Intensity interferometry (II) is an alternate form of creating images of distant objects. It is significantly less sensitive to atmospheric distortions and aberrations of telescope surfaces than conventional amplitude-based imaging. The deficiencies of II can be overcome as photodetectors’ read-out rates are becoming faster and computers more powerful. In recognition of the possibility of very large space-based imaging systems, this paper investigates how the deformation of a large, thin optical surface would influence the accuracy of II. Based on the theoretical foundation of II, an optical ray-tracing algorithm was used to examine how the statistics of a photon stream changes from the source to the detector. Ray-tracing and finite element analyses of the structure were thereafter integrated to quantify how the correlation of the intensity field changes as the reflective structure deforms. Varying the positions of the detector from the focal plane and the surface profile of the mirror provided an understanding and quantification of how the various scenarios affect the statistics of the detected light and the correlation measurement. This research and analysis provide the means to quantify how structural perturbations of focal mirrors affect the statistics of photon stream detections inherent in II instrumentation.
Using a single focal parabolic reflector of an intensity interferometer(II) system is simulated. The extent that focal properties amongst a parabolic reflector can change the statistics of the light at a detector is analyzed. Recent technological advances have increased the speed and sensitivity of photon detectors, developed large scale precision optics, and incorporated multi-spectral imaging techniques which have led the way to reexamine the usefulness of II for scientific measurements. A ray tracing algorithm is used to examine how the statistical variations of simulated monochromatic stellar light changes from the source to the detector. Changing the position of the detector from the focal plane and changing the surface profile of the mirror develops a metric to understand how the varying scenario’s affects the statistics of the detected light. Photon streams are evaluated for light distribution, time of flight, and statistical changes at a detector. This research and analysis is used as a tool to develop a metric to quantify how structural perturbation effect the statistics of photon stream detections inherent in II instrumentation and science.
Fluorozirconate glasses, such as ZBLAN (ZrF 4 -BaF 2 -LaF 3 -AlF 3 -NaF ), have the potential for optical transmission from 0.3 μm in the ultraviolet to 7 μm in the infrared regions. However, crystallites formed during the fiber-drawing process prevent this glass from achieving its desired transmission range. The temperature at which the glass can be drawn into a fiber is known as the working range, defined as (Tx-Tg), bounded by the glass transition temperature (Tg) and the crystallization temperature (Tx). In contrast to silica glasses, the working temperature range for ZBLAN glass is extremely narrow. Multiple ZBLAN samples were subjected to a heating and quenching test apparatus on the parabolic aircraft under a controlled μ-g and hyper-g environments and compared with 1-g ground tests. Optical microscopy examination elucidates that crystal growth in ZBLAN is suppressed and initiates at a later temperature when processed in a microgravity environment. Thus, the crystallization temperature, Tx, at which the crystals form has increased. The glass transition temperature, Tg, remains constant, as crystallization does not occur until approximately 360°C for this composition of ZBLAN. Therefore, the working temperature range for ZBLAN has been broadened.
Fluorozirconate glasses, such as ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF), have the potential for optical transmission from 0.3 μm in the UV to 7 μm in the IR region. However, crystallites formed during the fiber drawing process prevent this glass from achieving its desired transmission range. The temperature at which the glass can be drawn into a fiber is known as the working range, defined as (Tx - Tg), bounded by the glass transition temperature (Tg) and the crystallization temperature (Tx). In contrast to silica glasses, the working temperature range for ZBLAN glass is extremely narrow. Multiple ZBLAN samples were subject to a heating and quenching test apparatus on the parabolic aircraft, under a controlled 0-g and hyper-g environment and compared with 1-g ground tests. The microgravity duration on board Zero-G Corporation parabolic aircraft is approximately 20 seconds and the hyper-g intervals are approximately 56 seconds. Optical microscopy examination elucidates crystal growth in ZBLAN is suppressed when processed in a microgravity environment. The crystallization temperature, Tx, at which crystals form increased, therefore, significantly broadening the working temperature range for ZBLAN.
Vibration-based structural health monitoring could be a useful form of determining the health and safety of space
structures. A particular concern is the possibility of a foreign object that attaches itself to a satellite in orbit for adverse
reasons. A frequency response analysis was used to determine the changes in mass and moment of inertia of the space
structure based on a change in the natural frequencies of the structure or components of the structure. Feasibility studies
were first conducted on a 7 in x 19 in aluminum plate with various boundary conditions. Effect of environmental
conditions on the frequency response was determined. The baseline frequency response for the plate was then used as
the basis for detection of the addition, and possibly the location, of added masses on the plate. The test results were
compared to both analytical solutions and finite element models created in SAP2000. The testing was subsequently
expanded to aluminum alloy satellite panels and a mock satellite with dummy payloads. Statistical analysis was
conducted on variations of frequency due to added mass and thermal changes to determine the threshold of added mass
that can be detected.
The quickest method for generating a lightweight composite optic is to replicate an optical-quality glass tool onto a carbon-fiber-reinforced polymer (CFRP). However, fiber print-through creates an unacceptable sinusoidal surface roughness on replicated CFRP mirrors; chemical and thermal shrinkage during cure are commonly hypothesized to be the dominant causes. In order to mitigate fiber print-through, two methods of generating a polishable resin layer were investigated. The first method employs the application of a resin film to the CFRP surface. The second method, which is a more unconventional approach, generates a cocured resin layer using magnetic fibers. The latter approach is being developed to eliminate the application of additional resin layers to the CFRP surface, since additional layers present structural disadvantages.It was found that the magnetic fiber technique is comparable to the conventional approach in mitigating fiber print-through. Due to the presence of a 0.25-mm-thick buffer above the reinforcing phase, a final polishing step was used to attain optical quality features on all of the replicated specimens. CFRP and magnetic fiber samples were polished to within 50-Å rms roughness (1-µm to 1-mm bandwidth).
NASA and the U. S. Air Force are looking to improve space borne telescopes by reducing mirror
weight. One commonly attempted solution is to fabricate Carbon Fiber Reinforced Polymer
(CFRP) mirrors using a mirror replication technique. These attempts have been hindered by the
well-known fiber print-through phenomenon. The resulting sinusoidal surface distortion is fiber
print-through, where chemical and thermal shrinkage during cure have been hypothesized to be
the dominant causes. Although successful mitigation of fiber print-through via a polished resin
layer method has been proven, an additional resin layer reduces the heat transfer through the
mirror thickness that would be necessary for high-energy laser applications and also carries
structural disadvantages.
The purpose of this research was to quantify the dominating causes of fiber print-through and its
contribution to the total surface roughness of a composite (where total roughness includes the
elements of print-through and other surface anomalies that contribute to diffuse reflection). In
order to quantify the causes of fiber print-through, a number of CFRP samples with varying fiber
type, diameter and cure schemes were fabricated. The dominating causes of fiber print-through
were then found by measuring fiber print-through, via microscopic interferometry, and
determining which variables had the greatest influence on print-through.
The quickest method for generating a lightweight composite optic is to replicate an optical quality glass tool onto a carbon fiber reinforced polymer (CFRP). However, the effects of fiber print-through create an unacceptable surface roughness on replicated CFRP mirrors. In order to mitigate fiber print-through, two methods of generating a polishable resin layer were investigated. The first method employs the application of resin films to the CFRP surface. The second, unconventional method generates a co-cured resin layer using a magnetic fiber migration approach. A final polishing step was used to attain optical quality surface features on all of the replicated specimens. Replicated resin films with thicknesses ≥ 0.25 mm sufficiently mitigate fiber print-through. Room temperature and high temperature cure resins were polished below 50 Å rms surface roughness (1 μm to 1mm bandwidth) or better. The magnetic fiber migration technique was suitable for eliminating fiber print-through. Replicated magnetic fiber laminates were polished to within specular quality as well.
Materials and processes have been developed for production of polymer membranes with optical quality surface characteristics. These materials have been successfully used to manufacture large, high quality, ultra lightweight, optical flats for beam splitters, lens covers and other applications. These materials can potentially be used to develop large aperture primary mirrors with areal densities less than 1kg/m2. However, for curved mirrors it is more difficult to establish and maintain desired optical figure from the initial packaged configuration. This paper describes design analysis being performed to support fabrication of a membrane mirror test article. Modeling was performed to evaluate the effectiveness of several different boundary control concepts for correcting different types of figure aberrations. Analyses of different combinations of boundary displacement actuators, electrostatic force actuators, and pressure are presented.
Shape Memory Alloys (SMA) are increasingly used as smart devices ina erospace applicatons.Their priry advnatage over other smart materials z9piezo-ceramics adnpiezo-polymers) is in their ability to undergo large strains and dispalcements that enables the devlopment of smart mechanissm. The specific objective of this research was toinvestigate the use of embedded SMA for active shape correctionof flexibl emirror substrates.Active shape correction is a possiblemeans of mitigating thrmally induced distortions in space based optical iaging systems inorder to understand and quantify the designvalraibles that lead to that goal, embedded SMA wires we used toa cctuate a series of composite bmeasm.Themoemvent induced byactutionw as monitored with theMoire inteferietery method,and the rsutls wer compared witha n anlaytical model.The repeatabiltya nd rlaibltiyof a possible acutation systmeand the propertis of the SMa wires was also studied by testing the stress-strian anthe stress reo ery behavior under controlled conditons.110
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.
Electronic shearography (ES) is a laser based non-destructive testing method that has the potential to be developed into a commercial bridge monitoring technique. The primary advantage of ES over other similar techniques like electronic speckle pattern interferometry (ESPI) is its decreased sensitivity to in-plane rigid body movement and vibrations. Bridge inspection with ES has proven to be a daunting task so far. The main problem has been the inability of the method to handle the large deflections and vibrations that might be expected in a typical bridge subjected to normal service loads. Earlier research has shown that the extent of in-plane movement that can be tolerated by the system is dependent on the speckle size. The speckle size also affects the fringe quality by imposing resolution requirements on the imaging device. This article shall undertake the study of speckle size as a function of the focal length of the imaging lens, object distance and illumination wavelength using high resolution holographic film and a high magnification optical microscope.
Although a large number of nondestructive techniques based on laser interferometry have been developed over the last few decades, most of them are incapable of handling large structures. Shearography has shown potential as a full field nondestructive technique, capable of being used on large structures like bridges. The method however has been beset with the problem of poor fringe quality, and as any other interferometric technique, suffers from the problem of speckle de-correlation due to rigid body motions. This paper investigates the effect of rigid body motion on fringe visibility with an emphasis on applications in nondestructive testing of bridges. To this effect, a finite element and fracture mechanics analysis is carried out on a real bridge to identify the constraints on shearography in full scale structural testing.
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