The final finish and characterization of windows and domes presents a number of challenges in achieving desired
precision with acceptable cost and schedule. This becomes more difficult with advanced materials and as window and
dome shapes and requirements become more complex, including acute angle corners, transmitted wavefront
specifications, aspheric geometries and trending toward conformal surfaces. Magnetorheological Finishing (MRF®) and
Magnetorheological Jet (MR Jet®), along with metrology provided by Sub-aperture Stitching Interferometry (SSI®) have
several unique attributes that provide them advantages in enhancing fabrication of current and next generation windows
The advantages that MRF brings to the precision finishing of a wide range of shapes such as flats, spheres (including
hemispheres), cylinders, aspheres and even freeform optics, has been well documented. Recent advancements include
the ability to finish freeform shapes up to 2-meters in size as well as progress in finishing challenging IR materials. Due
to its shear-based removal mechanism in contrast to the pressure-based process of other techniques, edges are not
typically rolled, in particular on parts with acute angle corners. MR Jet provides additional benefits, particularly in the
finishing of the inside of steep concave domes and other irregular shapes. The ability of MR Jet to correct the figure of
conformal domes deterministically and to high precision has been demonstrated. Combining these technologies with
metrology techniques, such as SSI provides a solution for finishing current and future windows and domes in a reliable,
deterministic and cost-effective way. The ability to use the SSI to characterize a range of shapes such as domes and
aspheres, as well as progress in using MRF and MR Jet for finishing conventional and conformal windows and domes
with increasing size and complexity of design will be presented.
The design and manufacture of most optical systems revolves around the use of ideal optical surfaces. "Perfect" spheres
or flats are optimally combined and toleranced during the design phase, and the manufacturers attempt to get as close as
possible to these perfect optical surfaces during fabrication. One reason for this stems from the inherent capabilities of
the industry's oldest and most pervasive polishing tool: the full-aperture lap. The shape and motion of these tools
naturally produce spherical or flat geometries. More recently, a number of new manufacturing technologies based on
sub-aperture polishing tools have become available. Sub-aperture tools enable local, preferential removal: a controlled
way to polish more material at some locations and less at others. Magnetorheological Finishing (MRF<sup>(R)</sup> ) is one such
sup-aperture polishing technology, and when combined with an accurate measurement, can offer a precise method for
converging to the perfect surface: local removal based directly on measured surface height. This capability, however,
can also be leveraged in other, more creative, ways. For example, by replacing the typical surface-error measurement by
a transmitted wavefront measurement of an entire low-field optical system, a hitmap can be created for one surface in the
system that will perfectly compensate for errors of all the other surfaces. This paper will explore a number of examples
where "perfectly bad" surfaces have been exploited in actual optical systems to improve performance, improve
manufacturability, or reduce cost. In addition, we will ask the question: if making a "perfectly bad" surface was as easy
as making a perfectly good one, would this capability be used more widely by the precision optics industry?
Deterministic subaperture finishing technologies, such as Magnetorheological Finishing (MRF<sup>(R)</sup>) are becoming the
industry standard for finishing high precision optics with complex shapes, such as aspheres. However, astronomical or
very large optics were beyond the scale of existing capabilities and relied on traditional, artisan-based methods of
manufacture. It is not uncommon for these critical parts to spend a year or more in production. Recent developments
from QED Technologies(R) have expanded MRF technology to enable the manufacture of meter-scale aspheric optics.
QED, in conjunction with the Steward Observatory Mirror Laboratory (SOML) at the University of Arizona,
demonstrated the fabrication of an 840 mm diameter convex asphere with 1.3 mm of aspheric departure from a best-fit
sphere. Long-trace profilometry scans were initially performed at SOML to characterize the surface. A first figure
correction polishing iteration was conducted at QED Technologies in Rochester, NY on a meter-class MRF machine
(Q22-950F). The correction improved the surface to within the capture range of a full aperture interferometric test
performed at the Mirror Lab. A final polishing iteration at QED improved the surface to meet the optic specifications.
There are four fundamental steps to precision glass aspheric manufacturing: 1) grinding - to generate the rough shape, 2)
pre-polishing - to remove subsurface damage and smooth grinding residuals, 3) metrology - to quantify surface figure
errors and 4) finishing - to reach final figure and roughness specification. The aspheric pre-polish step is currently the
least deterministic process, as conventional sub-aperture tools (e.g. pitch, polyurethane pad) inherently have removal rate
variation due to tool misfit, pad wear, or slurry variation over time. This limits the final figure accuracy achievable or at
a minimum, leads to significant unpredictability in cycle time. Magnetorheological Finishing (MRF<sup>(R)</sup>) offers a very
deterministic finishing process, but is limited in its ability to smooth certain spatial frequencies. In this paper, we
present a complete polishing process (pre-polishing + finishing) that utilizes a novel combination of MRF and
conventional small-tool pitch polishing. This combined approach leverages the strengths of both processes, providing a
fast, deterministic, scalable aspheric finishing process to compliment existing grinding technology. Results for a 300
mm rectangular aperture, off-axis section will be presented showing final peak-to-valley quality of better than twentieth
New optical designs containing freeform optics have recently begun appearing in systems. Applications have
incorporated parts ranging in size from small (e.g.: ~5 – 10 mm rectangles) to large (e.g.: astronomical applications).
To meet these needs, QED Technologies recently introduced two solutions using its Q22-Y and Q22-950F platforms.
Magnetorheological Finishing® (MRF®) is a production proven technology for deterministically finishing symmetric
parts (flats, spheres, and on-axis aspheres) using a rotational toolpath, and rectangular flats and cylinders using a raster
toolpath. The new freeform toolpath expands the raster capabilities of the Q22-Y and Q22-950F machines to include
spheres, aspheres, off-axis sections, and true freeform geometries.
The freeform raster toolpath was first introduced on a meter-class optic platform, the Q22-950F. As optics grow in size,
the mass typically scales as well. This in turn increases the demands on the machine dynamics to meet rotational
polishing requirements. The raster freeform toolpath solution greatly reduces the machine dynamics and is employed to
polish a wide variety of part shapes, sizes, and geometries. A similar version of the toolpath was subsequently
implemented on the smaller Q22-Y platform. This paper will compare the implementations on each platform, describe
the benefits of the toolpath for existing and new applications, and present results from demonstrations on the two platforms.
Fabrication of large optics has been a topic of discussion for decades. As early as the late 1980s, computer-controlled
equipment has been used to semi-deterministically correct the figure error of large optics over a number of process
iterations. Magnetorheological Finishing, MRF®, was developed and commercialized in the late 1990's to predictably
and reliably allow the user to achieve deterministic results on a variety of optical glasses, ceramics and other common
optical materials. Large and small optics such as primary mirrors, conformal optics and off-axis components are
efficiently fabricated using this approach. More recently, specific processes, MR Fluids and equipment have been
developed and implemented to enhance results when finishing large aperture sapphire windows.
MRF, by virtue of its unique removal process, overcomes many of the drawbacks of a conventional polishing process.
For example, lightweighted optics often exhibit a quilted pattern coincident with their pocket cell structure following
conventional pad-based polishing. MRF does not induce mid-frequency errors and is capable of removing existing quilt
patterns. Further, odd aperture shapes and part geometries which can represent significant challenges to conventional
polish processing are simply and easily corrected with MRF tools. Similarly, aspheric optics which can often present
multiple obstacles-particularly when lightweighted and off-axis−typically have a departure from best-fit sphere that is
not well matched with to static pad-based polishing tools resulting in pad misfit and associated variations in removal.
The conformal subaperture polishing tool inherent to the QED process works as well on typical circular apertures as it
does on irregular shapes such as rectangles, petals and trapezoids for example and matches the surface perfectly at all
points. Flats, spheres, aspheres and off-axis sections are easily corrected. The schedule uncertainties driven by edge
roll and edge control are virtually eliminated with the MRF process.
This paper presents some recent results of the deterministic finishing typified by the QED product line and more
specifically of its large-aperture machines, presently capable of finishing optics up to one meter in size. Examples of
large sapphire windows and meter-class aspheric glass optics will be reviewed. Associated metrology concerns will also
Optics manufactured for infrared (IR) applications are commonly produced using single point diamond turning (SPDT).
SPDT can efficiently produce spherical and aspheric surfaces with microroughness and figure error that is often
acceptable for use in this region of the spectrum. The tool marks left by the diamond turning process cause high surface
microroughness that can degrade performance when used in the visible region of the spectrum. For multispectral and
high precision IR applications, surface figure may also need to be improved beyond the capabilities of the SPDT
process. Magnetorheological finishing (MRF®) is a deterministic, subaperture polishing technology that has proven to be
very successful at simultaneously improving both surface microroughness and surface figure on spherical, aspheric, and
most recently, freeform surfaces. MRF has been used on many diamond turned IR materials to significantly reduce
surface microroughness from tens of nanometers to below 1 nm. MRF has also been used to successfully correct figure
error on several IR materials that are not diamond turnable.
This paper will show that the combination of SPDT and MRF technologies enable the manufacture of high precision
surfaces on a variety of materials including calcium fluoride, silicon, and nickel-plated aluminum. Results will be
presented for microroughness reduction and surface figure improvement, as well as for smoothing of diamond turning
marks on an off-axis part. Figure correction results using MRF will also be presented for several other IR materials
including sapphire, germanium, AMTIR, zinc sulfide, and polycrystalline alumina (PCA).
The fabrication and metrology of astronomical optics are very demanding tasks. In particular, the large sizes needed for
astronomical optics and mirrors present significant manufacturing challenges. One of the long-lead aspects (and primary
cost drivers) of this process has traditionally been the final polishing and metrology steps. Furthermore, traditional
polishing becomes increasingly difficult if the optics are aspheric and/or lightweight.
QED Technologies (QED(r)) has developed two novel technologies that have had a significant impact on the production
of precision optics. Magnetorheological Finishing (MRF(r)) is a deterministic, production proven, sub-aperture polishing
process that can enable significant reductions in cost and lead-time in the production of large optics. MRF routinely
achieves surface figure accuracy of better than 30 nm peak-to-valley (better than 5 nm rms) and microroughness better
than 1 nm rms on a variety of glasses, glass ceramics and ceramic materials. Unique characteristics of MRF such as a
comparatively high, stable removal rate, the conformal nature of the sub-aperture tool and a shear-mode material
removal mechanism give it advantages in finishing large and lightweight optics. QED has, for instance, developed the
Q22-950F MRF platform which is capable of finishing meter-class optics and the fundamental technology is scalable to
even larger apertures. Using MRF for large optics is ideally partnered by a flexible metrology system that provides full
aperture metrology of the surface to be finished. A method that provides significant advantages for mirror manufacturing
is to characterize the full surface by stitching an array of sub-aperture measurements. Such a technique inherently
enables the testing of larger apertures with higher resolution and typically higher accuracy. Furthermore, stitching lends
itself to a greater range of optical surfaces that can be measured in a single setup. QED's Subaperture Stitching
Interferometer (SSI(r)) complements MRF by extending the effective aperture, accuracy, resolution, and dynamic range of
a standard phase-shifting interferometer. This paper will describe these novel approaches to large optics finishing, and
present a variety of examples.
Optical fabrication process steps have remained largely unchanged for decades. Raw glass blanks have been rough-machined, generated to near net shape, loose abrasive or fine bound diamond ground and then polished. This set of processes is sequential and each subsequent operation removes the damage and micro cracking induced by the prior
operational step. One of the long-lead aspects of this process has been the glass polishing. Primarily, this has been driven by the need to remove relatively large volumes of glass material compared to the polishing removal rate to ensure complete damage removal. The secondary time driver has been poor convergence to final figure and the corresponding polish-metrology cycles. The overall cycle time and resultant cost due to labor, equipment utilization and shop efficiency is increased, often significantly, when the optical prescription is aspheric. In addition to the long polishing cycle times, the duration of the polishing time is often very difficult to predict given that current polishing processes are not deterministic processes. This paper will describe a novel approach to large optics finishing, relying on several innovative technologies to be presented and illustrated through a variety of examples. The cycle time reductions enabled by this approach promises to result in significant cost and lead-time reductions for large size optics. In addition, corresponding increases in throughput will provide for less capital expenditure per square meter of optic produced. This process, comparative cycles time estimates and preliminary results will be discussed.
There is an increasing demand for large sapphire windows for a number of defense related programs. Some of these emerging requirements call for windows that are on the order of half a meter in size with tight surface figure and transmitted wavefront requirements. Magnetorheological Finishing (MRF<sup>®</sup>) is a deterministic polishing process capable of rapidly converging to the required surface figure. MRF finishing of sapphire has been demonstrated with surface accuracies better than 0.07 μm peak-to-valley (0.010 μm RMS) and surface microroughness less than 1.0 nm RMS on circular and square apertures. As a sub-aperture polishing technique, MRF provides a mechanism for effectively addressing and correcting a variety of optical surface features. This is of particular interest when correcting the transmitted wavefront on windows. The process allows for correction of the optical wavefront when it is aberrated due to inhomogeneity in the material in addition to the errors in the surface. Another benefit is that MRF has been shown to remove subsurface damage left from prior fabrication steps and can improve surface roughness of pre-polished sapphire. We report on a predictable, lower-cost process for fabricating large-scale sapphire windows.