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Recent advancements in MRF technology have improved the polishing performance expected for astronomical optics in low, mid and high spatial frequency regimes. Deterministic figure correction with MRF is compatible with most materials, including some recent examples on Silicon Carbide and RSA905 Aluminum. MRF also has the ability to produce ‘perfectly-bad’ compensating surfaces, which may be used to compensate for measured or modeled optical deformation from sources such as gravity or mounting. In addition, recent advances in MRF technology allow for corrections of mid-spatial wavelengths as small as ~1mm simultaneously with form error correction. Efficient midspatial frequency corrections make use of optimized process conditions including raster polishing in combination with a small tool size. Furthermore, a novel MRF fluid, called C30, has been developed to finish surfaces to ultra-low roughness (ULR) and has been used as the low removal rate fluid required for fine figure correction of mid-spatial frequency errors. This novel MRF fluid is able to achieve <4Å RMS on Nickel-plated Aluminum and even <1.5Å RMS roughness on Silicon, Fused Silica and other materials. C30 fluid is best utilized within a fine figure correction process to target mid-spatial frequency errors as well as smooth surface roughness 'for free' all in one step.
In this paper we will discuss recent advancements in MRF technology and the ability to meet requirements for precision optics in low, mid and high spatial frequency regimes and how improved MRF performance addresses the need for achieving tight specifications required for astronomical optics.
Classically, Magnetorheological Finishing (MRF) is implemented in production to correct low order errors generated by conventional polishing techniques on planos, spheres, on- and off-axis aspheres and freeform optics achieving figure errors as low as 1nm RMS while using careful metrology setups. MRF is also used routinely to turn a sphere into an asphere or freeform, or to print high resolution wavefront corrective patterns on optical surfaces to compensate for system errors or bulk material inhomogeneity.
Recent advancements enable correction of mid-spatial wavelengths as small as ∼1mm and smoothing of surface roughness to ∼1Å RMS. Using these new developments combined with correction of low order form error have improved MRF performance to manufacture higher precision optics with respect to the mid- and high-spatial frequency regimes.
Efficient mid-spatial frequency corrections utilize optimized process conditions; raster polishing with a small tool size. Furthermore, a novel MRF fluid, called C30, can finish surfaces to ultra-low roughness (ULR) and its low removal rate is optimal for fine figure correction of mid-spatial frequency errors. C30 MRF fluid is able to achieve <1.5Å RMS roughness on Silicon, CaF2, Fused Silica, glass and other materials. It is best utilized within a fine figure correction process to target mid-spatial frequency errors as well as smooth surface roughness ‘for free’ all in one step.
These expanded capabilities of MRF technology are well suited for producing high precision optics to be used for industrial, medical or semiconductor optics.
Fine figure correction and other applications using novel MRF fluid designed for ultra-low roughness
The subaperture and conformal nature of the MRF® polishing tool has proven its unmatched production capability and efficiency for more than a decade at leading optics manufacturers worldwide. The introduction of the third generation of manufacturing systems combined with newly developed MRF fluids pushes the limits of the technology to extend its benefits to very low roughness surfaces and high-precision freeform surfaces. In this article, after reviewing the benefits of the new platforms, two specific examples of very advanced capability will be discussed:
- A new super-fine MRF polishing fluid that is able to meet both form and roughness specifications for very demanding optics required for EUV applications and high power laser systems.
EUV optics, made of calcium fluoride or similar materials, ideally require sub-Angstrom surface roughness while achieving nm level form error. To achieve the above specifications, optics must undergo iterative global and local polishing processes. The new MRF polishing fluid minimizes the number of steps required if MRF® is used as the final step, or a reduction in the post-processing if a final smoothing step is performed.
- The total manufacturing process, including generation, pre-polishing, MRF and metrology, of a very steep freeform surface, highlighting the capabilities available in today’s optical fabrication companies.
Non-rotationally symmetric surfaces pose challenges to optical fabrication, mostly in the areas of polishing and metrology. The varying curvature of freeform surfaces drives the need for smaller, more “conformal” tools for polishing and reference beams for interferometry. In this paper, we present the fabrication results of a high-precision freeform surface.
Improvement of figure and finish of diamond turned surfaces with magneto-rheological finishing (MRF)
This course provides an overview of how aspheric surfaces are designed, manufactured, and measured. The primary goal of this course is to teach how to determine whether a particular aspheric surface design will be difficult to make and/or test. This will facilitate cost/performance trade off discussions between designers, fabricators, and metrologists.
We will begin with a discussion of what an asphere is and how they benefit optical designs. Next we will explain various asphere geometry characteristics, especially how to evaluate local curvature plots. We will also review flaws of the standard polynomial representation, and how the Forbes polynomials can simplify asphere analysis. Then we will discuss how various specifications (such as figure error and local slope) can influence the difficulty of manufacturing an asphere. Optical assembly tolerances, however, are beyond the scope of this course - we will focus on individual elements (lenses / mirrors).
The latter half of the course will focus on the more common technologies used to generate, polish, and/or measure aspheric surfaces (e.g. diamond turning, glass molding, pad polishing, interferometry). We'll give an overview of a few generic manufacturing processes (e.g. generate-polish-measure). Then we'll review the main strengths and weaknesses of each technology in the context of cost-effective asphere manufacturing.
This course explains the basic principles, operation and applications of Magnetorheological Finishing (MRF). Over the past 2 decades, MRF has gone from a cutting edge new technology to a standard used by optics shops around the world for the finishing of high-precision plano, spherical, aspherical and freeform optics. MRF not only enables higher-precision on complex optics, it can also be used to increase yields, increase throughput, improve on-time delivery, and otherwise increase your shop’s “manufacturing confidence”. Anyone who wants to answer questions such as, "how does MRF work?", “what can MRF be used for” or "how does MRF integrate with my other grinding, polishing, and/or diamond turning capabilities?" will benefit from taking this course.
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