MFT was mentioned in five of the papers given at the SPIE Conference on Telescopes in Edinburgh, Scotland at the end of June 2016 by authors from six different international research organizations.
The surface roughness measurements of a silicon plate with a multilayer coating to reflect Xrays taken with the MFT are consistent with each other going from 2.5x to 10x magnification, consistent with the atomic force microscope for higher spatial frequency measurements and consistent with the model of the surface roughness. The figure shows a map of the surface taken with the MFT, upper left, and a graph showing the consistency of the various measurements and the model over many orders of magnitude of spatial scale.
Shows the MFT sitting on the 4.2 m mirror and the lap top computer sitting along side it with a map of the mirror surface on the screen. Figure 17 shows 3 examples of the surface roughness taken at 3 different places on the mirror but all giving results in the 1 nm rms range over a sample area of 3 x 2.4 mm. Figure 18 shows that these surface roughness measurements converted into BRDF illustrate the mirror finish meeting the specification. It had previously been shown that the bare Zerodur mirror could not have been measured directly for BRDF because of the polycrystalline nature of the substrate but by measuring the surface roughness there was a path to measuring and meeting the specification (see Tayabaly, et. al., “Use of the surface PSD and incident angle adjustments to investigate near specular scatter from smooth surfaces”, Proc. SPIE 883805, 2013).
Roughness tolerances for Cherenkov telescope mirrors on p. 5-6 and Fig 2, gives a description of the MFT in detail and shows the MFT sitting on a Cherenkov mirror substrate and an example of the surface roughness of the dielectrically coated mirror that was about 1 nm rms. The paper goes on to show that a variety of mirror substrates and coatings were measured for surface roughness and the resulting surface roughness maps are shown in Fig. 3 that range from 0.58 nm rms to about 12 nm rms depending on coating and exposure to the environment.
Polishing and testing of the 3.4 m diameter f/1.5 primary mirror of the INO telescope, p. 7, Fig 9 shows the results of 156 separate measurements made on various parts of the mirror with an average roughness of 0.6 nm rms. This was a mirror made of Zerodur so the surface roughness could not be measured using scattering, and surface roughness had to be used to see that the mirror met spec. Also, because of the speed of the mirror it would have been difficult and time consuming to have made replicas to measure the surface roughness.
Ion beam figuring of thin glass plates: achievements and perspectives, P 4-5 and Figs. 5-6, indicates the MFT was used to measure the surface roughness of ion beam polished glass plates only 0.4 mm thick, yet because of the low mass of the MFT it was possible to set the MFT directly on the thin plates to make the measurements. Figs. 5 and 6 show the surface roughness maps after various amounts of material was removed by ion beam polishing and the resulting change in roughness. The places on the substrates were carefully aligned so that it was possible to show the evolution of the change in roughness. Again the data were highly consistent. Special mention was made of the low mass being particularly useful in this study.