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.