The smoothness criterion on the Daniel K. Inouye Solar Telescope primary mirror was a scatter specification. This was a problem in two respects, the mirror substrate was Zerodur and the scatter angle was too close to specular to measure with a scatterometer. The near zero expansion Zerodur is a microcrystalline glass ceramic that scatters light when illuminated so a scatter test could not be used during polishing. Also, the scatter angle in the specification was so close to specular that the detector in a scatterometer partially obscured the scattered light path and could not be used even if the substrate did not scatter.
Kashmira Tayabala, et. al., “Use of the PSD and incident angle adjustments to investigate near specular scatter from smooth surfaces”, Proc. SPIE, 8838 (2013) showed that the MicroFinish Topographer (MFT) could measure surface roughness over sufficiently long distances to infer the magnitude of the near specular angle scatter. Given this insight, the MFT was used to show the Inouye Solar Telescope primary met the near specular angle scatter specification, Chang Jin Oh, et. al., “Fabrication and Testing of 4.2m Off-Axis Aspheric Primary Mirror of Daniel K. Inouye Solar Telescope”, Proc. SPIE, 9912 (2016).