Run Huang

Aspheric and freeform surfaces metrology with software configurable optical test system: a computerized reverse Hartmann test

Abstract. 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.

High-accuracy aspheric x-ray mirror metrology using Software Configurable Optical Test System/deflectometry

Abstract. The Software Configurable Optical Test System (SCOTS) uses deflectometry to measure surface slopes of general optical shapes without the need for additional null optics. Careful alignment of test geometry and calibration of inherent system error improve the accuracy of SCOTS to a level where it competes with interferometry. We report a SCOTS surface measurement of an off-axis superpolished elliptical x-ray mirror that achieves <1  nm root-mean-square accuracy for the surface measurement with low-order term included.

Optical metrology of a large deformable aspherical mirror using software configurable optical test system

The software configurable optical test system (SCOTS) is an efficient metrology technology based on reflection deflectometry that uses only a liquid-crystal display and a camera to measure surface slope. The surface slope is determined by triangulation using the co-ordinates of the display screen, camera, and test mirror. We present our SCOTS test results concentrated on high dynamic range measurements of low order aberrations. The varying astigmatism in the 910-mm diameter aspheric deformable secondary mirror for the large binocular telescope was measured with SCOTS, requiring no null corrector. The SCOTS system was designed on-axis with camera and screen aligned on the optical axis of the test mirror with the help of a 6-inch pellicle beam splitter. The on-axis design provides better control of the astigmatism in the test. The high dynamic range of the slope provided a measurement of astigmatism within 0.2-μm root-mean-square accuracy in the presence of 231-μm peak-to-valley aspheric departure. The simplicity of the test allowed the measurements to be performed at multiple gravity angles.

X-ray mirror metrology using SCOTS/deflectometry

SCOTS is a high precision slope measurement technology based on deflectometry. Light pattern on a LCD display illuminates the test surface and its reflected image is used to calculate the surface slope. SCOTS provides a high dynamic range full field measurement of the optics without null optics required. We report SCOTS tests on X-ray mirrors to nm and even sub nm level with precise calibration of the test system. A LCD screen with dots/check board pattern was aligned into the system at the test mirror position to calibrate camera imaging distortion in-situ. System errors were further eliminated by testing and subtracting a reference flat which was also aligned at the same position as the test mirror. A virtual reference based on the ideal shape of the test surface was calculated and subtracted from the test raw data. This makes the test a ‘virtual null’ test. Two X-ray mirrors were tested with SCOTS. 0.1μrad (rms) slope precision and sub nm (rms) surface accuracy were achieved.

Measurement of a large deformable aspherical mirror using SCOTS (Software Configurable Optical Test System)

The software configurable optical test system (SCOTS) is an efficient metrology technology based on reflection deflectometry that uses only an LCD screen and a camera to measure surface slope. The surface slope is determined by triangulations using the coordinates of the display screen, camera and test mirror. We present our recent SCOTS test results concentrated on high dynamic range measurements of low order aberrations. The varying astigmatism in the 91 cm diameter aspheric deformable secondary mirror for the Large Binocular Telescope (LBT) was measured with SCOTS, requiring no null corrector. The SCOTS system was designed on axis with camera and screen aligned on the optical axis of the test mirror with the help of a 6 inch pellicle beam splitter. The on-axis design gives better control of the astigmatism in the test. The high dynamic range of slope provided a measurement of astigmatism with 0.2 μm rms accuracy in the presence of 231 μm peak-to-valley (PV) aspheric departure. The simplicity of the test allowed the measurements to be performed at multiple elevation angles.

Precision aspheric optics testing with SCOTS: A deflectometry approach

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.

Deflectometry measurement of Daniel K. Inouye Solar Telescope primary mirror

SCOTS (Software Configurable Optical Test System) is a high-precision slope measurement technique based on deflectometry. It utilizes a well-calibrated commercial LCD screen and a diffraction-limited camera to provide high dynamic range, non-contact and full-field metrology of reflective/refractive optics of high accuracy but low cost. Recently, we applied this metrology method on the fabrication of the primary mirror of Daniel K. Inouye Solar Telescope (DKIST), which is a 4.2 meter off-axis parabolic segment with more than 8 mm peak-to-valley aspheric departure. Sophisticated calibrations and compensations including camera mapping, screen nonlinearity and screen shape deformation are performed to achieve high accuracy measurement results. By measuring the mirror at different orientations, non-symmetrical systematic errors are eliminated. The metrology system also includes dual cameras that provide self- verification test. The measurement results are being used to guide the fabrication process.

Measuring large mirrors using SCOTS: the Software Configurable Optical Test System

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.

Measuring aspheric surfaces with reflection deflectometry

Interferometry is the approach widely favored for measuring aspheric specular surfaces, such as optics for large astronomy telescopes. This method produces highly accurate results, but has limited dynamic range. It also requires the use of null optics, including computer-generated holograms, for steep aspherical surfaces, which increases the complexity and expense of the procedure. We have developed a software-configurable optical test system (SCOTS) that rivals interferometry in accuracy, but is flexible and far more cost-effective. The system is based on reflection deflectometry: that is, measuring the reflected light pattern from a specular surface to calculate its shape.1 SCOTS is simple in setup, requiring only a liquid-crystal display (LCD) screen and a camera near the center of curvature of a test mirror. During a test, an intensity pattern is displayed on the LCD screen and the camera captures the reflected image on the mirror (see Figure 1). Knowing the coordinates of the pattern, mirror, and camera aperture, we can determine the mirror’s surface slope by geometric ray tracing. Because SCOTS measures local surface slope, it overcomes the dynamic range problems of interferometric tests when the surfaces are not spherical. Furthermore, our system camera provides a full field of view of the surface under test, which eliminates the need for stitching (combining overlapping images), a process that is complicated by nulling and alignment. We recently applied SCOTS to the measurement of a large deformable aspheric mirror, one of the adaptive secondary optics of the Large Binocular Telescope.2 We designed and built an onaxis SCOTS to investigate the possible sources of astigmatism in the secondary mirror when it is elevated with a telescope. The mirror is a 91cm-diameter ellipsoidal surface with 231 m peak-to-valley aspheric departure. Figure 2 shows the on-axis SCOTS system setup built for this procedure. We aligned the Figure 1. The schematic setup for our software configurable optical test system (SCOTS).

Fabrication and testing of 4.2m off-axis aspheric primary mirror of Daniel K. Inouye Solar Telescope

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.