The Wide Field X-ray Telescope (WFXT) is a medium class mission proposed to address key questions about cosmic origins and physics of the cosmos through an unprecedented survey of the sky in the soft X-ray band (0.2-6 keV) , . In order to get the desired angular resolution of 10 arcsec (5 arcsec goal) on the entire 1 degrees Field Of View (FOV), the design of the optical system is based on nested grazing-incidence polynomial profiles mirrors, and assumes a focal plane curvature and plate scale corrections among the shells. This design guarantees an increased angular resolution also at large off-axis positions with respect to the usually adopted Wolter I configuration. In order to meet the requirements in terms of mass and effective area (less than 1200 kg, 6000 cm2 @ 1 keV), the nested shells are thin and made of quartz glass. The telescope assembly is composed by three identical modules of 78 nested shells each, with diameter up to 1.1 m, length in the range of 200-440 mm and thickness of less than 2.2 mm. At this regard, a deterministic direct polishing method is under investigation to manufacture the WFXT thin grazing-incidence mirrors made of quartz. The direct polishing method has already been used for past missions (as Einstein, Rosat, Chandra) but based on much thicker shells (10 mm ore more). The technological challenge for WFXT is to apply the same approach but for 510 times thinner shells. The proposed approach is based on two main steps: first, quartz glass tubes available on the market are ground to conical profiles; second the pre-shaped shells are polished to the required polynomial profiles using a CNC polishing machine. In this paper, preliminary results on the direct grinding and polishing of prototypes shells made by quartz glass with low thickness, representative of the WFXT optical design, are presented.
The progressive transition from Excimer to EUV lithography is driving a need for flatter and smoother photomasks. It is
proving difficult to meet this next generation specification with the conventional chemical mechanical polishing
technology commonly used for finishing photomasks. This paper reports on the application of sub-aperture CNC
precessed bonnet polishing technology to the corrective finishing of photomask substrates for EUV lithography. Fullfactorial
analysis was used to identify process parameters capable of delivering 0.5 nm rms surface roughness whilst
achieving removal rates above 0.1 mm3/min. Experimental results show that masks pre-polished to 300~600 nm P-V flatness by CMP can then be improved down to 50~100 nm P-V flatness using the automated technology described in
this paper. A series of edge polishing experiments also hints at the possibility of increasing the quality area beyond the 5
mm defined in the official EUV photomask specification.
On one hand, the “float polishing” process consists of a tin lap having many concentric grooves, cut from a flat by single
point diamond turning. This lap is rotated above a hydrostatic bearing spindle of high rigidity, damping and rotational
accuracy. The optical surface thus floats above a thin layer of abrasive particles. But whilst surface texture can be
smoothed to ~0.1nm rms (as measured by atomic force microscopy), this process can only be used on flat surfaces. On
the other hand, the CNC “fluid jet polishing” process consists of pumping a mixture of water and abrasive particles to a
converging nozzle, thus generating a polishing spot that can be moved along a tool path with tight track spacing. But
whilst tool path feed can be moderated to ultra-precisely correct form error on freeform optical surfaces, surface finish
improvement is generally limited to ~1.5nm rms (with fine abrasives). This paper reports on the development of a novel
finishing method, that combines the advantages of “fluid jet polishing” (i.e. freeform corrective capability) with “float
polishing” (i.e. super-smooth surface finish of 0.1nm rms or less). To come up with this new “hybrid” method,
computational fluid dynamic modeling of both processes in COMSOL is being used to characterize abrasion conditions
and adapt the process parameters of experimental fluid jet polishing equipment, including: (1) geometrical shape of
nozzle, (2) position relative to the surface, (3) control of inlet pressure. This new process is aimed at finishing of next
generation X-Ray / Gamma Ray focusing optics.
Ultra-precision diamond turning can deliver very accurate form, often less than 100nm P-V. A possible manufacturing
method for thin Wolter type-1 mirrors in hard X-ray space telescopes thus involves generating electroless nickel plated
mandrels by diamond turning, before coating them with a reflective film and substrate. However, the surface texture
after turning falls far short from the requirements of X-ray and EUV applications. The machining marks need to be
removed, with hand polishing still widely employed. There is thus a compelling need for automated finishing of turned dies. A two step finishing method is presented that combines fluid jet and precessed bonnet polishing on a common 7-axis CNC platform. This method is capable of finishing diamond turned electroless nickel plated dies down to 0.28nm rms roughness, while deterministically improving form error down to 30nm P-V. The fluid jet polishing process, which consists of pressurizing water and abrasive particles for delivery through a nozzle, has been specially optimized with a newly designed slurry delivery unit and computer simulations, to remove diamond turning marks without introducing another waviness signature. The precessed bonnet polishing method, which consists of an inflated membrane rotated at an angle from the local normal to the surface and controlled by geometrical position relative to the work-piece, is subsequently employed with a novel control algorithm to deliver scratch-free surface roughness down to 0.28 nm rms. The combination of these two deterministic processes to finish aspheric and freeform dies promises to unlock new frontiers in X-ray and EUV optics fabrication.
The next generation wide-field X-ray telescope (WFXT), to be implemented beyond eRosita and proposed within the
NASA RFI call 2011, requires an angular resolution of less than 10 arcsec (with goal of 5”) constant across a wide field
of view (1 deg2). To achieve this requirement the design is based on nested modified grazing incidence Wolter-I mirrors with polynomial profiles. Our goals in terms of mass and stiffness can be meet with the use of fused silica glass, a wellknown material with good thermo-mechanical properties and polishability characteristics, together with an innovative polishing approach. Here we present the X-ray calibration results obtained for a prototypal shell tested in fullillumination mode at the Panter/MPE facility.
The next generation wide-field X-ray telescope (WFXT) will require an angular resolution of ~5-10 arcsec almost
constant across a wide field of view (~1 deg2 diameter). To achieve this goal, the design of the optical system has to be
based on mirrors characterized by short length and polynomial profiles, as well as focal plane curvature and plate scale
corrections. These concepts guarantee an improved angular resolution at large off-axis angle with respect to the normally
used Wolter-I configuration. These telescopes are therefore optimal for survey purposes. A significant increase of
effective area and grasp with respect to previous missions must also be achieved. This is possible with high precision but
at the same time thin (2-3 mm thickness for mirror diameters of 30-110 cm) glass mirror shells. To achieve the goal of 5
arcsec and improve further the technology, we are considering different materials. Fused silica, a well-known material
with good thermo-mechanical and polishability characteristics provide the best choice. To bring the mirror shells to the
needed accuracy, we are adopting a deterministic direct polishing method (already used for past missions as Einstein,
Rosat, Chandra). The technological challenge now is to apply it for almost ten times thinner shells.
Electroless Nickel (ENi) and binderless Tungsten Carbide (WC) are materials widely used in industry to make
replication moulds for precision optics, with applications ranging from consumer camera lenses to high accuracy X-ray
mirrors. The aspheric shape generation is generally performed by diamond turning in the case of Nickel, and micro-grinding
in the case of Tungsten Carbide. However, both machining methods fall short from meeting the ultra-precision
criteria required by an increasing number of applications, because of insufficient form accuracy and the frequency
content of the machining marks they leave on the surface. It is thus commonly observed in industry that moulds need to
be subsequently polished by hand, a usually slow and human resource intensive operation. The Zeeko 7-axis CNC
machine, equipped with sub-aperture fluid jet and precessed bonnet polishing technology, has been used to develop
deterministic finishing processes on both Electroless Nickel and Tungsten Carbide. Corrective polishing to less than λ/20
(<31nm PV) form error can be achieved, as well as the ability to smooth surface texture down to 1nm Ra or less, in a
time efficient manner.
The Wide Field X-ray Telescope (WFXT) is a medium class mission for X-ray surveys of the sky with an unprecedented
area and sensitivity. In order to meet the effective area requirement, the design of the optical system is based on very thin
mirror shells, with thicknesses in the 1-2 mm range. In order to get the desired angular resolution (10 arcsec requirement,
5 arcsec goal) across the entire 1x1 degree FOV (Field Of View), the design of the optical system is based on nested
modified grazing incidence Wolter-I mirrors realized with polynomial profiles, focal plane curvature and plate scale
corrections. This design guarantees an increased angular resolution at large off-axis angle with respect to the normally
used Wolter I configuration, making WFXT ideal for survey purposes. The WFXT X-ray Telescope Assembly is
composed by three identical mirror modules of 78 nested shells each, with diameter up to 1.1 m. The epoxy replication
process with SiC shells has already been proved to be a valuable technology to meet the angular resolution requirement
of 10 arcsec. To further mature the telescope manufacturing technology and to achieve the goal of 5 arcsec, a
deterministic direct polishing method is under investigation. The direct polishing method has already been used for past
missions (as Einstein, Rosat, Chandra): the technological challenge now is to apply it for almost ten times thinner shells.
Under investigation is quartz glass (fused silica), a well-known material with good thermo-mechanical and polishability
characteristics that could meet our goal in terms of mass and stiffness, with significant cost and time saving with respect
to SiC. Our approach is based on two main steps: first quartz glass tubes available on the market are grinded to conical
profiles, and second the obtained shells are polished to the required polynomial profiles by CNC (Computer Numerical
Control) polishing machine. In this paper, the first results of the direct grinding and polishing of prototypes shells made
by quartz glass with low thickness, representative of the WFXT optical design, are presented.
In this paper we address two interrelated issues important to primary mirror segments for extremely large telescopes - edge-control, and the detailed topography over the segment surface. Both affect the intensity and distribution of stray
light and infrared emissivity. CNC polishing processes typically deploy spiral or raster tool-paths, tending to leave
repetitive features. We compare and contrast two novel families of pseudo-random tool-paths for Precessions CNC
polishing. We then show how CNC control of the three-dimensional tool-path can optimize edge-profiles. Finally, we
demonstrate fluid-jet polishing used to clean up residual edge defects.
This paper describes a major advance in the post-treatment of diamond-turned surfaces to remove repetitive micro-structure;
a result which could have a major beneficial impact on fabrication of Walter-type X-ray mandrels, and metal
mirrors. Diamond-turning is highly deterministic and versatile in producing axially-symmetric forms, and through fast-tool
servos, non-axially symmetric, free-form and micro-structured surfaces. However, the fine turning marks left in the
metal surface limit performance. In this paper, we describe how fluid-jet polishing under CNC control can be used to
eliminate these structures, without significantly degrading the surface roughness or form produced by the prior turning
The 'Zeeko Classic' polishing process is implemented in a series of CNC machine-tools. The standard tooling utilizes
inflated membranes ('bonnet') covered with standard polishing cloths, and flooded by a supply of re-circulating
polishing slurry. The usual input quality is a part off a precision CNC grinding machine, and the process both polishes
and corrects form. In this paper we demonstrate how dynamic range can be substantially extended using three distinct
Zeeko Grolishing processes that are hybrids between loose-abrasive polishing and bound-abrasive grinding. The output
quality and volumetric removal rates of these processes are compared and contrasted. Finally, we note how these hybrid
processes can extend the capabilities of the machine from polishing and form control, to smoothing parts with inferior
input-quality, removing larger volumes of material during form control, and addressing harder materials.
This paper reports on the commissioning of the first of Zeeko's "IRP1200" 1.2m capacity 7-axis automated CNC polishing machines. These combo machines now support five different removal regimes, which are described. The machines differ substantially from Zeeko's more familiar 200mm machines on which we have focused before, in terms of overall architecture and detailed design. Large and small optics place different demands on part-fixturing, tooling, machine speeds and accelerations, metrology, slurry-handling, part-loading and access etc. These have had a profound effect on the development-path from 200 to 1.2m machines. Moreover, an advance in the kinematic design has extended the allowable range of surface slope-angles from typically 30° up to a hemisphere. The paper presents results from the pass-off trials, the first fluid-jet experiment, and the development of tooling to address a requirement to smooth a part with a local defect.
The requirements of space and defence optical systems and ground-based astronomy (especially extremely large telescopes) are providing optical fabricators with new challenges. These challenges particularly concern process speed, determinism and automation, and tighter tolerances on surface form and texture. Moreover, there is a growing demand for complex off-axis and 'freeform' surfaces and for larger components of the ~1m scale.
With this in view, we first report on form-correction on a smaller analogue of the IRP1200: an IRP400 in service in industry. We then report on the design, commissioning and preliminary process-development results from the first of the scaled-up 1.2m capacity CNC polishing machine from Zeeko, Ltd. This machine delivers the 'Classic' bonnet-based process, together with two new processes: fluid-jet polishing and the hybrid soft-grinding/polishing process called 'Zeeko-Grolish.' We indicate how this trio of processes running on the same machine platform with unified software can provide an unprecedented dynamic range in both volumetric removal rate and removal spot-size. This leads into a discussion of how these processes may be brought to bear on optimal control of texture and form. Preliminary performance of the 1.2m machine is illustrated with results on both axially-symmetric and more complex removal regimes. The paper concludes with an overview of the relevance of the technology to efficient production of instrumentation-optics, space optics and segmented telescope mirrors.
The recent upsurge in the demand for off-axis and complex "freeform" optical surfaces is driving the development of novel processes for their fabrication. This paper focuses on recent developments of the Precessions CNC polishing process for freeform surfaces, including off-axis as a special case. First, the surface-prescription and metrology-data, and their relation to the data-input for the polishing machines, are considered. The relevance of consistent coordinate frames is emphasised. An outline of how the process can 'polish' a ground freeform part (improve the texture), and then 'figure' the part (reduce the form errors) is given. Specific experimental case-studies are then presented, illustrating the versatility of the process on different materials and forms. Recent work is included in which the process-speed has been moderated in order to remove tens of nanometres of stock material, rather then the more usual hundreds of nanometres to tens of microns as in the standard Precessions process. The relevance of this to improving the ultimate surface-precision that should be achievable by this method is described. As a final illustration, the potential of the process to the rapid fabrication of the hundreds to thousands of 1-2 metre class mirror segments required for extremely large telescopes is considered.
Zeeko's Precession polishing process uses a bulged, rotating membrane tool, creating a contact-area of variable size. In separate modes of operation, the bonnet rotation-axis is orientated pole-down on the surface, or inclined at an angle and then precessed about the local normal. The bonnet, covered with standard polishing cloth and working with standard slurry, has been found to give superb surface textures in the regime of nanometre to sub-nanometre Ra values, starting with parts directly off precision CNC aspheric grinding machines. This paper reports an important extension of the process to the precision-controlled smoothing (or 'fining') operation required between more conventional diamond milling and subsequent Precession polishing. The method utilises an aggressive surface on the bonnet, again with slurry. This is compared with an alternative approach using diamond abrasives bound onto flexible carriers attached to the bonnets. The results demonstrate the viability of smoothing aspheric surfaces, which extends Precessions processing to parts with inferior input-quality. This may prove of particular importance to large optics where significant volumes of material may need to be removed, and to the creation of more substantial aspheric departures from a parent sphere. The paper continues with a recent update on results obtained, and lessons learnt, processing free-form surfaces, and concludes with an assessment of the relevance of the smoothing and free-form operations to the fabrication of off-axis parts including segments for extremely large telescopes.
Since the 2003 Annual Meeting, the Precessions process has become accepted as an efficient method for polishing and figuring moderate-sized axially-symmetric aspheric parts in industry. In this paper, we report on some very significant new advances beyond this capability. The first is the demonstration of the process on substantially larger diameter parts than worked hitherto - in particular, a precision-ground 500mm diameter deeply-concave aspheric mirror. We describe the consequences of polishing large parts with the axis of the part vertical, in contrast to the horizontal axis of the smaller machines. Issues include slurry puddling and settlement in concave forms, process-uniformity, adequate support of the part and handling. We then report on recent work developing the Precessions process for non axially-symmetric surfaces including free-form. The correct relationship of the process with metrology has proved to be complex on several fronts, one example being differing descriptions of form either along a surface or its projection. We present our experience using profilometry and interferometry on precision-ground and polished surfaces, and in achieving absolute form with known base radius. Finally, we remark on the potential power of a priori predictions of achievable surface quality when optimizing optical system designs.
The Precessions process uses an inflated membrane-tool that delivers near-Gaussian polishing spots. The tool-motion over the part can be constructed to preserve an aspheric form whilst removing damage from preceding processes, or control the form through a tool-path prescribed by numerical optimization. The process has previously been validated on surfaces up to 200mm diameter and used extensively in industrial environments. In this paper we report the first trials on a substantially larger part - a 500mm diameter f/1 ellipsoidal mirror - as part of the UK’s technology-development for Extremely Large Telescopes. We draw attention to subtle problems that have arisen along the way. We also report on developing the process for free-form surfaces, in contrast to the axially-symmetric parts worked hitherto. The paper concludes with an assessment of the lessons learnt from the experiments, as they may impact on realization in a practical ELT segment fabrication facility.
The Precessions process for producing aspheric and other optical surfaces is undergoing rapid development. In this paper, we summarise the considerable success achieved in controlling the repeatability of the process on both the 200mm and 600mm machines, and illustrate this with examples of aspherics that have been produced. We particularly describe our approach to fine form-control. This has required the development of various strategies to moderate the volumetric removal rates, in order to give the required sensitivity of removal. We conclude with a discussion of the scaling laws that apply when adapting the process to smaller and larger sized parts. This is illustrated by predicting the process-parameters for mass-producing segments for extremely large telescopes.
We summarize the reasons why aspheric surfaces, including non-rotationally-symmetric surfaces, are increasingly important to ground and space-based astronomical instruments, yet challenging to produce. We mainly consider the generic problem of producing aspheres, and then lightweight segments for the primary mirror of an Extremely Large Telescope. We remark on the tension between manufacturability of spherical segments, and performance with aspheric segments. This provides the context for our presentation of the novel Precessions process for rapid polishing and form-correction of aspheric surfaces. We outline why this is a significant step beyond previous methods to automate aspheric production, and how it has resulted in a generalized, scaleable technology that does not require high capital-value tooling customized to particular types of optical form. We summarize implementation in the first two automated CNC machines of 200mm capacity, followed by the first 600mm machine, and the current status of the process-development program. We review quantitative results of polishing trials, including materials relevant to large and instrumentation optics. Finally, we comment on the potential of the technology for space optics and for removing quilting in honeycomb substrates.
We first consider the potential impact of a technology that could deliver polished, accurate aspheric surfaces in a routine and automated manner. We then summarise the technical challenge, and present an appraisal of the performance of the novel 'Precessions' process, which is a major advance in this direction. We outline the design concepts behind the productionized CNC polishing machines which executes the process, and then describe the progress developing strategies to preserve form when polishing ground surfaces, and to correct form on both pre-ground and polished surfaces. Particular consideration is given to resolving the inherent difficulties of control of centres on rotationally-symmetric parts. We then report on experimental results achieved with the machines. Finally, we present our programme to extend the control-algorithms to handle fully free-form surfaces, and draw conclusions about the effectiveness and generality of the 'Precessions' technique.
This paper describes progress on the development of a new process for producing precision surfaces for the optics industry, and potentially for other sectors including silicon wafer fabrication and lapping and polishing of precision mechanical surfaces. The paper marks an important milestone in the development program, with the completion of the construction of the first fully-productionized machine and the first results from the commissioning process.
We report on progress developing the Precession Process, that has recently been embodied for the first time in a fully-productionised aspheric polishing machine. We describe how the process uses inflated polishing tools of continuously-variable size and hardness. Despite the rapid tool rotation needed to give high removal rates, the method produce well-behaved and near-Gaussian tool influence functions, by virtue of the precession of the spin axis. We then describe how form errors are controlled. The method takes influence-function data and an error map as input, together with, uniquely, weighting factors for height and slope residuals and process time. A numerical optimisation of the cost function with variable dwell time, tool path and tool size is then performed. The advantages of this new technique are contrasted with conventional deconvolution methods. Results of form control on aspheric surfaces are presented, with an interpretation in terms of spatial frequencies. We draw particular attention to control of form at the centre and periphery of a workpiece. Finally, we describe how Precession processing gives multi- directional rubbing of surfaces, and we present the superb texture achieved on samples.
We report on the development of a novel industrial process, embodied in a new robotic polishing machine, for automatically grinding an polishing aspheric optics. The machine is targeted at meeting the growing demand for inexpensive axially symmetric but aspherical lenses and mirrors for industry and science, non-axisymmetric and conformal optics of many kinds, the planarization of silicon wafers and associated devices, and for controlling form and texture in other artifacts including prosthetic joints. We describe both the physics and the implementation of the process. It is based on an innovative pressurized tool of variable effective size, spun to give high removal rate. The tool traverse and orientation are orchestrated in a unique (and patented) way to avoid completely the characteristic fast peripheral-velocity and center-zero left by conventional spinning tools. The pressurized tooling supports loose abrasive grinding and polishing, plus a new bound-abrasive grinding process, providing for a wide range of work from coarse profiling to fine polishing and figuring. Finally we discuss the critical control, data handling and software challenges in the implementation of the process, contrast the approach with alternative technologies, and present preliminary results of polishing trials.