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

doi:10.1117/12.857251

19 July 2010 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

Author Affiliations +

Proceedings Volume 7739, Modern Technologies in Space- and Ground-based Telescopes and Instrumentation; 77390N (2010) https://doi.org/10.1117/12.857251
Event: SPIE Astronomical Telescopes + Instrumentation, 2010, San Diego, California, United States

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.

Citation Download Citation

S. C. West, J. H. Burge, B. Cuerden, W. Davison, J. Hagen, H. M. Martin, M. T. Tuell, C. Zhao, and T. Zobrist "Alignment and use of the optical test for the 8.4-m off-axis primary mirrors of the Giant Magellan Telescope", Proc. SPIE 7739, Modern Technologies in Space- and Ground-based Telescopes and Instrumentation, 77390N (19 July 2010); https://doi.org/10.1117/12.857251

ACCESS THE FULL ARTICLE

INSTITUTIONAL
Select your institution to access the SPIE Digital Library.

SELECT YOUR INSTITUTION

PERSONAL
Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.

PERSONAL SIGN IN

No SPIE Account? Create one

PURCHASE THIS CONTENT

SUBSCRIBE TO DIGITAL LIBRARY

50 downloads per 1-year subscription

Members: $195

Non-members: $335 ADD TO CART

25 downloads per 1 - year subscription

Members: $145

Non-members: $250 ADD TO CART

PURCHASE SINGLE ARTICLE

Includes PDF, HTML & Video, when available