Optical Testing of Mirrors for Giant Telescopes

We have developed a metrology system that is capable of measuring rough ground and polished surfaces alike, has limited sensitivity to the nominal surface shape, and can accommodate surfaces up to 8.4 m in diameter. The system couples a commercial laser tracker with an advanced calibration technique and a system of stability references to mitigate numerous error sources. This system was built to guide loose abrasive grinding and initial polishing of the off-axis primary mirror segments for the Giant Magellan Telescope (GMT), and is also being used to guide the fabrication of the Large Synoptic Survey Telescope primary and tertiary mirrors. In addition to guiding fabrication, the system also works as a verification test for the GMT principal optical interferometric test of the polished mirror segment to corroborate the measurement in several low-order aberrations. A quantitative assessment of the system accuracy is presented, along with measurement results for GMT, including a comparison to the optical interferometric test of the polished surface.8

Sensitivity analysis of test methods for aspheric off-axis mirrors

The measurement of aspheres and freeforms poses several challenges to interferometric and other optical testing methods which are well established for spherical surfaces. Accuracy, measurement time and flexibility are requirements encountered in a production environment. The approach of tilted-wave-interferometry is to illuminate the specimen with a whole ensemble of wavefronts. Each of these wavefronts has a different amount of tilt with respect to the optical axis. The surface under test is completely covered by interferogram patches where rays from one source hit the surface such that resolvable fringe densities result. Yet, one still deviates from the null setup which requires that the interferometer is calibrated precisely. This is obtained by recording interferograms of a known reference object placed in the test space. The aberrations of the interferometer are described within a black box model. A sophisticated set of algorithms is used to reconstruct these coefficients with high accuracy. The non-null configuration prohibits a direct evaluation of the measurement from the interferograms. Instead, the surface is reconstructed by the solution of an inverse problem. A second step gives access to the remaining high-frequency errors. The key ideas and implementations in the process of measurement and calibration are explained and the differences to other common concepts in optical testing are elaborated. As an example, results of a freeform measurement, including both form and high-frequent deviations from design, are discussed.

The Richard F. Caris Mirror Lab at the University of Arizona continues production of 8.4 m lightweight honeycomb segments for the primary mirror of the Giant Magellan Telescope. GMT’s 25 m primary mirror consists of a center segment surrounded by six off-axis segments, with an additional off-axis segment to allow continuous operation as segments are removed for coating. We cast the sixth segment (5 off-axis segments + center segment) in March 2021. As of June 2022 we have tentatively completed polishing of the third off-axis segment, and we are in the process of grinding the optical surface of off-axis Segment 5. For Segment 3, we improved the measurement accuracy for small-scale structure near the edge of the mirror, which has been a challenge for the off-axis segments. In addition to full-aperture interferometry and deflectometry, we used a 20 cm test plate to obtain high-resolution interferometric measurements of the edge.

A Cassegrain telescope with a 450 mm clear aperture was developed for use in a spaceborne optical remote-sensing instrument. Self-weight deformation and thermal distortion were considered: to this end, Zerodur was used to manufacture the primary mirror. The lightweight scheme adopted a hexagonal cell structure yielding a lightweight ratio of 50%. In general, optical testing on a lightweight mirror is a critical technique during both the manufacturing and assembly processes. To prevent unexpected measurement errors that cause erroneous judgment, this paper proposes a novel and reliable analytical method for optical testing, called the bench test. The proposed algorithm was used to distinguish the manufacturing form error from surface deformation caused by the mounting, supporter and gravity effects for the optical testing. The performance of the proposed bench test was compared with a conventional vertical setup for optical testing during the manufacturing process of the lightweight mirror.

This chapter contains sections titled: Introduction Principles of Optical Nondestructive Testing Techniques (ONDTs) Optical Methods for NDT Conclusions and Perspectives Acknowledgments References

Accurate cavity volume testing based on micropressure differential gas inflation and deflation

Promising methods for quality control test of large convex aspherical mirrors

Temporal and spatial deep learning network for infrared thermal defect detection

Five interferometric tests were conducted at cryogenic temperatures on a lightweight, 50 cm diameter, hot isostatic pressed (HIP) beryllium mirror in the Ames Research Center (ARC) Cryogenic Optics Test Facility. The purpose of the tests was to determine the stability of the mirror's figure when cooled to cryogenic temperatures. Test temperatures ranged from room ambient to 8 K. One cycle to 8 K and five cycles to 80 K were performed. Optical and thermal test methods are described. Data is presented to show the amount of cryogenic distortion and hysteresis present in the mirror when measured with an earlier, Shack interferometer, and with a newly-acquired, phase-measuring interferometer.

In this work we report a method for testing a parabolic trough solar collector (PTSC) based on the null screen principles. For surfaces with symmetry of revolution a cylindrical null screen is used, now, for testing the PTSC we use a flat null screen. The design of the null screen with ellipsoidal spots is described; its image, which is formed by reflection on the test surface, becomes an exact square arra y of circular spots if the surface is perfect. Any departure from this geometry is indicative of defects on the surface. The flat null screen design and the surface evaluation algorithm are presented. Here the surface is tested in sections and the evaluation of the shape of the surface is performed with stitching method. Results of the evaluation for a square PTSC with 1000 mm by side (F/0.49) are shown. Keywords: Null screen, Parabolic Trough Solar Collector, Optical Testing 1. INTRODUCTION The optical performance of solar concentrating collectors is very sensitive to inaccuracies of components and assembly. Because of a finite size of the sun and present im precisions of the collector sy stem (e.g., tracking, receiver alignment, mirror alignment, mirror shape, and mirror secularity) the interception of light at the focal receiver is reduced. Among the principal methods for testing optical surfaces, the Ronchi and Hartmann tests have been popular for many years for testing slow (F/# >> 1) spherical and aspherical surfaces

Corundum-based abrasives are commonly used for cutting extremely hard materials like e.g. ceramics by means of abrasive waterjets. Due to a reduced lifetime of the used focusing tubes, this type of abrasive is only applied under special consideration of economic circumstances. The cutting centres which use this technology retain only a small amount of used abrasives because of the limited application of this grain type. Nevertheless, observing the disintegration of particles which have interacted with mostly advanced materials is of scientific interest. Garnet-based abrasives are commonly classified in different grain size classes after sieving to evaluate their recycling potential. Based on a feasibility study, this paper will present some results on achieved cutting data and also shows the geometrical change of the used focusing tubes which was realized by non-destructive examination. The mass loss factor of the used focusing tubes was calculated for different corundum materials and compared with the commonly used in AWJ machining abrasive, garnet. Additionally, cutting and recycling properties of the tested corundum-based abrasive was tested. The disintegration properties of the corundum-based abrasive were monitored by sieving and optical test methods. Particle identification after cutting process (grain or chip) was realized by energy-dispersive X-ray microanalysis (EDX). Finally, the results obtained were used to make a rough calculation for lifetime estimations of the focusing tubes related to the observed wear process.

Electric and optical fibre cables � Test methods for non-metallic materials

This test aimed to determine the effect of loaded and unloaded coconut shell charcoal briquettes with tapioca and starch adhesives on the AC electric motor RPM during the briquette-making process, and to obtain optimal hardness values for both coconut shell charcoal briquettes with tapioca and starch adhesives. The hardness testing method for coconut shell charcoal briquettes used the Brinell hardness test, using samples of coconut shell charcoal briquettes with tapioca and starch adhesives. The measurement used was an optical one, measuring the size of the indentation left by the indenter. Standard optical hardness testing methods include the Brinell hardness test (ISO 6506, ASTM E10). Based on the results of the briquette machine test for 7 minutes with a load of 15 kg, the coconut shell charcoal with starch adhesive produced an AC electric motor RPM of 702.9, while the coconut shell charcoal with tapioca adhesive produced an AC electric motor RPM of 884.7. The average hardness test results of coconut shell charcoal briquettes with tapioca adhesive have a value of 17.33 HRB, while coconut charcoal briquettes with starch adhesive have a value of 21.36 HRB. The hardness value of coconut shell charcoal briquettes with starch adhesive is harder or more optimal than coconut shell charcoal briquettes with tapioca adhesive.

In this study, the shift-rotation absolute testing method is extended to weakly aspheric surfaces, providing a direct, null-compensator-free solution for subnanometer figure testing. A key limitation of existing shift-rotation methods, namely the tradeoff between high-resolution and low-noise propagation, is addressed. Accordingly, a pixel-level-resolution absolute testing method based on multiple rotations and two orthogonal lateral shifts is proposed. Rotationally asymmetric errors are calibrated via rotation averaging, while rotationally symmetric errors are reconstructed by dividing shift-induced differential wavefronts into subapertures for least-squares fitting and stitching. These components are then fused to achieve pixel-level absolute surface figure reconstruction. The proposed method preserves high-frequency surface information while maintaining a low noise-propagation ratio, thereby achieving subnanometer accuracy. Numerical simulations and absolute testing experiments on a weakly aspheric surface validate the effectiveness and reliability of the proposed method for high-precision optical surface testing.