A durable silver coating was developed and applied to the Kepler Space Telescope primary mirror. The coating
was manufactured by an ion-assisted evaporation process and coating uniformity was better than 30-nm PTV over the
1.4-m mirror aperture. The protection scheme for silver was devised and patented twelve years ago by Lawrence
Livermore National Laboratory (LLNL) in the United States. An interference coating was added to the basic protected
silver design, to enhance the reflectance from 400-nm to the near infrared.
For many years, lightweighted sandwich-style mirror blanks made from Corning's Ultra-Low Expansion glass (ULE) have been used in space telescope systems that demand superior optical performance. Despite the superior performance of this technology, the historically high cost and long schedule to procure such a blank has limited their use to only the most elite missions. Future missions, such as JWST, will seek to dramatically reduce the historical cost/schedule paradigm for ULE blanks by capitalizing on economies-of-scale associated with a multi-segment design. However, for this blank technology to become accessible to a broader range of missions, fundamental changes in technical and business approaches are needed. Over the last four years, ATK COI has worked to develop the requisite technologies to produce ULE mirror blanks in-house, with an emphasis on reducing cost and schedule. Our focus has been in three areas: process development to enable reclamation of ULE glass residuals, glass fusion process qualification, and tooling cost reduction. The status of each of these areas is presented, and conclusions drawn about possible future costs of lightweight ULE mirror blanks.
The Herschel Space Observatory (formerly known as FIRST) consists of a 3.5 m space telescope designed for use in the long IR and sub-millimeter wavebands. To demonstrate the viability of a carbon fiber composite telescope for this application, Composite Optics Incorporated (COI) manufactured a fast (F/1), large (2 m), lightweight (10.1 kg/m2) demonstration mirror. A key challenge in demonstrating the performance of this novel mirror was to characterize the surface accuracy at cryogenic (70 K) temperatures. A wide variety of optical metrology techniques were investigated and a brief survey of empirical test results and limitations of the various techniques will be presented in this paper. Two complementary infrared (IR) techniques operating at a wavelength of 10.6 microns were chosen for further development: (1) IR Twyman-Green Phase Shifting Interferometry (IR PSI) and (2) IR Shack-Hartmann (IR SH) Wavefront Sensing. Innovative design modifications made to an existing IR PSI to achieve high-resolution, scannable, infrared measurements of the composite mirror are described. The modified interferometer was capable of measuring surface gradients larger than 350 microradians. The design and results of measurements made with a custom-built IR SH Wavefront Sensor operating at 10.6 microns are also presented. A compact experimental setup permitting simultaneous operation of both the IR PSI and IR SH tools is shown. The advantages and the limitations of the two key IR metrology tools are discussed.
The Herschel Space Observatory (formerly known as FIRST) consists of a 3.5 m space telescope. As part of a JPL- funded effort to develop lightweight telescope technology suitable for this mission, COI designed and fabricated a spherical, F/1, 2 m aperture prototype primary mirror using solely carbon fiber reinforced polymer (CFRP) materials. To assess the performance of this technology, optical metrology of the mirror surface was performed from ambient to an intended operational temperature for IR-telescopes of 70K. Testing was performed horizontally in a cryogenic vacuum chamber at Arnold Engineering Development Center (AEDC), Tennessee. The test incorporated a custom thermal shroud, a characterization and monitoring of the dynamic environment, and a stress free mirror mount. An IR-wavelength phase shifting interferometer (IR PSI) was the primary instrument used to measure the mirror surface. From an initial surface figure of 2.1 microns RMS at ambient, a modest 3.9 microns of additional RMS surface error was induced at 70K. The thermally induced error was dominated by low-order deformations, of the type that could easily be corrected with secondary or tertiary optics. In addition to exceptional thermal stability, the mirror exhibited no significant change in the figure upon returning to room temperature.
In the light of the recent successes in utilizing CFRP composites for fabrication of ultra-lightweight, micron- accuracy reflectors for space telescopes, this paper provides a recent assessment of the main factors influencing dimensional stability of composites. Two recent examples of all-composites reflector designs that demonstrate the validity of the composites choice for this type of space applications are presented.
The mass of the primary mirror has dominated the mass of larger aperture (> 1 m class) telescopes. Spaceborne telescopes have much to gain from a significant reduction in areal density. Areal density is often limited by the stiffness to weight ratio of the primary mirror. Two key factors drive this criteria: telescope structural characteristics (launch and deployment) and fabrication requirements. A new class of hybrid composite mirrors has been designed, prototyped, and fabricated to demonstrate the advantage of the high stiffness to weight ratio of carbon fiber composite materials and the superior optical fabrication for low expansion glasses. This hybrid mirror utilizes a unique `set and forget' fabrication technique. A thin meniscus of glass is mounted to a stiff composite support structure using composite flexure rods. The meniscus is lightweighted using waterjet pocket milling and is conventionally polished to a precise radius of curvature. This meniscus is then supported on the flexures and actuated to a precise figure. The flexures are fixed and the actuators are removed. The substrate is then ion figured to achieve the final figure. The areal density of this mirror is 10 kg/m2. Surface figure on a 0.25 m aperture prototype was demonstrated at better than (lambda) /4 (visible) prior to ion figuring. Two 0.6 m mirrors are under fabrication. The design of the mirror and results of the fabrication and testing will be discussed.
Composite materials are an ideal choice for the FIRST Telescope, since they provide dimensional stability, excellent stiffness to weight ratios, near zero thermal expansion, and manufacturing flexibility. The most challenging aspect of producing an all-composite FIRST telescope, is the development of the lightweight primary mirror. The design of the primary mirror must satisfy requirements for surface accuracy to operating temperatures of 80 +/- K as well as stiffness and strength considerations during launch.
The Far Infrared and Submillimeter Telescope (FIRST), is an ESA cornerstone mission, that will be used for photometry, imaging and spectroscopy in the 80 to 670 micrometer range. NASA, through the Jet Propulsion Laboratory (JPL), will be contributing the telescope and its design to ESA. This paper will discuss the work being done by JPL and Composite Optics, Incorporated (COI), the developer of the primary mirror technology. Optical and mechanical constraints for the telescope have been defined by ESA and evolved from their trade studies. Design drivers are wave front error (10 micrometer rms with a goal of 6 micrometer rms), mass (260 kg), primary mirror diameter (3.5 m) and f number (f/0.5), and the operational temperature (less than 90 K). In response to these requirements a low mass, low coefficient of thermal expansion (CTE) telescope has been designed using carbon fiber reinforced polymer (CFRP). This paper will first present background on the JPL/COI CFRP mirror development efforts. After selection of the material, the next two steps, that are being done in parallel, are to demonstrate that a large CFRP mirror could meet the requirements and to detail the optical, thermal and mechanical design of the telescope.
Recent developments in the design and fabrication of very light-weight all-composite mirrors have made possible extremely well balanced, thermally stable, structures which distort very little when cooled. One such mirror is the Composite Optics, Incorporated all-composite mirror, M4, which has a 45.7 cm diameter and 3 cm thickness and a spherical surface of radius-of-curvature 2.92 meters. Relative figure measurements of this mirror were made with the Steward Observatory Light Weight Mirror Low Temperature Test Chamber over a temperature range from 20 C to -60 C using a 10.6 μm interferometer. The measurements show a remarkably small increase in the rms figure departure from a spherical surface of fixed radius-of-curvature of 0.27 μm over the 80 C temperature change. The effective coefficient of thermal expansion over this temperature range derived from the focus change is 0.66 x 10-6/C, close to that of fused silica.
The objective of this paper is to report the recent developments in lightweight mirror technology that have occurred at Composite Optics, Inc. The developments occurred as a result of the activities being conducted in support the Next Generation Space Telescope, microwave limb sounder, and small business innovative research programs. Our sponsors on these programs are the Marshall and Goddard Space Flight Centers and the JPL. The requirements, design approach, actual performance, and the technology status for each program are summarized in the following sections. The emergence of composite designs provides exciting potential for nontraditional, accurate, lightweight, stable, stiff, and high strength composite mirrors. This evolving technology promises significant improvement in reducing weight, cost and cycle time for future IR, visible, and ex- ray systems. Customers currently embracing composite mirror technology for radiometric use are already reaping substantial system performance benefits. Other customers interested in LIDAR, IR, visible, and grazing incidence x- ray applications are eagerly awaiting successful completion of current technology development and demonstration efforts.