Chang Jin Oh

Development of a wide field spherical aberration corrector for the Hobby Eberly Telescope

A 4-mirror prime focus corrector is under development to provide seeing-limited images for the 10-m aperture Hobby- Eberly Telescope (HET) over a 22 arcminute wide field of view. The HET uses an 11-m fixed elevation segmented spherical primary mirror, with pointing and tracking performed by moving the prime focus instrument package (PFIP) such that it rotates about the virtual center of curvature of the spherical primary mirror. The images created by the spherical primary mirror are aberrated with 13 arcmin diameter point spread function. The University of Arizona is developing the 4-mirror wide field corrector to compensate the aberrations from the primary mirror and present seeing limited imaged to the pickoffs for the fiber-fed spectrographs. The requirements for this system pose several challenges, including optical fabrication of the aspheric mirrors, system alignment, and operational mechanical stability.

Swing arm optical CMM for aspherics

A profilometer for in situ measurement of the topography of aspheric mirrors called the Swing arm Optical CMM (SOC) was built, and has been used for measuring the figure of 1.4 m convex aspheric mirrors with a performance rivaling full aperture interferometric tests. Errors in the SOC that have odd symmetry are self-calibrated due to the test geometry. Even errors are calibrated against a full aperture interferometric test.

Swing arm optical coordinate-measuring machine: high precision measuring ground aspheric surfaces using a laser triangulation probe

The swing arm optical coordinate-measuring machine (SOC), a profilometer with a distance measuring interferometric sensor for in situ measurement of the topography of aspheric surfaces, has shown a precision rivaling the full aperture interferometric test. To further increase optical manufacturing efficiency, we enhance the SOC with an optical laser triangulation sensor for measuring test surfaces in their ground state before polishing. The calibrated sensor has good linearity and is insensitive to the angular variations of the surfaces under testing. Sensor working parameters such as sensor tip location, projection beam angle, and measurement direction are calibrated and incorporated in the SOC data reduction software to relate the sensor readout with the test surface sag. Experimental results show that the SOC with the triangulation sensor can measure aspheric ground surfaces with an accuracy of 100 nm rms or better.

Swing-arm optical coordinate measuring machine: modal estimation of systematic errors from dual probe shear measurements

The swing-arm optical coordinate measuring machine (SOC), a profilometer with a distance-measuring interferometric probe for in situ measurement of the topography of aspheric surfaces,has been used for measuring highly aspheric mirrors with a performance rivaling full aperture interferometric tests. Recently, we implemented a dual probe, self-calibration mode for the SOC. Data from the dual probes can be used to calibrate the swing-arm air bearing errors since both probes see the same bearing errors while measuring different portions of the test surface. Bearing errors are reconstructed from modal estimation of the sheared signal.

Swing arm optical CMM: self calibration with dual probe shear test

Swing arm optical CMM (SOC), a profilometer with distance measuring interferometric probe for in situ measurement of the topography of aspheric has been used for measuring highly aspheric mirrors with a performance rivaling full aperture interferometric tests. Recently, we implemented a dual probe self calibration mode for the SOC. Data from the dual probes can be used to calibrate the swing-arm air bearing errors since both probes see the same bearing errors while measuring different portions of the test surface. Bearing errors are reconstructed from the shear signal with a modal estimation.

Fabrication and testing of 1.4-m convex off-axis aspheric optical surfaces

New developments in fabrication and testing techniques at the College of Optical Sciences, University of Arizona have allowed successful completion of 1.4-m diameter convex off-axis aspherics. The optics with up to 300 μm aspheric departure were finished using a new method of computer controlled polishing and measured with two new optical tests: the Swingarm Optical CMM (SOC) and a Fizeau interferometer using a spherical reference surface and CGH correction. This paper shows the methods and equipment used for manufacturing these surfaces.

Algorithms for surface reconstruction from curvature data for freeform aspherics

Increasing demand for highly accurate freeform aspheric surfaces requires accurate and efficient measurement techniques. One promising possibility uses a sub-aperture scanning system that measures local curvature variations across the part. In this paper, we develop and demonstrate two different data processing algorithms, a zonal approach using Southwell integration method and a modal approach leveraging Zernike curvature basis, that reconstruct the surface 3-dimensional profiles from the curvature data. The performance of suggested methods and the sensitivity to noise is diagnosed for various SNR (Signal-to-Noise Ratio) cases.

Optical testing for meter size aspheric optics

Several meter size steep aspheric optics, with aspheric departure ranging from 100μm to 2mm, have been successfully fabricated at the College of Optical Sciences at University of Arizona. Optical metrology systems have been developed for measuring the optical surfaces efficiently and accurately. These systems include laser tracker surface profiler, swing arm optical CMM with different type of sensors, slope measurements with SCOTS, the Software Configurable Optical Test System (SCOTS) and interferometry with computer generated holograms. We summarize the test methods and provide comparison of the relative strengths and weaknesses.

Fabrication of 4-meter class astronomical optics

The 8-meter mirror production capacity at the University of Arizona is well known. As the Arizona Stadium facility is occupied with giant mirrors, we have developed capability for grinding, polishing, and testing 4-m mirrors in the large optics shop in the College of Optical Sciences. Several outstanding capabilities for optics up to 4.3 meters in diameter are in place: A 4.3-m computer controlled grinding and polishing machine allows efficient figuring of steeply aspheric and nonaxisymmetric surfaces. Interferometry (IR and visible wavelengths) and surface profilometry making novel use of a laser tracker allows quick, accurate in-process measurements from a movable platform on a 30-m vertical tower. A 2-meter class flat measured with a 1-m vibration insensitive Fizeau interferometer and scanning pentaprism system; stitching of 1-m sub-apertures provides complete surface data with the technology ready for extension to the 4 m level. These methods were proven successful by completion of several optics including the 4.3-m Discovery Channel Telescope primary mirror. The 10 cm thick ULE substrate was ground and polished to 16 nm rms accuracy, corresponding to 80% encircled energy in 0.073 arc-second, after removing low order bending modes. The successful completion of the DCT mirror demonstrates the engineering and performance of the support system, ability to finish large aspheric surfaces using computer controlled polishing, and accuracy verification of surface measurements. In addition to the DCT mirror, a 2-meter class flat was produced to an unprecedented accuracy of <10 nm-rms, demonstrating the combined 1-m Fizeau interferometer and scanning pentaprism measurement techniques.

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.