Measurement of a large deformable aspherical mirror using SCOTS (Software Configurable Optical Test System)
2013
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.
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