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Phasing the mirror segments of the Keck telescopes: the broadband phasing algorithm
1Department of Physics and Astronomy, University of California, Irvine, California 92697, USA.
Applied Optics
|February 13, 2008
Summary
Keck telescope primary mirrors require precise phasing (<100 nm piston errors) for infrared diffraction limits. A novel Shack-Hartmann test variation effectively corrects large segment errors, achieving 30 nm precision.
Area of Science:
- Astronomy
- Optical Engineering
Background:
- The Keck telescopes' primary mirrors are segmented, requiring precise alignment for optimal infrared performance.
- Piston errors between mirror segments must be below 100 nm to reach the diffraction limit.
Purpose of the Study:
- To develop and validate a method for accurately phasing the Keck telescope's primary mirror segments.
- To reduce residual piston errors to below 100 nm.
Main Methods:
- A wave optics variation of the Shack-Hartmann test was employed.
- The degree of coherence of subimages, rather than centroid, was used as the signal.
- Filters with varying coherence lengths were utilized to manage initial piston errors up to +/-30 micrometers.
Main Results:
- The method successfully reduced initial piston errors as large as +/-30 micrometers to 30 nm.
- This represents a dynamic range of three orders of magnitude in error correction.
- Residual errors of approximately 75 nm were observed, largely due to segment aberrations.
Conclusions:
- The developed wave optics Shack-Hartmann test is effective for precise phasing of large segmented telescope mirrors.
- Achieving the full diffraction limit in the infrared requires addressing both piston errors and segment aberrations.
