If one is to choose a single physical property of importance in optical applications in which metals differ markedly from other mirror substrate materials, the obvious choice is thermal conductivity. The overwhelming reason for this difference is that the valence electrons in metals are free of the influence of the individual atoms. In dielectric solids the valence electrons are an integral part of the interatomic bond; they are tightly captured by pairs or very small groups of neighboring atoms. The transport properties of metals can be fairly accurately calculated by assuming that the electrons behave as a dense gas cloud and by applying appropriate statistical mechanics theories.
This paper reviews the problem facing today's optical designer who must select materials and proper fabrication processes for metal mirrors that are to be used in challenging high energy laser programs. The materials in use today, the properties of these materials, the environmental constraints and the fabrication options open to the designer are reviewed in depth.
This paper summarizes development of large, lightweight, beryllium optical components pursued for nearly three years under a U.S. Air Force contract. The entire process of large beryllium optical technology is described, beginning with materials development, through the design and fabrication process, and final optical surface figuring. The results achieved to date are presented, and finally, the future direction and expectations of ultralightweight optical technologies that will eventually derive from this initial effort are addressed.
Diamond turning and precision generation of aspheric metal surfaces has promoted a change in lapping techniques due to the extremely close figure tolerances and surface finishes that have been achieved. In order to polish the unusual aspheric figures, we utilized special tooling, diamond abrasive, and silicon oil and techniques which we will describe in detail. Our studies include small flat diamond turned samples of copper, electroplated copper, electroplated silver, electroplated nickel and silver as well as large aspheres such as an f/0.75, 35 cm diameter copper ellipse. Results from cleaning studies on flat samples using ultrasonics and vapor degreasers will also be summarized. Interferograms of wavefront distortion and analysis of focal volume i ll ll be included as well as 10.6 pm reflectivity and a summary of laser damage experiments.
A lightweighted Beryllium mirror, with a weight of 32 pounds and an elliptical shape of 32 inches by 34 3/4 inches, was fabricated to a flat surface. The final figuring on the mirror was accomplished using computer controlled polishing. The removal tool consisted of small Kemet pads which individually conformed to the surface and were maintained at constant pressure. This tool was rotated at constant velocity as it transversed the piece in a raster scan pattern. Control over material removal was accomplished by varying the linear velocity of the tool along its path. Velocity data for the polisher was derived from analysis of a series of sub-aperture interferograms. The abrasive elements for the polishing operation were diamond particles suspended in a slurry. The lightweighted Beryllium mirror surface was fabricated with a O.03 RMS optical path difference from flat, where λ is 0.6328
This study adds evidence to the hypothesis that polishing can degrade the IR absorption of bare metal surfaces. The alloys of Al and Ti as well as the metallographic polishing techniques are described. Characterization of the samples included 10.6 pm absorption and reflectivity, total-integrated-scatter at 0.6 01, Nomarski microscopy,Taleystep surface roughness, Knoop hardness, and an x-ray nondispersive analysis attachment to a scanning electron microscope. The titanium's hardness did not significantly change during polishing and the absorption was reasonably correlated to the surface roughness for a range of 0.127 to 0.16 and 28 to 71 Å (rms) respectively. The absorption of the Al samples was not correlated to its roughness. The best polished sample had an absorption of 0.053 as compared to an unpolished (as received) sample which had an absorption of 0.022. The absorption of aluminum was well correlated to the microhardness indicating that the increase in absorption during the polishing procedure may be due to work hardening.
Many applications using metal optics require that the optical elements have very low scattering surfaces. A scatterometer suitable for measuring the scattering properties of metal mirror has been developed at AEDC. The scatterometer operates at two wavelengths, 0. 6328 and 10. 6 μm. Tests required to evaluate the performance of the scatterometer are described, and the directional scattering data off a high quality metal mirror are presented.
An evaluation of the off-specular scatter from a metal mirror is required for those radiometric systems where high off-axis rejection is achieved through low-scatter primary optics. This paper addresses the testing procedures for determination of the bi-directional reflectance distribution function (BRDF) at 10.64m, handling techniques for the prevention of degraded mirror performance through contamination, and the low-scatter performance achieved by the current fabrication technology for large, aspheric samples. The statistics of mirror surface sampling is also presented, as well as the distribution of the resultant measurements for a significant sampling of the surface area. The latter are used to demonstrate the impact that various types of surface imperfections have on low-scatter performance, as well as indicate typical behavior for the better low-scatter mirrors. These statistics also indicate the best performance that can be anticipated under the present technology once these imperfections are eliminated.
Data is presented which details investigations of substrate throughprint in low scatter nickel plated aluminum mirrors. Effects of fabrication procedures are examined. A novel method is described for photographically recording the visible light scattering of mirror surfaces at near specular angles. Application of the technique to quantitative scatter measurement is considered. Use of the test for inprocess inspection is described.
The state of the art in manufacturing large one piece metal mirrors has advanced dramatically in the past 10 years, following important pioneer work by NASA's Jet Propulsion Laboratory in 1965. Since that time, several additional large solar simulator mirrors have been manufactured. Careful mechanical and thermal design analysis and structure material selection are important elements in a successful finished product. Manufacturing techniques for mirror structure fabrication borrow from the heavy metal industry, while optical processing still follows relatively classical methods. Requirements for electroless nickel plating, sea transportation, and improved long term optical surface protection are challenges that have resulted in innovative practical solutions. The recently completed 5.5 meter (18 foot) diameter mirror for the Japanese National Space Development Agency's new space center at Tsukuba, Japan, exemplifies much of the technical advancement achieved over the last 10 years.
This paper describes the vacuum cryogenic test program conducted on a 32-inch by 34-inch elliptical, flat, beryllium mirror over a temperature range of 300°K to 150°K. It discusses the arrangement of optical test equipment for in-situ measurements of mirror optical quality, describes the thermal environmental control and data system, and covers the test setup and operational activity. The optical measurements were made using a scat-ter plate interferometer in conjunction with a 36-inch diameter parabolic mirror and appropriate transfer optics arranged to work over a 65-foot optical path length. The main-tenance of optical alignment and the real-time adjustments of the interferometer were carried out remotely under the test environment using a TV display of the test interferogram.
A three-mirror electroless nickel- slated beryllium optical system is described which is used as the collecting telescope for the Visible Infrared Spin-Scan Radiometer (VISSR). The 16-inch diameter aper-ture, 114.7-inch focal length system is comprised of a flat, elliptically-shaped object-space scan mirror and a Ritchey-Chretien primary-secondary mirror pair. Hot pressed beryllium is used as the substrate material because of its light weight, structural rigidity, and thermal stability. Specific selection criteria include precision elastic limit (PEL), homogeneity, and isotropy. PEL is measured mechanically. Homo geneity and freedom from voids and inclusions are ensured by X-ray inspection. Anisotropy is evaluated from thermal expansion coefficients measured along three perpendicular axes. The scan mirror uses a structure of hollow triangular ribs to provide maximum strength and rigidity with minimum weight. The primary and secondary mirrors are three-point tangent bar mounted. All mirrors are plated with low-stres s electroless nickel. Phosphorus content analysis is used as a low stress indicator. Maximum plating stress levels are specified and are calculated from test strip measurements. Reflective surfaces are optically figured and coated with evaporated aluminum overcoated with silicon oxide. The finished system is designed for near diffraction limited performance in the visible.
Electroforming as a method of producing metal optics offers some advantages and disadvantages when compared to optics produced by mechanical forming methods or conventional grinding and polishing techniques. These advantages and disadvantages are discussed together with various methods of tooling for electroforming. Some key process control procedures are described for control of optical accuracy.
This paper describes some of the first work done to gather machinability data in order to better understand the mechanics of the diamond-turning process. Basic force data have been measured for the most common metals used in optical fabrication (electroplated gold, silver, OFHC copper, and 6061-T6 aluminum). In addition, force and surface deformation data were gathered on large and small-grained electroplated copper and electroplated nickel. Residual stress was measured as a function of the depth of cut in order to begin to understand how the history of the machining process could influence surface quality.
The Air Force Weapons Lab (AFWL) has a multiple-bounce reflectometer(1-3) which can measure 10.6 pm re-flectivities. This article will describe the capabilities of the reflectometer, discuss results of recent measurements and summarize what reflectivity one can expect for various mirror surfaces. Some of the mirrors have been used in operational systems. Many of the mirrors were polished and/or coated by the Developmental Optical Facility (DOF) at AFWL.