Matt Novak

Analysis of a micropolarizer array-based simultaneous phase-shifting interferometer.

Recent technological innovations have enabled the development of a new class of dynamic (vibration-insensitive) interferometer based on a CCD pixel-level phase-shifting approach. We present theoretical and experimental results for an interferometer based on this pixelated phase-shifting technique. Analyses of component errors and instrument functionality are presented. We show that the majority of error sources cause relatively small magnitude peak-to-valley errors in measurement of the order of 0.002-0.005lambda. These errors are largely mitigated by high-rate data acquisition and consequent data averaging.

Modern approaches in phase measuring metrology (Invited Paper)

The measurement accuracy of an interferometric optical test is generally limited by the environment. This paper discusses two single-shot interferometric techniques for reducing the sensitivity of an optical test to vibration; simultaneous phase-shifting interferometry and a special form of spatial carrier interferometry utilizing a micropolarizer phase-shifting array. In both techniques averaging can be used to reduce the effects of turbulence and the normal double frequency errors generally associated with phase-shifting interferometry.

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.

Distortion mapping correction in aspheric null testing

We describe methods to correct both symmetric and asymmetric distortion mapping errors induced by null testing elements such as holograms or null lenses. We show experimental results for direct measurement and correction of symmetric mapping distortion, as well as an example result for analytical mapping performed using an orthogonal set of vector polynomials for asymmetric correction. The empirical determination of symmetric distortion is made via calculation from predicted and measured changes to aberrations induced via known changes to the testing point.

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.

Fabrication and testing of large flats

Flat mirrors of around 1 meter are efficiently manufactured with large plano polishers and measured with Fizeau interferometry. We have developed technologies and hardware that allow fabrication and testing of flat mirrors that are much larger. The grinding and polishing of the large surfaces uses conventional laps driven under computer control for accurate and systematic control of the surface figure. The measurements are provided by a combination of a scanning pentaprism test, capable of measuring power and low order irregularity over diameters up to 8 meters, and subaperture Fizeau interferometry. We have developed a vibration insensitive Fizeau interferometer with 1 meter aperture and software to optimally combine the data from the subaperture tests. These methods were proven on a 1.6 m flat mirror that was finished to 6 nm rms irregularity and 11 nm rms power.

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.

Real-time scanner error correction in white light interferometry

3D microscopes based on white light interferometry (WLI) with vertical scanning have been widely used in many areas of surface measurements and characterizations for decades. This technology provides fast, non-contact, and full-field surface 3D measurements with vertical resolution as low as the sub-nanometer range. Its applications include measurements of step height, surface roughness, film thickness, narrow trench and via depths as well as other geometric and texture parameters. In order to assure the highest accuracy of the measurement, scanner linearity needs to be maintained or monitored so that the nonlinearity can be accounted for during the measurement. This paper describes a method that accounts for nonlinearities in real time without the need to store frame data; in addition this method is shown to be less sensitive to vibrations than previous methods described. The method uses an additional interferometer, a distance measuring interferometer to measure the actual scanner position at each scan step.

3D interferometric microscope: color visualization of engineered surfaces for industrial applications

3D microscopes based on white light interference (WLI) provide precise measurement for the topography of engineering surfaces. However, the display of an object in its true colors as observed under white illumination is often desired; this traditionally has presented a challenge for WLI-based microscopes. Such 3D color display is appealing to the eye and great for presentations, and also provides fast evaluation of certain characteristics like defects, delamination, or deposition of different materials. Determination of color as observed by interferometric objectives is not straightforward; we will present how color imaging capabilities similar to an ordinary microscope can be obtained in interference microscopes based on WLI and we will give measurement and imaging examples of a few industrial samples.