but other smaller chromatic eﬀects are also shown to be present and these are not yet understood. If the purely chromatic eﬀects can be eliminated, we show that the intrinsic accuracy of the narrowband algorithm is about 6 nm (surface).
Gary Chanan and Mitchell Troy, "Chromatic effects in narrowband phasing of the Keck Telescope segments: theory and numerical simulations," Proc. SPIE 10700, Ground-based and Airborne Telescopes VII, 107001E (Presented at SPIE Astronomical Telescopes + Instrumentation: June 14, 2018; Published: 6 July 2018); https://doi.org/10.1117/12.2310157.
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