Scanning pentaprism measurements of off-axis aspherics II

Su, Peng; Burge, James H.; Cuerden, Brian; Allen, Richard G.; Martin, Hubert M.

doi:10.1117/12.828497

Cited by 7 publications

(8 citation statements)

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“…[10], [11], [12] It uses the natural telescope geometry, illuminating the segment with a narrow collimated beam parallel to the telescope's optical axis, which the segment focuses on a detector at the telescope's prime focus. The collimated beam is scanned across the segment by moving a pentaprism along a rail perpendicular to the optical axis.…”

Section: Measuring the Center Segmentmentioning

confidence: 99%

Status of mirror segment production for the Giant Magellan Telescope

et al. 2016

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The Richard F. Caris Mirror Lab at the University of Arizona is responsible for production of the eight 8.4 m segments for the primary mirror of the Giant Magellan Telescope, including one spare off-axis segment. We report on the successful casting of Segment 4, the center segment. Prior to generating the optical surface of Segment 2, we carried out a major upgrade of our 8.4 m Large Optical Generator. The upgrade includes new hardware and software to improve accuracy, safety, reliability and ease of use. We are currently carrying out an upgrade of our 8.4 m polishing machine that includes improved orbital polishing capabilities. We added and modified several components of the optical tests during the manufacture of Segment 1, and we have continued to improve the systems in preparation for Segments 2-8. We completed two projects that were prior commitments before GMT Segment 2: casting and polishing the combined primary and tertiary mirrors for the LSST, and casting and generating a 6.5 m mirror for the Tokyo Atacama Observatory.

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Section: Measuring the Center Segmentmentioning

confidence: 99%

Status of mirror segment production for the Giant Magellan Telescope

et al. 2016

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“…In addition, there are field aberrations consisting of astigmatism, coma and trefoil in fixed ratios. [7] The field aberrations represent slope errors that vary with pupil position and therefore show up in the result of the pentaprism measurement. Any change in field angle among the different scans causes variable field aberrations that cannot easily be disentangled from surface errors.…”

Section: Effects Specific To Off-axis Mirrorsmentioning

confidence: 99%

“…These are the insensitivity of in-scan spot displacements to orientation of the pentaprisms, and the use of the reference prism to remove system misalignments including the tilt of the beam projector. Both effects are quantified in Reference [7]. In order to make use of the first feature-insensitivity to prism orientation-we must determine the in-scan direction in the focal plane to an accuracy of 0.5 mrad.…”

Section: Effects Specific To Off-axis Mirrorsmentioning

confidence: 99%

“…In NST testing this ratio varied from 1 to 0.91 in the worst case (scan 1 in Figure 1). [7] In practice, in-scan displacements of the reference spot are less than 50 μrad. To reach a measurement accuracy of 0.5 μrad in wavefront slope, the ratio of displacements is calculated at least to an accuracy of 0.01, and the reference spot's displacement is scaled by this ratio before subtracting it from the scanning spot's displacement.…”

Section: Effects Specific To Off-axis Mirrorsmentioning

confidence: 99%

“…To obtain this accuracy with an off-axis mirror, it is necessary to carefully account for field aberrations that result when the collimated beam is not exactly parallel to the optical axis. These effects have been analyzed in previous papers [6], [7] , and the results have been used in testing the 1.7 m off-axis primary mirror of the New Solar Telescope (NST). In this paper, we summarize the principles of the pentaprism test, including the field effects that are important for off-axis mirrors.…”

Section: Introductionmentioning

confidence: 99%

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Scanning pentaprism test for the GMT 8.4-m off-axis segments

Allen

Burge

et al. 2010

SPIE Proceedings

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The scanning pentaprism system for testing the 8.4 m off-axis segments for the Giant Magellan Telescope has recently been completed. The system uses a fiber source and a carriage mounted pentaprism to scan a 40 mm collimated beam across the surface of the segment under test. Since the scanning beam is parallel to the optical axis of the parent mirror, it comes to focus on a detector at the telescope's prime focus, where displacement of the spot is proportional to the slope error. A second collimated beam from a stationary reference pentaprism is used to compensate for any changes in the relative positions of the optical components during testing. The optical components are suspended over the mirror on a rail system that can be rotated so that scans can be made across any diameter of the segment. The test is capable of measuring wavefront slope errors to 1 μrad rms, adequate to verify that power, astigmatism, coma, and other low-order aberrations are small enough to be corrected easily at the telescope with the segment's active support system.

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