Ground based testing is a critical and costly part of component, assembly, and system verifications of large space telescopes. At such tests, however, with integral teamwork by planners, analysts, and test personnel, segments can be included to validate specific analytical parameters and algorithms at relatively low additional cost. This paper presents analytical verification and validation segments currently added to ambient and vacuum cryogenic testing of Advanced Mirror System Demonstrator (AMSD) assemblies for the Next Generation Space Telescope (NGST) project. The test segments for workmanship testing, cold survivability, and cold operation optical throughput are supplemented by segments for analytical verifications of structural, thermal, and optical parameters. Utilizing integrated modeling and separate materials testing, the paper continues with analyses to be performed for AMSD testing, currently slated for calendar year 2003. These segments form a well-verified portion of the integrated modeling being conducted on AMSD for NGST performance predictions.
An Optical Testing System (OTS) has been developed to measure the figure and radius of curvature of Next Generation Space Telescope (NGST) developmental mirrors in a vacuum, cryogenic environment using the X-Ray Calibration Facility (XRCF) at Marshall Space Flight Center (MSFC). The OTS consists of a WaveScope Shack-Hartmann sensor from Adaptive Optics Associates as the main instrument and a Leica Disto Pro distance measurement instrument. Testing is done at the center of curvature of the test mirror and at a wavelength of 632.8 nm. The error in the figure measurement is <EQ(lambda) /13 peak-to-valley (PV). The error in radius of curvature is less than 5 mm. The OTS has been used to test the Subscale Beryllium Mirror Demonstrator (SBMD), a 0.532-m diameter spherical mirror with a radius of curvature of 20 m. SBMD characterization consisted of three separate cryogenic tests at or near 35 K. The first two determined the cryogenic changes in the mirror surface and their repeatability. The last followed cryo-figuring of the mirror. This paper will describe the results of these tests. Figure results will include full aperture results as well as an analysis of the mid-spatial frequency error results. The results indicate that the SBMD performed well in these tests with respect to the requirements of (lambda) /4 PV (full aperture), (lambda) /10 PV (mid-spatial, 1-10 cm), and +/- 0.1 m for radius of curvature after cryo-figuring.
The successful augmentation of NASA's X-Ray Cryogenic Facility (XRCF) at the Marshall Space Flight Center (MSFC) to an optical metrology testing facility for the Sub-scale Beryllium Mirror Development (SBMD) and NGST Mirror Sub-scale Development (NMSD) programs required significant modifications and enhancements to achieve useful and meaningful data. In addition to building and integrating both a helium shroud and a rugged and stable platform to support a custom sensor suite, the sensor suite was assembled and integrated to meet the performance requirements for the program. The subsequent evolution from NMSD and SBMD testing to the Advanced Mirror System Demonstrator (AMSD) program is less dramatic in some ways, such as the reutilization of the existing helium shroud and sensor support structure. However, significant modifications were required to meet the AMSD program's more stringent test requirements and conditions resulting in a substantial overhaul of the sensor suite and test plan. This overview paper will discuss the instrumentation changes made for AMSD, including the interferometer selection, null optics, and radius of curvature measurement method. The error budgeting process will be presented, and the overall test plan developed to successfully carry out the tests will be discussed.
The design analysis and preliminary testing of a prototype AFOCL is described. The AFOCL is an active optical component composed of solid state lead lanthanum-modified zirconate titanate (PLZT) ferroelectric ceramic with patterned indium tin oxide (ITO) transparent surface electrodes that modulate the refractive index of the PLZT to function as an electro- optic lens. The AFOCL was developed to perform optical re- alignment and wavefront correction to enhance the performance of Ultra-Lightweight Structures and Space Observatories. The AFOCL would be an active optical component within a larger optical system. Information from a wavefront sensor would be processed to provide input to the AFOCL to drive the sense4d wavefront tot he desired shape and location. While offering variable and rapid focusing capability similar to liquid crystal based spatial light modulators, the AFOCL offers some potential advantages because it is a solid-stat, stationary, low-mass, rugged, and thin optical element that can produce wavefront quality comparable to the solid refractive lens it replaces. The AFOCL acts as a positive or negative lens by producing a parabolic phase-shift in the PLZT material through the application of a controlled voltage potential across the ITO electrodes. To demonstrate the technology, a 4 mm diameter lens was fabricated to produce 5-waves of optical power operating at 2.051 micrometers wavelength. Optical metrology was performed on the device to measure focal length, optical quality, and efficiency for a variety of test configurations. Preliminary data was analyzed and compared to idealized performance available from computer-based models of the AFOCL.
Over the past 7 years, NASA Marshall Space Flight Center (MSFC) through the Global Hydrology and Climate Center (GHCC) has been working; in collaboration with the University of Alabama in Huntsville (UAH) Center for Applied Optics (CAO), and others; towards demonstrating a solid state coherent Doppler lidar instrument for space-based global measurement of atmospheric winds. The Space Readiness Coherent Lidar Experiment (SPARCLE) was selected by NASA's New Millennium Program to demonstrate the feasibility and technology readiness of space-based coherent wind lidar. The CAO was responsible for the design, development, integration, and testing of the SPARCLE optical system. Operating at 2-micron wavelength, SPARCLE system performance is dominated by the optical quality of the transmitter/receiver optical system. The stringent optical performance requirements coupled with the demanding physical and environmental constraints of a space-based instrument necessitate extensive characterization of the telescope optical performance that is critical to predicting the lidar system efficiency and operation in space. Individual components have been measured prior to assembly and compared to the designed specifications. Based on the individual components, the telescope design was optimized to produce a suitable telescope. Once the telescope is completed, it will be tested and evaluated and the data shall be used to anchor computer based models of the optical system. Commercial optical modeling codes were used to evaluate the performance of the telescope under a variety of anticipated on-orbit environments and will eventually be compared to environmental tests conducted in the course of qualifying the telescope for flight. Detailed analysis using the "as built" data will help to reduce uncertainties within the lidar system model and will increase the accuracy of the lidar performance predictions.
NASA is intent on exploiting the unique perspective of space-based remote optical instruments to observe and study largescale environmental processes. Emphasis on smaller and more affordable missions continues to force the remote sensing instruments to find innovative ways to reduce the size, weight, and cost of the sensor package. This is a challenge because many of the proposed instruments incorporate a high quality meter-class telescope that can be a significant driver of total instrument costs. While various methods for telescope weight reduction have been achieved, many of the current approaches rely on exotic materials and specialized manufacturing techniques that limit availability or substantially increase costs. A competitive lightweight telescope technology that is especially well suited to space-based coherent Doppler wind lidar has been developed through a collaborative effort involving NASA Marshall Space Flight Center (MSFC) through the Global Hydrology and Climate Center (GHCC) and the University of Alabama in Huntsville (UAH) at the Center for Applied Optics (CAO). The new lightweight optics using metal alloy shells and surfaces (LOMASS) fabrication approach is suitable for high quality metal mirrors and meter-class telescopes. Compared to alternative materials and fabrication methods the new approach promises to reduce the areal density of a meter-class telescope to less than 15-kg/m2; deliver a minimum ?/1O-RMS surface optical quality; while using commercial materials and equipment to lower procurement costs. The final optical figure and finish is put into the mirrors through conventional diamond turning and polishing techniques. This approach is especially advantageous for a coherent lidar instrument because the reduced telescope weight permits the rotation of the telescope to scan the beam without requiring heavy wedges or additional large mirrors. Ongoing investigations and preliminary results show promise for the LOMASS approach to be successful in demonstrating a novel alternative approach to fabricating lightweight mirrors with performance parameters comparable with the Space Readiness Coherent Lidar Experiment (SPARCLE). Development and process characterization is continuing with the design and fabrication of mirrors for a 25-cm telescope suitable for a lidar instrument.
The measurement of winds from a space borne platform is of significant scientific importance to both weather prediction and climate research. One of the key technologies embodied in coherent detection of winds from space is the use of large aperture, compact, lightweight, high-quality wavefront, photon-efficiency optics. This paper discusses the optical design, the mechanical design, material preference, diamond turning issues, polishing requirements, and coating selections for the primary mirror of a 25X afocal beam expander intended for use in space-based coherent lidar systems.
The SPAce Readiness Coherent Lidar Experiment (SPARCLE) is the first demonstration of a coherent Doppler wind lidar in space. Coherent lidars can accurately measure the wind velocity by extracting the Doppler frequency shift in the back-scattered signal from the atmosphere through optical heterodyne (coherent) detection. Coherent detection is therefore highly sensitive to aberrations in the signal phase front, and to relative alignment between the signal and the local oscillator beams. The telescope and scanning optics consist of an off-axis Mersenne telescope followed by a rotating wedge of silicon and a window of fused silica. The wedge is in very close proximity to the experiment window, and is essentially in contact with the scanner motor/encoder system. The can environment temperature is nominally 20 degrees Celsius, the window ranges from -20 degrees Celsius to 0 degrees Celsius, and the scanner motor/encoder system alone could generate temperatures as high as 35 degrees Celsius. This thermal environment, coupled with the relatively large sensitivity of silicon's refractie index to temperature, has required careful thermal design and compensation techniques. This paper discusses the optical issues of these thermal effects and a variety of methods used to ameliorate them.
A critical component in the 2-micrometer coherent spaced-based lidar system (SPARCLE) is the compact, off-axis, 25-cm aperture telescope. The stressing optical performance demanded from this telescope coupled with the difficulty associated with aligning such a fast, off-axis system; has created the need for a multiple-axis alignment stage for the secondary mirror. Precision micrometer kinematic mounts were used in the laboratory to demonstrate the ability to successfully align the telescope. For the flight configuration, a more robust and considerably smaller stage (both in size and weight) had to be designed in order to fit within the space shuttle packaging constraints. The new stage operates with multiple degrees of freedom of motion to achieve micrometer precision alignment and then uses a mechanical multiple point support to lock-in the alignment and provide stability. The optomechanical design of the flight stage is described.
This paper presents the test results on a compact, off-axis telescope which is the precursor projector/receiver for a NASA Shuttle-based coherent lidar system operating at a wavelength of 2 microns to measure atmospheric wind profiles. The afocal telescope has an entrance pupil diameter of 25 cm, and an angular magnification of 25x. To determine the transmitted and returned optical wavefront quality, the telescope was tested in a Twyman-Green configuration at the operational wavelength. Interferograms were obtained via an infrared camera, and analyzed using a digitizing tablet and WYKO WISP software. Interferograms were obtained with and without an 11.7 degree wedged silicon window located in the entrance pupil. This window, which rotates orthogonal to the telescope optical axis, serves as the lidar system scanner. The measured wavefront information from the interferometer was used in a GLAD heterodyne receiver model to predict the effect of the optical system on the lidar performance. The experimental setup and procedures will be described, and the measurement results of the coherent lidar optical subsystem will be presented in this paper.
The results of a design study for the development of an eye- safe (near-infrared wavelength), compact, multichannel optical interconnect system appropriate for integration with electronics and to be used for short distance communication are discussed. There are potential advantages to using optical interconnects instead of current hardwire interconnections for data transmission over short distances. This technology also has potential applications to data transmission for computing applications. This design study focused on the development of an optical interconnect module to function much like a conventional data cable. The module must be rugged, small, easily integrated into current data transfer, and must have the potential to be produced in volume and at lost cost. The desired system level performance of the optical interconnects was evaluated and design specifications were determined for the optical design. Trade studies involving current technologies were performed to determine suitable hardware configurations. These requirements pointed toward the application of microfabrication technology and micro-optics in order to accomplish the design goals. A pseudo-monolithic silicon-based optical system has been proposed involving diffractive and microrefractive optics along with integrated sensors and emitters. The device emphasizes the use of existing technologies gathered from different disciplines and integrated into one system.
The results of a comprehensive design study for the development of a compact infrared zoom lens suitable for use in guided munitions are discussed. The continuously variable zoom of the lens offers significant operational performance benefits to weapon systems using fixed or switchable FOV optics. Two practical zoom lens systems were designed that showed potential to meet typical guided munitions system requirements by utilizing in the first system conventional surfaces and a combination of conventional and diffractive surfaces in the second system. Significant weight savings, enhanced optical performance, and excellent athermalization over conventional lenses were realized. The optical performance over the entire 4:1 zoom range and 5-20 degrees field-of-view is near-diffraction limit while maintaining a constant F-number.
In this paper some practical observations related to the fabrication of multifocal IOLs are presented from the viewpoint of a diffractive optics design and fabrication group whose experience lies mostly outside the area of ophthalmic optics.
In support of ongoing investigations into turbulence generated aero-optic effects, Teledyne Brown Engineering (TBE) has conducted a series of laboratory based experiments on high velocity turbulent mixing/shear layers. The cornerstone of this effort was the TBE designed and fabricated Dual Injection Nozzle Aero-Optic Simulator for Endoatmospheric Research (DINASER). The DINASER was used to generate flow conditions that simulate the aero-optic effects encountered in flight. The goal of the investigation was to collect data that could be used to validate computational fluid dynamic (CFD) codes, evaluate turbulence models, and anchor aero-optic propagation codes, all of which are essential components in flight simulations. From previous experiments and analysis, the unknown quantities necessary to validate the turbulence models and CFD code have been separated from the essential optical quantities. This investigation has primarily focused on the quantification of the optical parameters, measured along the line-of-sight.
Characterization of small scale structures within high speed turbulent flow fields requires instrumentation that is capable of acquiring high speed data at rates exceeding one megahertz. From experimental studies performed by the Teledyne Brown Engineering (TBE) Experimental Aero-Optics Group in conjunction with SY Technology, it has been observed that structures within a high speed turbulent flow have a limited lifetime. With the development of the Ultranac computer controlled high speed camera, the collection of high speed images was possible. The camera was capable of 8 to 24 short sub-microsecond exposure times and fast MHz frequency frame rates, all of which was variable and could be set independently for each frame recorded by the camera. An application for this system was demonstrated using a collimated beam of HeNe laser light to record shadowgraphs of turbulent flow structures generated by TBE's Aero-Optic Simulator (AOS). Argon gas was exhausted at a low speed from one nozzle and neon gas was exhausted at a higher speed from the other nozzle to give a calculated shear layer flow velocity of approximately 450 m/s. Frame-by- frame comparisons were made and flow structures were observed to persist for periods on the order of a microsecond. Based on experience from this preliminary demonstration, improvements for future experiments have been suggested. These tests clearly demonstrate the potential of the Ultranac camera to aid in the characterization of high speed turbulent flows.
A simple technique has been developed to record both the high and low frequency fluctuations contained within an aerooptically distorted 2-D point spread function. A collimated beam of light from an Nd:YAG laser, operated at 1.064 jim, was passed through a high velocity wrbulent flow field and imaged on the focal plane of a 128 x 128 array CCD camera (60 jun square pixels). Extremely short duration (50 nsec) pulses from the laser, synched with a high speed (92 frames per second), video data acquisition system captured one pulse per frame and effectively froze all motion in the flow. Post test averaging of single pulsed frames made it possible to visualize the full range of frequencies contained in the Fourier plane. Investigations are currently being made into the mathematical relationship between these high resolution images and the 2-D power spectrum for the refractive index fluctuations. The technique has the advantages of being non-intrusive and capable of acquiring multiple samples in a short period of time to insure statistical validity. Experimental measurements were performed on the center of a turbulent mixing shear layer (—8 mm thick), generated by back-to-back supersonic nitrogen/argon gas nozzles, with a mean flow velocity of ''380 rn/sec. Calculations were made using horizontal scans through several frames from a single test run. Turbulent scale lengths down to 0.6 mm were resolved.
Experiments were conducted by Teledyne Brown Engineering (TBE), that simultaneously recorded flow field shadowgraphs and imaging data, to investigate the relationship between high velocity turbulence and aero-optic image distortion. A laboratory based Dual Nozzle Aero-Optic Simulator (DNAOS) was used to produce a turbulent flight level aero-optic environment similar to that encountered by a hypersonic vehicle. An Nd:YAG laser, operating at 1.064 micrometers , was expanded to 5 mm, collimated, and directed through the turbulent flow field to serve as a point source at infinity. On the opposite side of the flow field, the beam was split into two components and directed towards two 60 micrometers square pixelated, 128 X 128 CCD array cameras. One camera had a bare focal plane and was used to record the turbulence induced scattering field (shadowgraph), while the other had a 3.4 m focal length lens to image this field, producing a point-spread-function (PSF) on the CCD array. A 50 nsec duration laser pulse at a frequency of 92.5 Hz (frame rate of the CCD cameras) was recorded by each of the cameras and the data was digitized by a high speed data acquisition system. The shadowgraphs and imaging data were compared frame-for-frame to determine the similarities between the flow field events and the image distortion. Based on this analysis, a procedure has been proposed to numerically transform shadowgraphs to obtain pseudo-images that could be compared to experimentally recorded images.
An experimental investigation was undertaken to examine the aero-optic performance of multi- aperture windows for use in hypersonic endo-atmospherical vehicles. A series of imaging tests was conducted through a two-dimensional flat plate model of a multi-aperture window that was incorporated into the Teledyne Brown Dual Nozzle Aero-Optic Simulator (DNAOS). This simulator brought two high-velocity gas streams together in an enclosed test region to form an approximate Mach 2 mixing/shear layer, creating the turbulent properties found in hypersonic flight. The same series of tests was conducted looking through a monolithic flat plate window. The images recorded through both window schemes were analyzed to determine image distortion and results were compared to demonstrate the various optical phenomenon associated with multi-aperture windows.
Mixing/shear layer turbulence is the major contributor to optical degradation effects experienced by a windowed hypersonic vehicle. A critical component in the prediction of these aero-optic effects, is the distribution, relative sizes, and velocities of the turbulent structures found within the mixing/shear layer. Previous attempts have had difficulty in measuring these high frequency, small scale turbulent properties. Therefore, a novel non-intrusive optical technique called the fiber optic flow monitor, was developed. This device was used in conjunction with a dual nozzle aero-optic simulator to experimentally determine turbulent flow properties and investigate their relationship to image distortion. The flow field studied was a dual species mixing/shear layer that had a mean flow velocity of approximately 430 m/s with a calculated mean turbule size of 0.7 mm. It was observed that the turbulent structures redistributed incident collimated energy into unique patterns of light. By monitoring these patterns, it was possible to measure several flow field properties. Data, gathered from this technique, was used to compute a statistical distribution of turbule velocities that was compared to theoretical predictions and image distortion parameters. Close correlation between experimental and theoretical values confirms that the technique provides a non- intrusive method of accurately characterizing small scale, high velocity turbulent structures.
A nonintrusive optical technique has been developed that measures the velocity components of high velocity mixing/shear layers. Within the high velocity turbulent media, flow structures exist that can randomly concentrate and/or redistribute incident light into unique intensity patterns. By observing the motion of these patterns over short time intervals, it is possible to deduce the velocity of the flow. A series of laboratory experiments was conducted to demonstrate the technique using the Teledyne Brown Engineering Dual Nozzle Aero-Optic Simulator (DNAOS). A binary gas, classical mixing/shear layer with a mean flow velocity of approximately 460 m/sec was generated for the tests. Two independent Q-switched Nd:YAG laser beams (1.064 micrometers ) were colinearly aligned, directed through the flow, and then recorded with a high speed CCD square array. Each laser was fired once during a camera frame, with a measured time delay of 1.36 microsecond(s) between the two laser pulses. The frames were taken at 92.5 Hz and stored for post-test analysis. By identifying the projected flow structure patterns and measuring the displacement of the patterns as recorded by the camera, two dimensional velocity components were calculated. These values were in fair agreement with mean flow velocities predicted with an empirical flow field prediction code.
A nonintrusive experimental method, utilizing two Quad Cell detectors and a cross beam correlation (CBC) technique, was applied to Teledyne Brown Engineering (TBE) to investigate the relationship between fluctuating optical properties and image distortion caused by high velocity turbulence. A laboratory based Dual Nozzle Aero-Optic Simulator (DNAOS) was used to produce a mixing/shear layer that simulated a flight level aero-optic environment. Two Quad Cells were used to simultaneously measure the centroidal shift of two orthogonal laser beams that were located in a plane normal to the direction of flow and were coincident at only one point within the mixing/shear layer. The 2D angular deviations of the laser beams were calculated from the recorded centroidal fluctuations. From this data, the cross correlations could be calculated to determine the turbulence induced optical deviation (wavefront distortion) and density related index of refraction fluctuations for the flow field. The experimentally measured angular deviations were found to compare reasonably well to theoretically predicted values. This demonstrates that Quad Cell CBC can provide a nonintrusive method of accurately characterizing the fluctuating optical properties resulting from small scale hypersonic turbulent structures.
An investigation has been undertaken which utilizes nonintrusive optical interferometric techniques to visualize the turbulent structure found in a high-velocity flow field and thereby characterize the resulting optical distortion. Experiments were conducted on a 7.68 mm by 7.68 mm cross section of a high velocity, dual gas, mixing/shear layer, and the preliminary results are presented. The experimental apparatus consisted of a dual beam Mach-Zehnder interferometer with a customized high-speed CCD camera data acquisition system. A series of time varying images of the gas flow were captured and digitized with the interferometer configured in both a finite and an infinite fringe mode. By correlating the initial tare run wavefront (gas off condition) to any subsequent distorted wavefront (gas on condition), the turbulent flow field structure and the relative phase shift across the test region was analyzed. Both classical and nonclassical approaches were taken in analyzing the interferometric data to obtain an understanding of the high velocity flow field. In addition, the experimental results were compared to theoretical predictions for RMS wavefront distortion.