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6 July 2018 Interferometric characterization of Keck segment edge errors
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Abstract
The Keck telescope segments were manufactured by stressed mirror polishing of large circular pieces of Zerodur that were then cut into hexagons and finished by Ion Beam Figuring (IBF). It has long been believed that this process results in segments with little or no edge effects. As a result, this same general approach is planned for segment manufacturing for the Thirty Meter Telescope (TMT) and the European Extremely Large Telescope (E-ELT). However, recent measurements at the Keck telescope suggest that at least some of the Keck segments have significant aberrations within 60 mm of the edge. These aberrations impact the telescope phasing and the overall telescope image quality. We present interferometric measurements of multiple Keck segments, characterizing the surface errors near the edges over spatial periods from ~5 cm down to ~1 mm. We show that the largest phasing and image quality effects are due to plateaus of unremoved material, left behind after IBF as a result of obscuration by the IBF supports. Apart from these plateaus, the edge quality is relatively good, though not as good as in the segment interiors. Some residual phasing and image quality effects remain, and these are not currently understood.
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© (2018) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Mitchell Troy, Gary Chanan, Mark Colavita, and Stephen J. Martinek "Interferometric characterization of Keck segment edge errors ", Proc. SPIE 10700, Ground-based and Airborne Telescopes VII, 107000M (6 July 2018); https://doi.org/10.1117/12.2314568
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