High Resolution Optical Surface Metrology with the Slope Measuring Portable Optical Test System

Abstract

New optical designs strive to achieve extreme performance, and continually increase the complexity of prescribed optical shapes, which often require wide dynamic range and high resolution. SCOTS, or the Software Configurable Optical Test System, can measure a wide range of optical surfaces with high sensitivity using surface slope. This dissertation introduces a high resolution version of SCOTS called SPOTS, or the Slope measuring Portable Optical Test System. SPOTS improves the metrology of surface features on the order of sub-millimeter to decimeter spatial scales and nanometer to micrometer level height scales. Currently there is no optical surface metrology instrument with the same utility. SCOTS uses a computer controlled display (such as an LCD monitor) and camera to measure surface slopes over the entire surface of a mirror. SPOTS differs in that an additional lens is placed near the surface under test. A small prototype system is discussed in general, providing the support for the design of future SPOTS devices. Then the SCOTS instrument transfer function is addressed, which defines the way the system filters surface heights. Lastly, the calibration and performance of larger SPOTS device is analyzed with example measurements of the 8.4-m diameter aspheric Large Synoptic Survey Telescope's (LSST) primary mirror. In general optical systems have a transfer function, which filters data. In the case of optical imaging systems the instrument transfer function (ITF) follows the modulation transfer function (MTF), which causes a reduction of contrast as a function of increasing spatial frequency due to diffraction. In SCOTS, ITF is shown to decrease the measured height of surface features as their spatial frequency increases, and thus the SCOTS and SPOTS ITF is proportional to their camera system's MTF. Theory and simulations are supported by a SCOTS measurement of a test piece with a set of lithographically written sinusoidal surface topographies. In addition, an example of a simple inverse filtering technique is provided. The success of a small SPOTS proof of concept instrument paved the way for a new larger prototype system, which is intended to measure subaperture regions on large optical mirrors. On large optics, the prototype SPOTS is light weight and it rests on the surface being tested. One advantage of this SPOTS is stability over time in maintaining its calibration. Thus the optician can simply place SPOTS on the mirror, perform a simple alignment, collect measurement data, then pick the system up and repeat at a new location. The entire process takes approximately 5 to 10 minutes, of which 3 minutes is spent collecting data. SPOTS' simplicity of design, light weight, robustness, wide dynamic range, and high sensitivity make it a useful tool for optical shop use during the fabrication and testing process of large and small optics.