Deflectometry is a metrology method able to measure large surface slope ranges that can achieve surface reconstruction accuracy similar to interferometry, making it ideal for freeform metrology. While it is a non-null method, deflectometry previously required a precise model of the unit under test to accurately reconstruct the surface. However, there are times when no such model exists, such as during the grinding phase of an optic. We developed a model-free iterative data processing technique which provides improved deflectometry surface reconstruction of optics when the correct surface model is unknown. The new method iteratively reconstructs the optical surface, leading to a reduction in error in the final reconstructed surface. Software simulations measuring the theoretical performance limitations of the model-free processing technique as well as a real-world test characterizing actual performance were performed. The method was implemented in a deflectometry system and a highly freeform surface was measured and reconstructed using both the iterative technique and a traditional non-iterative technique. The results were compared to a commercial interferometric measurement of the optic. The reconstructed surface departure from interferometric results was reduced from 44.39 μm RMS with traditional non-iterative deflectometry down to 5.20 μm RMS with the model-free technique reported.
Daniel K. Inouye Solar Telescope (formerly known as Advanced Technology Solar Telescope) will be the largest optical solar telescope ever built to provide greatly improved image, spatial and spectral resolution and to collect sufficient light flux of Sun. To meet the requirements of the telescope the design adopted a 4m aperture off-axis parabolic primary mirror with challenging specifications of the surface quality including the surface figure, irregularity and BRDF. The mirror has been completed at the College of Optical Sciences in the University of Arizona and it meets every aspect of requirement with margin. In fact this mirror may be the smoothest large mirror ever made.
This paper presents the detail fabrication process and metrology applied to the mirror from the grinding to finish, that include extremely stable hydraulic support, IR and Visible deflectometry, Interferometry and Computer Controlled fabrication process developed at the University of Arizona.
Low order aberration was founded when focused Gaussian beam imaging at Kodak KAI -16000 image detector, which is integrated with lenslet array. Effect of focused Gaussian beam and numerical simulation calculation of the aberration were presented in this paper. First, we set up a model of optical imaging system based on previous experiment. Focused Gaussian beam passed through a pinhole and was received by Kodak KAI -16000 image detector whose microlenses of lenslet array were exactly focused on sensor surface. Then, we illustrated the characteristics of focused Gaussian beam and the effect of relative space position relations between waist of Gaussian beam and front spherical surface of microlenses to the aberration. Finally, we analyzed the main element of low order aberration and calculated the spherical aberration caused by lenslet array according to the results of above two steps. Our theoretical calculations shown that , the numerical simulation had a good agreement with the experimental result. Our research results proved that spherical aberration was the main element and made up about 93.44% of the 48 nm error, which was demonstrated in previous experiment. The spherical aberration is inversely proportional to the value of divergence distance between microlens and waist, and directly proportional to the value of the Gaussian beam waist radius.
Large telescope mirrors are typically measured using interferometry, which can achieve measurement accuracy of a few nanometers. However, applications of interferometry can be limited by small dynamic range, sensitivity to environment, and high cost. We have developed a range of surface measurement solutions using SCOTS, the Software Configurable Optical Test System, which illuminates the surface under test with light modulated from a digital display or moving source. The reflected light is captured and used to determine the surface slope which is integrated to provide the shape. A range of systems is presented that measures nearly all spatial scales and supports all phases of processing for large telescope mirrors.
ITF is usually over looked during the deflectometry measurements, especially when low spatial frequency errors are the main test focus. However, real data shows that the effect of ITF cannot be ignored to reach high accuracy measurements of high spatial frequency features. We illustrated with simulation that ITF of SCOTS is proportional to the camera imaging MTF. We then applied this result to the edge measurement data of a large mirror, where a better agreement is achieved between SCOTS test and a test-plate interferometric test after the compensation. Experimental verification of the ITF theory for deflectometry is preliminary performed. The results will be summarized in our following paper.
A software configurable optical test system (SCOTS) based on deflectometry was developed at the University of Arizona for rapidly, robustly, and accurately measuring precision aspheric and freeform surfaces. SCOTS uses a camera with an external stop to realize a Hartmann test in reverse. With the external camera stop as the reference, a coordinate measuring machine can be used to calibrate the SCOTS test geometry to a high accuracy. Systematic errors from the camera are carefully investigated and controlled. Camera pupil imaging aberration is removed with the external aperture stop. Imaging aberration and other inherent errors are suppressed with an N -rotation test. The performance of the SCOTS test is demonstrated with the measurement results from a 5-m-diameter Large Synoptic Survey Telescope tertiary mirror and an 8.4-m diameter Giant Magellan Telescope primary mirror. The results show that SCOTS can be used as a large-dynamic-range, high-precision, and non-null test method for precision aspheric and freeform surfaces. The SCOTS test can achieve measurement accuracy comparable to traditional interferometric tests.
Absolute measurement with SCOTS/deflectometry is a calibration problem. We use a laser tracker to calibrate the test
geometry. The performance id demonstrated with the initial measurement results from the Large Synoptic Survey
Telescope tertiary mirror. Systematic errors from the camera are carefully controlled. Camera pupil imaging aberration is
removed with an external aperture stop. Imaging aberration and other inherent errors are suppressed with a rotation test.
Results show that the SCOTS can act as a large dynamic range, high precision, non-null test method for precision
aspheric optics. The SCOTS test can achieve measurement accuracy comparable with the traditional interferometric
testing.
A software configurable optical test system (SCOTS) based on fringe reflection was implemented for measuring the
primary mirror segments of the Giant Magellan Telescope (GMT). The system uses modulated fringe patterns on an
LCD monitor as the source, and captures data with a CCD camera and calibrated imaging optics. The large dynamic
range of SCOTS provides good measurement of regions with large slopes that cannot be captured reliably with
interferometry. So the principal value of the SCOTS test for GMT is to provide accurate measurements that extend
clear to the edge of the glass, even while the figure is in a rough state of figure, where the slopes are still high.
Accurate calibration of the geometry and the mapping also enable the SCOTS test to achieve accuracy that is
comparable measurement accuracy to the interferometric null test for the small- and middle- spatial scale errors in
the GMT mirror.
The scanning long-wave optical test system (SLOTS) is under development at the University of Arizona to provide rapid
and accurate measurements of aspherical optical surfaces during the grinding stage. It is based on the success of the
software configurable optical test system (SCOTS) which uses visible light to measure surface slopes. Working at long
wave infrared (LWIR, 7-14 μm), SLOTS measures ground optical surface slopes by viewing the specular reflection of a
scanning hot wire. A thermal imaging camera collects data while motorized stages scan the wire through the field.
Current experiments show that the system can achieve a high precision at micro-radian level with fairly low cost
equipment. The measured surface map is comparable with interferometer for slow optics. This IR system could be
applied early in the grinding stage of fabrication of large telescope mirrors to minimize the surface shape error imparted
during processing. This advantage combined with the simplicity of the optical system (no null optics, no high power
carbon dioxide laser) would improve the efficiency and shorten the processing time.
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