David D. Walker

Thirty Meter Telescope Site Testing I: Overview

As part of the conceptual and preliminary design processes of the Thirty Meter Telescope (TMT), the TMT site-testing team has spent the last five years measuring the atmospheric properties of five candidate mountains in North and South America with an unprecedented array of instrumentation. The site-testing period was preceded by several years of analyses selecting the five candidates: Cerros Tolar, Armazones and Tolonchar in northern Chile; San Pedro Martir in Baja California, Mexico; and the 13 North (13N) site on Mauna Kea, Hawaii. Site testing was concluded by the selection of two remaining sites for further consideration, Armazones and Mauna Kea 13N. It showed that all five candidates are excellent sites for an extremely large astronomical observatory and that none of the sites stands out as the obvious and only logical choice based on its combined properties. This is the first article in a series discussing the TMT site-testing project.

Edges in CNC polishing: from mirror-segments towards semiconductors, paper 1: edges on processing the global surface

Segment-edges for extremely large telescopes are critical for observations requiring high contrast and SNR, e.g. detecting exo-planets. In parallel, industrial requirements for edge-control are emerging in several applications. This paper reports on a new approach, where edges are controlled throughout polishing of the entire surface of a part, which has been pre-machined to its final external dimensions. The method deploys compliant bonnets delivering influence functions of variable diameter, complemented by small pitch tools sized to accommodate aspheric mis-fit. We describe results on witness hexagons in preparation for full size prototype segments for the European Extremely Large Telescope, and comment on wider applications of the technology.

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.

Novel automated process for aspheric surfaces

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.

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.

Thirty Meter Telescope Site Testing VI: Turbulence Profiles

The results on the vertical distribution of optical turbulence above the five mountains which were investigated by the site testing for the Thirty Meter Telescope (TMT) are reported. On San Pedro Martir in Mexico; the 13 North site on Mauna Kea; and three mountains in northern Chile: Cerro Tolar, Cerro Armazones, and Cerro Tolonchar; MASS-DIMM turbulence profilers have been operated over at least two years. Acoustic turbulence profilers—SODARs—were also operated at these sites. The obtained turbulence profiles indicate that at all sites the lowest 200 m are the main source of the total seeing observed, with the Chilean sites showing a weaker ground layer than the other two sites. The two northern hemisphere sites have weaker turbulence at altitudes above 500 m, with 13N showing the weakest turbulence at 16 km, responsible for the large isoplanatic angle at this site. The influence of the jetstream and wind speeds close to the ground on the clear sky turbulence strength throughout the atmosphere are discussed, as well as seasonal and nocturnal variations. This is the sixth article in a series discussing the TMT site testing project.

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.

Final tests and commissioning of the UCL echelle spectrograph

The optical design and performance of the UCLES echelle spectrograph, installed at the f/37.7 coude focus of the 3.9-m AAT in June 1988, are described and illustrated with extensive diagrams, drawings, photographs, and sample spectra. The UCLES operates at 300-1100 nm with resolution 30,000-115,000 and adjustable collimated beam size; it employs either 31.6-g/mm or 79-g/mm echelle gratings and a train of UV-transmitting fused-silica prisms for cross-dispersion. Also discussed are the focal modifier lenses; the Bowen-Walraven image slicer; the commissioning procedures; and preliminary observations of Zeta Oph, planetary nebulae, and Seyfert galaxies.

Subsurface damage in precision ground ULE(R) and Zerodur(R) surfaces.

The total process cycle time for large ULE® and Zerodur® optics can be improved using a precise and rapid grinding process, with low levels of surface waviness and subsurface damage. In this paper, the amounts of defects beneath ULE® and Zerodur® surfaces ground using a selected grinding mode were compared. The grinding response was characterised by measuring: surface roughness, surface profile and subsurface damage. The observed subsurface damage can be separated into two distinct depth zones, which are: ‘process’ and ‘machine dynamics’ related.