John Simms

Zeeko/UCL process for polishing large lenses and prisms

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.

First aspheric form and texture results from a production machine embodying the precession 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.

Precessions process for efficient production of aspheric optics for large telescopes and their instrumentation

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.

Precessions aspheric polishing: new results from the development program

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.

New results from the Precessions polishing process scaled to larger sizes

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.

Recent developments of precessions polishing for larger components and free-form surfaces

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.