Modern high technology optical surface manufacture makes use of one or more of four basic manufacturing technologies: loose abrasive grinding and polishing, single-point diamond turning, multi-point (fixed-abrasive) machining, and position-dependent material alteration. Each is described and a general comparison made with respect to achievable results, versatility and applicability to astrophysical, surveillance, and other advanced system applications. Such manufacturing technologies also offer a number of natural extensions which serve to provide, at least, a dim view down the development pathway. Such things as diamond-turned visible light optics, single-point machining of glass, precision fixed-abrasive grinding, precise computer-controlled polishing, and altered index materials are explored as offering exciting possibilities in the future.
Conventionally, aspherical surfaces of revolution are produced by numerically controlled machines of the XYZ axis configuration or by correction of best fitting spherical surfaces using lapping and polishing techniques. Described here is a machine and means for converting this machine to achieve best fitting toroidal surfaces to a desired aspherical surface. The common chordal generator is a simple machine used to produce spherical surfaces. By revision, it may be made to produce aspherical surfaces of revolution.
Aspheric surface generation and precision machining have been important technologies at Hughes Optical Products, Inc. (formerly Optical Division, Bell & Howell Company) for over twenty years. Present machining capabilities and supporting services which are available on a custom basis are described. A variety of applications of diamond machining are illustrated, involving not only the usual reflective materials such as aluminum, copper, and electroless nickel but also such IR refractive materials as germanium, silicon, and chalcogenide glasses.
A number of papers have been presented on the merits of CNC (computer numerical control) diamond turning of aspheric optical components. This paper will describe the design and operating characteristics of a CNC single point diamond fly cutter, showing how CNC can be equally effective for producing highly accurate piano optical components.
During the summer of 1976, one of the co-authors reported on his view of the suitability and potential benefits of the application of diamond machining to the volume production of optical elements for infrared systems. Since that time, Honeywell has actively pursued and acquired the machining and associated technologies necessary to support a production optical component product line. Advances in machine-fabrication and test technologies have been reported in the areas of optical characterization, holographic interferometry, system assembly and alignment, infrared interferometry and applications to aspheric refractors. These efforts were all directed towards the establishment of a production-oriented fabrication capability covering a wide range of high-accuracy optical designs and applications which now resides within Diamond Electro-Optics, Inc. The purpose of this paper is to demonstrate the component accuracies that are available in large quantity via examples from a production history covering over 12,000 precision optical elements and the contribution of assorted optical testing techniques to their fabrication.
After presenting a rule of thumb to evaluate the economical benefit of diamond turning compared to conventional polishing, applications of spinner mirrors, injection molding masters, and X-ray optics are discussed. Non-optical applications are included as well as precision grinding materials incompatible with diamond turning. KEY WORDS: Diamond Turning, Precision Machining, Precision Grinding, Quartz Grinding, X-ray Optics.
The following discussion will cover the details of a study on 304 and 316 stainless steel. The purpose of the study was to attempt to optimize a hybrid method for producing specular mirrors in stainless steel and other ferrous materials like Be. The manufacturing approach taken involved the selection of a single point turning tool that would produce the minimum sub-surface damage condition. The criteria for sub-surface condition superseded that of an optimal surface condition due to the need for a post-polish of the materials. The reasons for this choice will become more clear.
A 64-inch swing, vertical spindle axis precision lathe has been constructed. The machine incorporates a multiple-path laser feedback system, capacitance gauges, a 32-bit computer and capstan drives to provide two axes of tool motion in a 32-inch radius by 20-inch length working volume. Dimensional stability of critical components is achieved through the use of low coefficient-of-thermal-expansion materials and temperature-controlled heat sinks. Projected accuracy of the machine is approximately one microinch rms.
LODTM, built by LLNL for DOD, is a precision vertical lathe used to diamond turn large optical parts (to 64" diameter) to 1 μinch rms figure error. The LODTM motion controls require precision sensors, precision servos, and a wideband (330 Hz) real time computer. Position is sensed to 1/40 μinch resolution by laser interferometers and differential capacitance gauges. Precision servos operating at 1/10 μinch resolution and at low velocities with zero backlash are required. A unique Fast Tool Servo (FTS), located close to the diamond tool, adjusts the final tool position. The LODTM controls coordinate the X, Z, and FTS servos to cause the tool to move on a specified contour and at a specified feedrate in the X-Z plane. The part floppy diskette commands this motion with a linear CNC and real time (32-bit) computer. During each 1.5 ms sample time, the computer must DMA 14 sensor readings into memory, calculate the X and Z Tool Coordinates, calculate the FTS input, output the following errors, output the FTS input, store "flight recorder" information, and check for anomalies.
A quantitative description of the deformation of an annular optical element subject to external forces has been developed. Expressions applicable when the width of the element is small compared to its radius provide distortion data for both free rings and rings supported on thin-wall cylinder segments ('tangent flanges'). This data may be used to guide the design of fixtures for diamond turning large annular optics.
High quality air temperature control can provide an excellent means for minimizing the thermal drift of machine tools and inspection instruments when other means are not practical. Improvements in air temperature control has been improved in several machining areas by as much as ten to one by the careful identification of heat loads and the application of some fundamental classical control theory.
This presentation focuses on the evolution of Computerized Numerical Controls (CNC) for precision machine tools. It starts with a discussion of some early designs and progresses through the design of the controls for POMA and LODTM. The final system discussed utilizes the knowledge gained in those projects to develop a more generalized CNC with many of the features contained in the more elaborate systems.
Diamond machining of optical surfaces, especially of metals, is a well-established commercial technology. It is appreciated that the conditions of machining, including tool geometry and machining fluid, can affect the quality of the resulting surfaces. It is not widely known, however, that this variation can be used to advantage in tailoring the properties of the resulting surface to a specific application. This paper describes the controllable variation in optical absorption, scatter, and mechanical properties that are possible on diamond single-point machined OFHC copper. The interrelation of these properties is verified with an evaluation using pulsed laser-induced damage.
I am sure that there are those of you who are saying, well, here we go again another commercial about Diamond Turning; or those machines are just too expensive! - we don't know the first thing about Single-Point Diamond Turning, and we could never afford the learning curve. Or how about this argument, Diamond Turning? Not enough business, too little return on investment; and some of you are wondering, "Is there anything really new going on?"
The stressed mirror polishing method is being used to produce two thin two-meter diameter off-axis parabola mirrors. These mirrors are prototypes for a large segmented mirror telescope and the initial objective is to achieve 0.025 micron (RMS) surface accuracy. The fabrication set-up will be described along with improvement modifications that have proven to be necessary. Polishing results to date will be presented.
A method for the absolute calibration of a flat optical surface is described. An algorithm which utilizes the orthogonal properties of the Zernike polynomials was used on interferometric data to fabricate and certify three 10-inch Zerodur reference flats to λ/100 peak-to-valley quality.
The University of Arizona has initiated the construction of a precision generator for larger diameter optical surfaces (large optical generator). This LOG machine will be capable of generating a rotationally symmetric primary mirror of up to 7.3 meters diameter, and segments of mirrors to 10 meters and larger diameters. Initially the machine will be used for generation of molds for a sub-millimeter telescope array, but is eventually intended for wide application mirrors of visual wavelength accuracy.
A flexible, bound diamond material, recently developed for the stone surfacing industry, promises to be useful in the optics industry. Grinding rates and surface damage measurements have been made and compared to conventional grinding methods.
The increased demand for large, aspheric, optical components requires an interferometer that can accurately measure phase errors during the fabrication process. To meet this need, a phase-shifting infrared interferometer is being developed. This paper describes the design of the interferometer head.
This paper discusses the uses of ultrasonic non-destructive test procedures in evaluating water-cooled laser mirrors. The advantages, techniques and results of this method are presented, and some examples are included. Possible future applications are also mentioned.
An experimental carbon-fiber, graphite-epoxy, aluminum Flexcore sandwich panel roughly 1-m square was made by Dornier System, Friedrichshafen, West Germany. The panel was a pre-prototype of the panels to be used in the dish of the 10-m diameter Sub-Millimeter Telescope, a joint project of the Max-Planck-Institute fur Radioastronomie, Bonn, West Germany, and Steward Observatory, the University of Arizona in Tucson.
This paper outlines the fabrication process for the panel and indicates the surface accuracy of the panel replication process. To predict the behavior of the panel under various environmental loads, the panel was modeled structurally using anisotropic elements for the core material. Results of this analysis along with experimental verification of these predictions are also given.
Using two-wavelength holographic interferometry and a recyclable thermoplastic holographic camera, we developed a technique for real-time contouring of aspherical surfaces. Polished, fine-ground and coarse-ground surfaces were measured. Incorporating phase measurement interferometry with two-wavelength holographic interferometry, we have developed phase measurement two-wavelength holographic interferometry.
One method of correcting the aberrations of a mirror system is to add refractive corrector elements to the design. These elements provide the degrees of freedom required to achieve a corrected solution. An example of this approach is the Maksutov catadioptric system. This system consists of a primary mirror, in front of which is located a highly meniscus negative element. The meniscus element provides the compensation of the spherical aberration introduced by the spherical mirror. Many systems have been designed and constructed with more than one mirror and with multi-element corrector arrangements. The Maksutov system, consisting of a single spherical mirror and a single refractive element, is an example of the simplest form of correction using meniscus elements. In the fabrication of a well-corrected catadioptric system, which is required to perform near its theoretical limit, it is necessary to test the corrector elements to a high degree of accuracy. This paper describes the design of a special purpose tester for a particular refractive meniscus corrector element, the Strong Shell of the Perkin-Elmer Micralign Model 500 Projection Mask Alignment System. The Model 500 is a catadioptric system used by the semiconductor industry for optical microlithography on silicon wafers. A key factor in the optical design of the special purpose tester was to arrive at a solution that would allow all elements within their constructional tolerances for radius, thickness, and wedge to be evaluated with little further adjustment to the tester. This characteristic greatly accelerates testing and improves test data repeatability. This was accomplished by the selection of the testing conjugate point, a key factor in the overall design of the tester. A brief description of the Model 500 optical design, including the function of the strong shells is provided. Also provided is a description of the tester as well as the methods for alignment and certification.
A brief review of results from recent cryogenic tests of fused-silica mirrors is given with consideration of the implications for the design of cooled infrared telescopes. Implications include optical performance with a discussion of the top-down optical error budgeting for the Shuttle Infrared Telescope Facility (SIRTF), thermal properties of the mirrors, and mirror mounting.
Corrector plates for Schmidt-Cassegrain systems are subjected to equivalent focal length changes if the neutral zone varies from the established 0.86 zone. Fabricating a plano-convex element first, before changing to a corrector plate (C .P.) removes almost 50% slope departure. Shifting of the neutral zone is caused by the non-control of the grinding with glass tool rings. The control of the neutral zone can be accomplished by other fabrication methods.