AO4ELT5 Proceedings

Keck Segment Surface Artifacts: Impacts on Segment Phasing and Image Quality

Troy, Mitchell (Jet Propulsion Laboratory), Chanan, Gary (University of California Irvine), Brock, Neal (4D Technology)

We show that the segments of the Keck telescopes suffer from small but significant surface artifacts that limit segment phasing accuracy. Both the segment surface artifacts themselves and the limits they place on segment phasing have important implications for adaptive optics, high contrast imaging, and seeing limited observations. These surface artifacts are likely by-products of the segment polishing, most likely the Ion Beam Figuring (IBF) process that constitutes the final step of the segment surface figuring. The surface artifacts diffract a significant amount of light out past +/-3.5 arcseconds and generate chromatic effects that limit segment phasing accuracy. We present on-sky measurements of the diffracted intensity. Segment surface roughness measurements are also presented, which show that surface roughness is not the source of the problem. These results have direct relevance to segment manufacturing for the Thirty Meter Telescope and other segmented mirror telescopes.

DOI: 10.26698/AO4ELT5.0012- Proceeding PDF

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