The MFT MicroFinish Topographer is a small, lightweight and simple system for interferometrically measuring surface roughness on all sizes of optics from a few mm to meters. For large optics the MFT sits directly on the surface requiring measurement thus eliminating the need for replicas.
For small optics the MFT is inverted and a sample plate is added onto which the sample is placed on 3 nylon balls. In this way once the fringes are initially found, sample after sample maybe measured without further adjustment. Also, since the sample is directly coupled to the MFT via the 3 balls, the MFT is largely immune to vibration.
FLEXIBILITY IN MEASUREMENT
The small size and light weight of the MFT make it possible to measure substrates and configurations not possible with other instruments. The MFT has been used to make direct measurements of large substrates used in Cherenkov reflectors with delicate dielectric coatings and cylindrical X-ray mirrors where the MFT sat inside the barrel shaped mirror that was only 3 mm thick.
Further the MFT is small and light enough to mount directly on a diamond turning machine for in-situ measurement of roughness and figure.
IMMEDIATE FINISH FEEDBACK
The MFT gives immediate finish feedback because it can be set on an optic during polishing without ever having to remove the optic from the polishing machine. This ability permits greater process control and evidence of meeting specification without excessive polishing. Scroll down for bibliography of archival MFT papers.
Recently the MFT was used for this purpose on two large telescope mirror projects.
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Bibliography: Archival papers describing a variety of applications of the MFT for surface roughness
“Calculating BTDF from window surface roughness”, J. Stover, et. al., The Scatter Works, College of
Optical Sciences, Univ. of Arizona and Moscow State University, Proc. SPIE 883804, doi:10.1117/12.2024599. Paper discusses the use of the MFT to take surface topography maps of window and lens surfaces.
“Fabrication and testing of 4.2m off-axis aspheric primary mirror of Daniel K. Inouye Solar Telescope”, C. J. Oh, et. al., College of Optical Sciences, Univ. of Arizona, Proc. SPIE 99120O, doi:10.1117/12.2229324. Paper shows the use of the MFT to measure the surface topography of the mirror by setting the MFT directly on the surface of the mirror while on the polishing machine.
“Final acceptance testing of the LSST monolithic primary/tertiary mirror”, M. Tuell, et. al., Steward Observatory, Univ. of Arizona and National Optical Astronomy Observatory, Proc. SPIE 91510W, doi:10.1117/12.2057076. Paper discusses the use of the MFT to measure the surface roughness directly on the mirror surface.
“Ion beam figuring of thin glass plates: achievements and perspectives”, M. Civitani, et. al., INAF Astronomical Observatory of Brera, Proc. SPIE 990578, doi:10.1117/12.2233821. Paper discusses the use of the MFT to monitor the in-process surface roughness of large, thin glass samples during ion beam figuring.
“Manufacturing of super-polished large aspheric/freeform optics”, D. W. Kim, et. al., ., College of Optical Sciences and Steward Observatory, Univ. of Arizona, Proc. SPIE 99120O, doi:10.1117/12.2229324. Paper shows direct surface roughness PSD measurements made with the MFT on the mirror surface are in good agreement with PSD measurements made with other instruments at larger spatial scales.
“Point spread function computation in normal incidence for rough optical surfaces”, K. Tayabaly, et. al., INAF/Brera Astronomical Observatory and Politecnico di Milano, Proc. SPIE 99111X, doi:10.1117/12.2232320. Paper discusses the use of the MFT to help verify the theoretical calculations of the psf at small physical dimensions.
“Polishing and testing of the 3.4 m diameter f/1.5 primary mirror of the INO telescope”, T. Korhonen, et. al., Opteon Oy, Finland and Iran and Iranian National Observatory, Proc. SPIE 99120Q, doi:10.1117/12.2232691. Paper discusses the use of the MFT to show that the finished mirror met the specification for surface roughness by direct measurement on the mirror while still on the polishing machine.
“Roughness tolerances for Cherenkov telescope mirrors”, K. Tayabaly, et. al., INAF/Brera Astronomical Observatory and Politecnico di Milano, Proc SPIE 960307, doi: 10.1117/12.2187025. Paper describes the use of the MFT to directly measure the surface roughness of large, thin mirrors including diamond turned and dielectric coated ones by setting the MFT on the mirror surface.
“Simulation and modeling of silicon pore optics for the ATHENA X-ray telescope”, D. Spiga, et. al., INAF/Brera Astronomical Observator, Politecnico di Milano, DTU Space, Techn. Univ. of Denmark, and European Space Agency, ESTEC, Proc SPIE 990550, doi: 10.1117/12.2232230. Paper shows that surface roughness measurements made with the MFT match up well with AFM measurements made at finer spatial scales.
“Super-smooth optical fabrication controlling high-spatial frequency surface irregularity”, J. Del Hoyo, College of Optical Sciences, Univ. of Arizona, Proc. SPIE 88380T, doi:10.1117/12.2022924. Paper shows the use of the MFT to monitor surface roughness and degree of polish as a function of polishing compound and time through direct surface measurement while the optic is still on the polishing machine.
“Use of the surface PSD and incident angle adjustments to investigate near specular scatter from smooth surfaces”, K. Tayabaly and J. Stover, College of Optical Sciences, Univ. of Arizona and The Scatter Works, Proc. SPIE 883805, doi:10.1117/12.2024612. Paper discusses the match between surface roughness measured with the MFT and BRDF scatter measurements. In cases where scatter cannot be measured directly because it is too close to specular or the substrate material scatters, surface roughness measurements can be used in lieu of scatter.
The PSM is an ideal tool for finding the center of curvature of a ball or the axis of a cylinder. The question is for how small a ball or cylinder can the PSM do this?
The smallest article that was readily available was a piece of monofilament 8 pound test fishing line that was 290 μm in diameter. There was no problem finding the axis of the fishline, and separating the Cat’s eye reflection from the surface from the confocal reflection of the axis. The experiment was done with a 5x objective, and the result would have been even more definitive using a 10x objective.
Here is a case of a very happy customer due to better optics.
A few days ago an astronomer friend of mine commented that he had gotten the optics of his telescope improved and the improvement reduced the time it took to get data by a factor of 3. For an astronomer this is a dramatic improvement since observing time on large telescopes can cost thousands of dollars an hour.
My friend did not say how the optics had been improved, but the important point is that better optics, whether due to figure errors, mounting or alignment mean more productive optics. I generally think of better optics as a better product leaving the manufacturing facility without thinking about how much the better optics mean to the productivity of the customer.