This paper presents the results of an experimental study of the nanometer-level mechanics of a precision deployable structure. The test article is a single bay of a deployable truss, which has both a high structural efficiency (stiffness to mass ratio), and high packaging efficiency. Illustrative data are presented which assess the suitability of the test article for use as an optical support structure, or optical bench. The results show new and unusual microdynamics, namely viscoelasticity and harmonic distortion. However, the structure otherwise exhibits an elastic, stable response within the nanometer-level resolution of the test apparatus. These results are the first experimental evidence that a high efficiency, joint dominated deployable structure might be suitable for optical applications. To the extent this supposition holds true for other structures, these results may have significant implications for the architecture of planned deployable optical spacecraft missions.
NASA Langley Research Center, Composite Optics, Inc., and Nyma/ADF have developed jointly a deployable primary mirror for space telescopes that combines over five years of research on deployment of optical-precision structures and over ten years of development of fabrication techniques for optical-precision composite mirror panels and structures. The deployable mirror is directly applicable to a broad class of non-imaging `lidar' (light direction and ranging) telescopes whose figure-error requirements are in the range of one to ten microns RMS. Furthermore, the mirror design can be readily modified to accommodate imaging-quality reflector panels and active panel-alignment control mechanisms for application to imaging telescopes. The present paper: (1) describes the deployable mirror concept; (2) explains the status of the mirror development; and (3) provides some technical specifications for a 2.55-m- diameter, proof-of-concept mirror.
Different secondary mirror support towers (pedestals) were modeled for the NGST Optical Telescope Assembly (OTA) B. Transmittance analyses at two levels were performed on the secondary mirror support towers. Detailed transmittances were accomplished by the use of the CODE V optical design/analysis program and were compared to a quick approximation method that required only tracing four rays. A way to obtain simple, very quick approximate transmittances without raytrace also developed by the same author, is laid out. Point spread function (PSF) calculations, including both diffraction and aberration effects, were performed on CODE V. PSF contours can be delineated on CODE V down to about 10-8 times the peak intensity, fine detailing the differences in the wings of the PSF. As one goes out from the center of the blur (for a point source), two types of support towers showed little difference between their PSF intensities until one reaches about the 3% level. Between the first type of tower and the hexapod tower below 3% of the peak intensity, the PSF contour maps well delineated significant differences in the outer portions of the PSFs. The hexapod tower provided the best PSF. Such differences in the wings of the PSF significantly impact the resolution of brown dwarfs or extra solar planets from their star.
BATC has developed a laboratory testbed specifically designed to evaluate the relative merits of wavefront sensing and correction algorithms for active optical systems. The testbed includes a point source, a segmented aperture input with four independently alignable rigid segments, and a sensor assembly that simultaneously provides numerous wavefront and image sensing capabilities. This Wavefront Control Testbed has been used to conduct a series of experiments designed to compare and quantify various techniques for measuring and removing the wavefront errors in a segmented aperture optical system. The results of these experiments are used to support the top-level definition of the wavefront control system that can be used for ensuring the optical performance of large space-based observatories such as the Next Generation Space Telescope. A short description of the experimental parameters and results is presented, along with the expected future use of the testbed.
This paper reports the results of the Space Infrared Telescope Facility (SIRTF) prototype telescope build conducted by Ball Aerospace and Technologies Corporation. The components for the SIRTF prototype telescope were designed and built under NASA's Infrared Telescope Technology Testbed program; the build and subsequent test activities were conducted under NASA's SIRTF/CTA flight program. The flight mission of the telescope will be astronomical observations in the infrared spectrum, 3.5 micrometers to 180 micrometers . To facilitate these observations the telescope will be cooled to 5.5 Kelvin and placed in a solar orbit aboard the SIRTF spacecraft. The prototype was built to verify the structural and cryogenic performance of the telescope.
The international Rosetta mission, now planned by ESA to be launched in January 2003, will provide a unique opportunity to directly study the nucleus of comet 46P/Wirtanen and its activity from a heliocentric distance of 3.2 AU to the perihelion passage at 1.06 AU in July 2013. We describe here the design, the development and the performances of the telescope of the Narrow Angle Camera of the OSIRIS experiment which will give high resolution images of the cometary nucleus in the visible spectrum.
The upgraded 3.8 m UK Infrared Telescope is now provided with: (1) tip-tilt image stabilization by a light-weighted secondary mirror on piezo-electric actuators, controlled by a fast guider sampling at >= 40 Hz on guide stars V m6; (2) active primary mirror figure and secondary mirror alignment control, via a regularly-maintained look-up table; (3) active focus measurements and correction by the fast guider, supplementing a focus maintenance model which corrects for elastic and thermal changes; (4) ventilation of the 2600 m3 dome by 16 apertures totalling 50 m2; (5) insulation of the underside of the concrete dome floor; and (6) internal air circulation during the day, to reduce heating of the upper telescope steelwork.
This paper reports the experiment results of ultrathin hexagonal optical flat (a small sample of LAMOST MA submirror, its diagonal is 310 mm with a thickness of 7 mm), on (Phi) 1M annular continuous polisher, and its merit, technology, testing method and testing result (rms < 0.03(lambda) ). This paper also analyzes the questions that exist in processing now and suggests to build processing and testing equipment, if we prepare for polishing 1100 mm MA submirrors of LAMOST.
We describe here an off-axis design for a 6.5 m astronomical telescope optimized for low scattered light and low emissivity. This is part of a new concept for an instrument which we call the New Planetary Telescope. We show how the geometric optical performance can equal that of an on-axis conventional telescope while the diffractive performance fundamentally surpasses conventional telescopes because of the absence of pupil obstruction. The decentered concept also allows wide-field and versatile instrumentation configurations that are not possible with more conventional design.
The QuickBird telescope is a large-aperture, high-resolution instrument that produces panchromatic and multispectral images of the earth. It has been successfully aligned and is now undergoing final performance verification tests. To help create this unique instrument, Ball Aerospace & Technologies Corp. has invested in the development of a facility to help reduce the time and expense typically associated with the assembly, alignment, and test of complex systems. This facility offers an integrated capability for interferometric alignment and testing of large telescopes, end-to-end image characterization including flight focal plane and electronics, Modulation Transfer Function testing, effective focal length and distortion testing, and radiometric calibration. This paper describes the overall capability of this facility and uses actual data from the alignment and test of the QuickBird telescope to demonstrate the successful completion of that instrument.
The Optical Monitoring Camera (OMC) is a part of the scientific payload being developed for the ESA INTEGRAL mission, scheduled to be launched in 2001. The OMC is a imager that will monitor star variations in the V-band in a 5 X 5 degree(s) field of view. This paper describes the acceptance tests for 3 sub-systems of OMC: the optical system, the baffle and the cover system.
Large ground-based observatories and future space-based astronomical observatories will rely increasingly on optical systems containing active image maintenance. A near-term example of a space-based system that will rely on this technique for ensuring adequate imaging performance is the Next Generation Space Telescope. In this case, the need for a telescope aperture larger than anything supportable as a monolith within existing launch capabilities necessitates the need for a segmented deployable primary mirror. To collect the desired science, it is necessary to maintain the wavefront to about the 50 nanometer RMS level after deployment. In addition, it is necessary to isolate global telescope alignment errors from the segmented-induced wavefront errors and bring the telescope into a globally optimized alignment. Several techniques have been proposed for sensing the wavefront error in the resulting collected image, with the intent of adjusting the opto-mechanical system to reduce the errors to acceptable limits. In addition, a small set of image examinations have been simulated to determine global misalignments in deployed systems. BATC has developed a testbed to support evaluation of the various techniques for autonomously measuring and correcting wavefront errors and for isolating misalignments in large telescope systems. The testbed is designed to be modular, with separate subassemblies providing the segmented input wavefront, the control capability, and the imaging and sensing capabilities. It is also designed to be an evolving asset, providing several levels of testing enhancements over time and supporting the development of test facilities that will be integral to future observatory integration. This paper describes the initial and future top-level requirements, design parameters, and performance capabilities of the testbed.
Development of a high-resolution space telescope is a challenge associated with solution of a number of technical problems, among them being the problem of delivery of the high-precision primary mirror of the telescope to outer space. This paper deals with some problems related to integration of an ultra-light-weight elastic membrane mirror into a telescope with image correction based on a nonlinear- optical corrector and a diffraction optical element mounted immediately in front of the primary mirror.
For quality surveillance of large-sized astronomical optics in observatory conditions a Hartmann method is widely used. In the given article problems of wavefront and deformations restoration of a main mirror are examined by use of a Hartmann technique with small-sized mask in a converging beam for the telescope testing. Is shown, that by virtue of weak conditionality of restoration task the most suitable decision of a problem is Gram-Shmidt process, allowing at appropriate modification to receive best conditionality and consequently to minimize thus error of restoration.
The paper describes the manufacture and testing of a lightweighted Zerodur secondary mirror for the United Kingdom Infrared Telescope on Mauna Kea, Hawaii. The 313 mm diameter mirror is mounted on a Piezo platform for fast tip/tilt corrections. Therefore, the mirror mass has to be minimized to achieve high dynamic properties of the adaptive tip/tilt platform. The goal was to test the convex secondary without large auxiliary optics (Hindle sphere). We measured the mirror through the back surface using a small null lens system. A special transparent and highly homogeneous Zerodur was used for this purpose. We demonstrate that grinding a honeycomb structure and acid-etching of the back side of the mirror does not affect the figure of the polished convex surface.
We demonstrate the feasibility of glass membrane deformable mirror (DM) support structures intended for very high order low-stroke adaptive optics systems. We investigated commercially available piezoelectric ceramics. Piezoelectric tubes were determined to offer the largest amount of stroke for a given amount of space on the mirror surface that each actuator controls. We estimated the minimum spacing and the maximum expected stroke of such actuators. We developed a quantitative understanding of the response of a membrane mirror surface by performing a Finite Element Analysis (FEA) study. The results of the FEA analysis were used to develop a design and fabrication process for membrane deformable mirrors of 200 - 500 micron thicknesses. Several different values for glass thickness and actuator spacing were analyzed to determine the best combination of actuator stoke and surface deformation quality. We considered two deformable mirror configurations. The first configuration uses a vacuum membrane attachment system where the actuator tubes' central holes connect to an evacuated plenum, and atmospheric pressure holds the membrane against the actuators. This configuration allows the membrane to be removed from the actuators, facilitating easy replacement of the glass. The other configuration uses precision bearing balls epoxied to the ends of the actuator tubes, with the glass membrane epoxied to the ends of the ball bearings. While this kind of DM is not serviceable, it allows actuator spacings of 4 mm, in addition to large stroke. Fabrication of a prototype of the latter kind of DM was started.
Understanding segmented mirror operation is crucial to the development of future space-based optical systems such as the Next Generation Space Telescope. Several non-standard effects must be understood and experienced through simulation and experimentation, including segment-to-segment edge discontinuities, edge diffraction, and segment lateral displacement. Simulation provides an initial understanding of the imaging impacts of these effects, while experimentation supplies the necessary operational experience of measuring and correcting them. An integral assembly within the BATC Wavefront Control Testbed is a quadrant segmented spherical concave mirror that provides the input to the wavefront error sensing subassembly. The segments are individually mounted, with 4 degrees of freedom of automated commandable movement. This paper discusses the design requirements, components, operation, and performance of the mirror and mirror mount assembly.
Active and adaptive imaging systems rely on accurate measurement, correction, and maintenance of the wavefront error in a collected signal. The performance of different sensing techniques will depend on the characteristics of the input optical signal, including such parameters as spatial and temporal disturbance bandwidths, absolute wavefront error, source spectral and radiometric content, and image sampling parameters. To determine the relative performance of error sensing techniques, it is useful to conduct the error sensing on identical inputs. This leads to the desire to have multiple simultaneous sensing capabilities within a given architecture. The BATC Wavefront Control Testbed error sensing assembly provides several types of wavefront sensing within a single sensor subassembly, allowing different techniques to be evaluated operating on the same input. Currently, the configuration provides the capability to sense wavefront errors through shearing interferometry, direct image position and intensity measurement, edge sampling, and Hartmann-based mask operations. This paper discusses the imaging and sensing capabilities of the assembly, describes the types of sensing that have been currently evaluated in sensing and correction experiments, and defines the parameters for extending the capability to include other types of sensing.
Silicon carbide may well be the best known material for the manufacture of high performance optical components. A combination of extremely high specific stiffness (r/E), high thermal conductivity and outstanding dimensional stability make silicon carbide superior overall to beryllium and low- expansion glass ceramics. A major impediment to wide use of silicon carbide in optical systems has been the costs of preliminary pressing, casting, shaping and final finishing of silicon carbide. Diamond grinding of silicon carbide is a slow and expensive process even on machines specially designed for the task. The process described here begins by machining the component from a special type of graphite. This graphite is easily machined with multi-axis CNC machine tools to any level of complexity and lightweighting required. The graphite is then converted completely to silicon carbide with very small and very predictable dimensional change. After conversion to silicon carbide the optical surface is coated with very fine grain silicon carbide which is easily polished to extreme smoothness using conventional optical polishing techniques. The fabrication process and a 6 inch diameter development mirror is described.