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 (TMT) and other segmented mirror telescopes.
[1]
C. Ohara,et al.
Phasing the mirror segments of the Keck telescopes II: the narrow-band phasing algorithm.
,
2000,
Applied optics.
[2]
T. Mast,et al.
Phasing the mirror segments of the Keck telescopes: the broadband phasing algorithm.
,
1998,
Applied optics.
[3]
Gary Chanan,et al.
W.M. Keck Telescope phasing camera system
,
1994,
Astronomical Telescopes and Instrumentation.
[4]
Gary Chanan,et al.
Phasing the segments of the Keck and Thirty Meter Telescopes via the narrowband phasing algorithm: chromatic effects
,
2016,
Astronomical Telescopes + Instrumentation.
[5]
Timothy S. Lewis,et al.
Surface error correction of a Keck 10-m telescope primary mirror segment by ion figuring
,
1992,
Optics & Photonics.
[6]
Lynn N. Allen.
Progress in ion figuring large optics
,
1995,
Laser Damage.