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Focus errors from tracking sodium layer altitude variations with laser guide star adaptive optics for the Thirty Meter Telescope

Herriot, Glen and Hickson, Paul and Ellerbroek, Brent and Véran, Jean-Pierre and She, Chiao-Yao and Clare, Richard and Looze, Doug (2006) Focus errors from tracking sodium layer altitude variations with laser guide star adaptive optics for the Thirty Meter Telescope. In: Advances in Adaptive Optics II. Proceedings of SPIE. No.6272. Society of Photo-Optical Instrumentation Engineers (SPIE) , Bellingham, WA, Art. No. 62721I. ISBN 9780819463371. http://resolver.caltech.edu/CaltechAUTHORS:20190521-095812777

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Abstract

Laser guide star (LGS) adaptive optics systems for extremely large telescopes must handle an important effect that is negligible for current generation telescopes. Wavefront errors, due to improperly focusing laser wavefront sensors (WFS) on the mesospheric sodium layer, are proportional to the square of the telescope diameter. The sodium layer, whose mean altitude is approximately 90 km, can move vertically at rates of up to a few metres per second; a few seconds lag in refocusing can substantially degrade delivered image quality (15 m of defocus can cause 120 nm residual wavefront error on a 30-m telescope.) As well, the range of temporal frequencies of sodium altitude focus, overlaps the temporal frequencies of focus caused by atmospheric turbulence. Only natural star wavefront sensors can disentangle this degeneracy. However, applying corrections with representative focus mechanisms having modest control bandwidths causes appreciable tracking errors. In principle, electronic offsets measured by natural guide star detectors could be rapidly applied to laser WFS measurements, but to provide useable sky coverage, integrating sufficient photons causes an unavoidable time delay, again resulting in potentially serious focus tracking errors. However, our analysis depends on extrapolating to temporal frequencies greater than 1 Hz from power spectra of sodium profile time series taken at 1-2 minute intervals. In principle, with a pulsed laser, (e.g. 3-μs pulses) and dynamic refocusing on a polar-coordinate CCD, this focus tracking error may be eliminated. This result is an additional benefit of dynamic refocusing beyond the commonly recognized amelioration of LGS WFS spot elongation.


Item Type:Book Section
Related URLs:
URLURL TypeDescription
https://doi.org/10.1117/12.672081DOIArticle
Additional Information:© 2006 Society of Photo-Optical Instrumentation Engineers (SPIE). The authors gratefully acknowledge the support of the TMT partner institutions. They are the Association of Canadian Universities for Research in Astronomy (ACURA), the Association of Universities for Research in Astronomy (AURA), the California Institute of Technology and the University of California. This work was supported, as well, by the Canada Foundation for Innovation, the Gordon and Betty Moore Foundation, the National Optical Astronomy Observatory, which is operated by AURA under cooperative agreement with the National Science Foundation, the Ontario Ministry of Research and Innovation, and the National Research Council of Canada.
Group:Thirty Meter Telescope
Funders:
Funding AgencyGrant Number
Association of Canadian Universities for Research in Astronomy (ACURA)UNSPECIFIED
Association of Universities for Research in Astronomy (AURA)UNSPECIFIED
CaltechUNSPECIFIED
University of CaliforniaUNSPECIFIED
Canada Foundation for InnovationUNSPECIFIED
Gordon and Betty Moore FoundationUNSPECIFIED
National Optical Astronomy Observatory (NOAO)UNSPECIFIED
NSFUNSPECIFIED
Ontario Ministry of Research and InnovationUNSPECIFIED
National Research Council of CanadaUNSPECIFIED
Subject Keywords:Adaptive Optics, sodium layer, focus tracking, TMT, pulsed laser, dynamic refocusing
Record Number:CaltechAUTHORS:20190521-095812777
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20190521-095812777
Official Citation:Glen Herriot, Paul Hickson, Brent Ellerbroek, Jean-Pierre Véran, Chiao-Yao She, Richard Clare, and Doug Looze "Focus errors from tracking sodium layer altitude variations with laser guide star adaptive optics for the Thirty Meter Telescope", Proc. SPIE 6272, Advances in Adaptive Optics II, 62721I (28 June 2006); doi: 10.1117/12.672081; https://doi.org/10.1117/12.672081
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:95635
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:21 May 2019 17:44
Last Modified:21 May 2019 17:44

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