Optical devices are extremely important since they play a critical role in optical recording and display. Single point diamond turning is one of the most common methods to create plastic optics. Diamond turning of plastics is influenced by a wide variety of factors such as the glass transition temperature of the polymer, other material properties and operator controlled cutting conditions. Since diamond is one of the hardest materials in nature and polymers are relatively soft, little tool wear is expected. But the optics industry claims that tool wear is a major problem. Most of the optical industry uses Poly (methyl methacrylate) (PMMA) and Polycarbonate (PC) for creating optics. The objective of this research is to optimize machining parameters (such as feed, depth of cut, cutting speed and rake angle) to produce optical surface quality (RMS Surface finish < 10 nm) while minimizing tool wear for the two materials. A wide range of experiments were performed on the two materials by varying machining parameters and measuring worn tools using the Electron Beam Induced Deposition (EBID) technique in the Scanning Electron Microscope (SEM). In the experimental conditions used, PMMA was found to have better surface finish than PC when machined with a zero-rake angle diamond tool. Polycarbonate was found to wear the tool more than PMMA under similar cutting conditions. It was also found that Polycarbonate is more sensitive to chip management and chip geometry than PMMA. Detailed effects of all machining parameters for the two materials were studied.
The NASA Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII) system is designed to study the infrared emissions from star formation and active galactic nuclei through a double-Fourier Michelson interferometer located on a balloon at an altitude of 37 km. The BETTII external optics include a pair of identical beam-reducing, four-mirror telescopes, each with a 522-mm aperture, nonrotationally symmetric primary mirror. These telescopes were designed and assembled at the North Carolina State University Precision Engineering Consortium and are composed entirely of thin-walled aluminum components. The mounting structure is designed to be light weight and stiff to reduce thermal equilibration time in the rarified air at the edge of space and to maintain robust alignment of the optical elements. The mounts also prevent deformation of the large optical elements via custom-built kinematic Kelvin couplings and fixed-load clamps; the maximum form error of the optical surfaces are 300 nm RMS. This work details the design of the thin mirrors and mounting structure as well as validation of the mount assembly process, mount stiffness, and the kinematic couplings.
Freeform Optical surfaces are defined as any non-rotationally symmetric surface or a symmetric surface that is rotated about any axis that is not its axis of symmetry. These surfaces offer added degrees of freedom that can lead to lower wavefront error and smaller system size as compared to rotationally symmetric surfaces. Unfortunately, freeform optics are viewed by many designers as more difficult and expensive to manufacture than rotationally symmetric optical surfaces. For some freeform surfaces this is true, but a designer has little or no feedback to quantify the degree of difficulty for manufacturing a surface. This paper describes a joint effort by Optical Research Associates (ORA) and the Precision Engineering Center (PEC) at North Carolina State University to integrate metrics related to the cost and difficulty of manufacturing a surface into the merit function that is used during the design of an optical system using Code V. By incorporating such information into the merit function, it is possible to balance optical performance and manufacturability early in the design process.
We present highlights from the American Society for Precision Engineering’s 2004 Winter Topical Meeting entitled "Free-Form Optics: Design, Fabrication, Metrology, Assembly." We emphasize those papers that are most relevant to astronomical optics. Optical surfaces that transcend the bounds of rotational symmetry have been implemented in novel optical systems with fantastic results since the release of Polaroid’s first instant camera. Despite these successes, free-form optics have found only a few niche applications and have yet to enter the mainstream. The purpose of this meeting was to identify the state of the art of free-form optics design, fabrication, metrology and assembly and to identify the technical and
logistical challenges that inhibit their widespread use. Issues that were addressed include: What are free-form optics? How can optical systems be made better with free-form optics? How can designers use free-form optics? How can free-form optics be fabricated? How can they be measured? How are free-form optical systems assembled?
Challenges in fabrication and testing have historically limited the choice of surfaces available for the design of reflective optical instruments. Spherical and conic mirrors are common, but, for future science instruments, more degrees of freedom will be necessary to meet performance and packaging requirements. These instruments will be composed of surfaces of revolution located far off-axis with large spherical departure, and some designs will even require asymmetric surface profiles. We describe the design and diamond machining of seven aluminum mirrors: three rotationally symmetric, off-axis conic sections, one off-axis biconic, and three flat mirror designs. These mirrors are for the Infrared Multi-Object Spectrometer instrument, a facility instrument for the Kitt Peak National Observatory’s Mayall Telescope (3.8 m) and a pathfinder for the future Next Generation Space Telescope multi-object spectrograph. The symmetric mirrors include convex and concave prolate and oblate ellipsoids, and range in aperture from 92 x 77 mm to 284 x 264 mm and in f-number from 0.9 to 2.4. The biconic mirror is concave and has a 94 x 76 mm aperture, (formula available in paper) and is decentered by -2 mm in x and 227 mm in y. The mirrors have an aspect ratio of approximately 6:1. The fabrication tolerances for surface error are < 63.3 nm RMS figure error and < 10 nm RMS microroughness. The mirrors are attached to the instrument bench using semi-kinematic, integral flexure mounts and optomechanically aligned to the instrument coordinate system using fiducial marks and datum surfaces. We also describe in-process profilometry and optical testing.