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

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

doi:10.1117/12.857901

Cited by 13 publications

(5 citation statements)

References 5 publications

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“…At the present time the rail and all of the components for the pentaprism test have been mounted in the test tower, and measurements taken with the complete system should become available shortly. A detailed account of the pentaprism test is presented by Allen et al 6 Very recently a new test has been implemented which relies on a small video camera and LCD monitor placed near the combined focus of the primary mirror segment and 3.8 m fold sphere. The LCD monitor is illuminated with a series of lines in two orthogonal directions and the resulting images as viewed in the primary mirror segment are sensed by the video camera.…”

Section: Opticsmentioning

confidence: 99%

GMT overview

Shectman¹,

Johns²

2010

SPIE Proceedings

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The 25-meter Giant Magellan Telescope (GMT) is one of the next generation of extremely large ground-based optical/infrared telescopes. GMT is currently under development by a consortium of major US and international university and research institutions. The telescope will be located at the Las Campanas Observatory in Chile. The GMT Project is mid-way through its Design Development Phase. This paper summarizes the organizational structure and status of the GMT Project and recent progress in the technical development of the various GMT subsystems.

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Section: Opticsmentioning

confidence: 99%

GMT overview

Shectman¹,

Johns²

2010

SPIE Proceedings

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“…[11], [12] The slopes are integrated to give a full surface measurement with high accuracy for periods up to 2-3 m. • The scanning pentaprism test is the main independent verification of off-axis geometry and large-scale figure. [13]- [15] It synthesizes a star test by illuminating the segment with a narrow collimated beam parallel to the telescope's optical axis and scanning the beam across the segment in multiple cuts. The beam comes to a focus on a detector at the telescope's prime focus, where the spot motion is proportional to the slope error on the segment.…”

Section: Validatiomentioning

confidence: 99%

Manufacture of primary mirror segments for the Giant Magellan Telescope

Martin

Allen

Gasho

et al. 2018

Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation III

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The Richard F. Caris Mirror Lab at the University of Arizona continues the production of 8.4 m lightweight honeycomb segments for the primary mirror of the Giant Magellan Telescope. GMT will have a center segment surrounded by six identical off-axis segments, plus an additional off-axis segment to allow continuous operation as segments are removed for coating. Production highlights of the last two years include the spin-casting of Segment 5, preliminary polishing of Segment 2, and completion of the rear surface processing for Segments 3 and 4. We completed a preliminary design of the significant modifications of the test systems required for Segment 4, the center segment. We finished an upgrade of the 8.4 m polishing machine; both the upgrade and experience gained with Segment 1 have contributed to much faster polishing convergence for Segment 2. Prior to polishing Segment 2, we verified the stability and accuracy of the measurement systems by re-measuring Segment 1, achieving good agreement among multiple independent tests as well as good agreement with the original acceptance tests of Segment 1.

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“…The pentaprism test is a slope measurement and it measures the low-order aberrations up to the 4th order spherical aberration. It is a verification test for the low-order aberrations [6][7][8].…”

Section: Optical Setupmentioning

confidence: 99%

Spot centroid sensitivity to angle of intensity on image detector with lenslet array

Hui

Zhou

et al. 2015

Appl. Opt.

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Lenslet array was introduced to an image detector to compensate for low sensitivity. These lenses deviate the light from different incident angles and potentially introduce errors when subpixel accuracy is needed. We investigated the spot centroid position because the angle of incidence changes on a Kodak KAI-16000 image detector with lenslet array. In our experiment, we noticed that there is a cubic dependency on the incident angle. The experimental results show that dependence on the angle of incidence is related to the lenslet array in the Kodak detector used for the pentaprism test. This situation caused an error in spherical aberration on the test surface after integration. The magnitude of the cubic component at incident angle of 14° (equivalent to F/2) is 11.6 μm, which corresponds to a 48 nm rms spherical aberration for the test surface and brings the scanning pentaprism test closer to the principal test while there is a 56 nm rms discrepancy. The discrepancy in spherical aberration between the two tests reduced to 8 nm after this calibration. It also showed the contrast measurement results for the Kodak detector and PointGrey detector. We performed experiments with two different detectors to quantify this effect.

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

Cited by 13 publications

References 5 publications

GMT overview

GMT overview

Manufacture of primary mirror segments for the Giant Magellan Telescope

Spot centroid sensitivity to angle of intensity on image detector with lenslet array

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