Alignment and use of the optical test for the 8.4 m off-axis primary mirrors of the Giant Magellan Telescope

S. C. West, J. H. Burge, B. Cuerden, W. Davison, J. Hagen, H. M. Martin, M. T. Tuell, C. Zhao, T. Zobrist

Research output: Chapter in Book/Report/Conference proceedingConference contribution

17 Scopus citations

Abstract

The Giant Magellan Telescope has a 25 meter f/0.7 near-parabolic primary mirror constructed from seven 8.4 meter diameter segments. Several aspects of the interferometric optical test used to guide polishing of the six off-axis segments go beyond the demonstrated state of the art in optical testing. The null corrector is created from two obliquelyilluminated spherical mirrors combined with a computer-generated hologram (the measurement hologram). The larger mirror is 3.75 m in diameter and is supported at the top of a test tower, 23.5 m above the GMT segment. Its size rules out a direct validation of the wavefront produced by the null corrector. We can, however, use a reference hologram placed at an intermediate focus between the two spherical mirrors to measure the wavefront produced by the measurement hologram and the first mirror. This reference hologram is aligned to match the wavefront and thereby becomes the alignment reference for the rest of the system. The position and orientation of the reference hologram, the 3.75 m mirror and the GMT segment are measured with a dedicated laser tracker, leading to an alignment accuracy of about 100 microns over the 24 m dimensions of the test. In addition to the interferometer that measures the GMT segment, a separate interferometer at the center of curvature of the 3.75 m sphere monitors its figure simultaneously with the GMT measurement, allowing active correction and compensation for residual errors. We describe the details of the design, alignment, and use of this unique off-axis optical test.

Original languageEnglish (US)
Title of host publicationModern Technologies in Space- and Ground-Based Telescopes and Instrumentation
DOIs
StatePublished - Sep 23 2010
EventModern Technologies in Space- and Ground-Based Telescopes and Instrumentation - San Diego, CA, United States
Duration: Jun 27 2010Jul 2 2010

Publication series

NameProceedings of SPIE - The International Society for Optical Engineering
Volume7739
ISSN (Print)0277-786X

Other

OtherModern Technologies in Space- and Ground-Based Telescopes and Instrumentation
CountryUnited States
CitySan Diego, CA
Period6/27/107/2/10

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Keywords

  • Aspheres
  • Interferometry
  • Null lens
  • Optical alignment
  • Optical testing
  • Telescopes

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics
  • Computer Science Applications
  • Applied Mathematics
  • Electrical and Electronic Engineering

Cite this

West, S. C., Burge, J. H., Cuerden, B., Davison, W., Hagen, J., Martin, H. M., Tuell, M. T., Zhao, C., & Zobrist, T. (2010). Alignment and use of the optical test for the 8.4 m off-axis primary mirrors of the Giant Magellan Telescope. In Modern Technologies in Space- and Ground-Based Telescopes and Instrumentation [77390N] (Proceedings of SPIE - The International Society for Optical Engineering; Vol. 7739). https://doi.org/10.1117/12.857251