Search NASA⌕ Search

SEARCH · Search NASA

Results for “linear array”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

The AXAF CCD imaging spectrometer

The current status of the instrument design and the status of the CCDs being fabricated for the AXAF CCD Imaging Spectrometer (ACIS) are summarized. The instrument consists of an image recording array of CCDs and a linear arrangement of CCDs to record the spectra formed by the objective grating spectrometer. Both arrays employ CCDs with pixel dimensions which correspond to about 0.5 arcsec samples of the image. The CCDs provide moderate spectral resolution and good detection efficiency over the energy range 0.5 to 10 keV. Spectral resolution of 200 or more is achievable using the objective grating with the grating array. Radiation damage effects are shown to degrade the energy resolution of CCDs. Specially designed CCD pixel architecture is employed together with shielding and low temperature operation to slow the effects of radiation damage.

Garmire, G. P.↗
NASA NTRS2002

The Fourier-Kelvin Stellar Interferometer Mission Concept

The Fourier-Kelvin Stellar Interferometer (FKSI) is a mission concept for an imaging interferometer for the mid-infrared spectral region (5-30 microns). FKSI is conceived as a scientific and technological precursor to TPF as well as Space Infrared Interferometric Telescope (SPIRIT), Submillimeter Probe Evolution of Cosmic Structure (SPECS), and Single Aperture for Infrared Observatory (SAFIR). It will also be a high angular resolution system complementary to Next Generation Space Telescope (NGST). The scientific emphasis of the mission is on the evolution of protostellar systems, from just after the collapse of the precursor molecular cloud core, through the formation of the disk surrounding the protostar, the formation of planets in the disk, and eventual dispersal of the disk material. FKSI will also search for brown dwarfs and Jupiter mass and smaller planets, and could also play a very powerful role in the investigation of the structure of active galactic nuclei and extra-galactic star formation. We are in the process of studying alternative interferometer architectures and beam combination techniques, and evaluating the relevant science and technology tradeoffs. Some of the technical challenges include the development of the cryocooler systems necessary for the telescopes and focal plane array, light and stiff but well-damped truss systems to support the telescopes, and lightweight and coolable optical telescopes. The goal of the design study is to determine if a mid-infrared interferometry mission can be performed within the cost and schedule requirements of a Discovery class mission. At the present time we envision the FKSI as comprised of five one meter diameter telescopes arranged along a truss structure in a linear non-redundant array, cooled to 35 K. A maximum baseline of 20 meters gives a nominal resolution of 26 mas at 5 microns. Using a Fizeau beam combination technique, a simple focal plane camera could be used to obtain both Fourier and spectral data simultaneously for a given orientation of the array. The spacecraft will be rotated to give sufficient Fourier data to reconstruct complex images of a broad range of astrophysical sources.

Danchi, W. C.↗
DOE OSTI2016

A high performance neutron powder diffractometer at 3 MW Triga Mark-II research reactor in Bangladesh

A high performance neutron diffractometer called Savar Neutron Diffractometer (SAND) was built and installed at radial beam port-2 of TRIGA Mark II research reactor at AERE, Savar, Dhaka, Bangladesh. Structural studies of materials are being done by this technique to characterize materials crystallograpohically and magnetically. The micro-structural information obtainable by neutron scattering method is very essential for determining its technological applications. This technique is unique for understanding the magnetic behavior in magnetic materials. Ceramic, steel, electronic and electric industries can be benefited from this facility for improving their products and fabrication process. This instrument consists of a Popovicimonochromator with a large linear position sensitive detector array. The monochromator consists of nine blades of perfect single crystal of silicon with 6 mm thickness each. The monochromator design was optimized to provide maximum flux on 3 mm diameter cylindrical sample with a relatively flat angular dependence of resolution. Five different wave lengths can be selected by orienting the crystal at various angles. A sapphire filter was used before the primary collimator to minimize the first neutron. The detector assembly is composed of 15 linear position sensitive proportional counters placed at either 1.1 m or 1.6 m from the sample position and enclosed in a air pad supported high density polythene shield. Position sensing is obtained by charge division using 1-wide NIM position encoding modules (PEM). The PEMs communicate with the host computer via USB. The detector when placed at 1.1 m, subtends 30° (2θ) at each step and covers 120° in 4 steps. When the detector is placed at 1.6 m it subtends 20° at each step and covers 120° in 6 steps. The instrument supports both low and high temperature sample environment. The instrument supports both low and high temperature sample environment. The diffractometer is a state-of-the art technology for diffraction study in our country.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Evaluation of Computed Tomography Techniques for Material Identification in Low Level and Intermediate Level Waste - 16648

Measures required to assure long term safe storage of Low-level and intermediate-level short-lived (LIL/SL) waste generated by the nuclear-power industry in France include the characterization of the waste and identification of materials within the waste. Digital Radiography (DR) techniques using x-ray at energies in the 6 MeV range have been shown to be effective for validation of the integrity of the concrete containers, and it was considered that the use of this technique together with the addition of Computed Tomography (CT) techniques could effectively identify materials within the waste. This paper describes the initial evaluation of two DR imaging techniques and two CT techniques using x-ray energy spectra up to 9 MeV to inspect a surrogate waste package, which comprised an outer concrete cylindrical container of 1400 mm diameter with 148 mm wall thickness, and an overall volume of 2 meters cubed, within which a 200 liter drum containing representative heterogeneous inactive waste, was placed in the center and the annulus between the 200 liter drum and the outer cylindrical package was filled with cement grout. The surrogate waste within the 200 liter drum comprised three separate layers of high density, medium density and low density mixed materials. The equipment used was a linear accelerator producing high energy spectra at either 6 MeV or 9 MeV and a high-precision CT manipulator with X, Y, Z and rotation motions, which were common to all techniques investigated. Two different digital detectors were used in the DR and CT performance evaluations, a Digital Detector Array (DDA) comprising a columnar Thallium-doped Cesium-Iodide scintillator screen in intimate contact with a 2 k by 2 k array of amorphous silicon detectors, 200 μm x 200 μm in area, and a Linear Diode Array (LDA) comprising a line of Cadmium-Tungstate columns, 1 cm thick and 400 μm x 400 μm in area. The work comprised evaluations of linear accelerator spectra and the attenuation effects of the waste container and contents, image acquisition performance of each detector, and CT techniques using area detector and linear detector. The results presented in this paper include mathematical evaluation of the two CT techniques, and analysis of measurement data, digital images, CT reconstruction images, and estimated material densities. The data was compared with recorded details and photographs of the actual configuration of materials within the surrogate waste. The Digital Detector Array can generate CT inspection volumes with considerable resolution and detail for the low-density and medium-density sections of the surrogate waste, however, the high-density sections were not imaged with sufficient detail and the images contained a large number of artifacts mainly due to the detectors lack of high energy photon stopping power. Materials identification with the Digital Detector Array, while possible for certain materials, for materials in the steel to plastic density range the attenuation values of the voxels in the reconstruction are too similar and difficult to resolve from the artifacts present. The Linear Diode Array can generate CT inspection volumes of regions of interest of the low-density and medium-density sections with considerable resolution and contrast, and potentially acceptable levels of resolution and contrast of the high-density section of the waste. While a few artifacts were present in the reconstructions, the majority are not due to the fundamental properties of the detector and are considered to be correctable. Materials identification with the LDA inspection data appear to be viable, with steel, copper, and higher-density metals being readily be distinguished from other materials on the basis of voxel value. Future work is needed to obtain a better definition of optimum spatial resolution and inspection speed, and to develop a system of 'calibration rods' which would consist of a series of 50 mm diameter cylinders of 7 materials; Plastic, Teflon, Concrete, Aluminum, Copper, Tungsten and Lead. The cylinders will be distributed at a certain radius in the surrogate waste and will be used as reference points in the evaluation and demonstration of the contrastive performance for the low, medium and high-density cases. The ability to identify the materials within a sealed waste container, by nondestructive examination, enables the verification of waste acceptance criteria without the need to open the waste containers. This eliminates the need for complex and expensive equipment to open and remotely handle the intermediate level waste, avoids potential hazards and risk of personnel dose uptake, and does not increase the volume of waste caused by repackaging of materials after examination and the decontamination or disposal of equipment used.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
NASA NTRS2004

Characterizing the Energy Scale of the XRS on Astro-E2

The XRS on Astro-E2 consists of a microcalorimeter array with 31 active pixels. The XRS energy scale, which converts the measured pulse heights to the energies of the photons incident on the array, is a non-linear function of the thermal properties of the pixels and the power and noise conditions on the array. Each of the pixels operates independently and requires an individual energy scale. Analytic models of the XRS pulses are currently being developed that will be used to model the energy scale. However, because of the inherent complexity, a full empirical calibration of the energy scale is being conducted. The XRS energy scale has been calibrated for a range of operating conditions including variations in the detector temperature, bath temperatures, and photon flux. This allows us to characterize the functional dependence of the energy scale on these variables and to then generate a semi-empirical model that can be used to account for variations in operating conditions in space. We will present the results of the empirical characterization and comparisons to the analytic models.

Cottam, J.↗
NASA NTRS2013

NASA Tech Briefs, Februrary 2013

Topics covered include: Measurements of Ultra-Stable Oscillator (USO) Allan Deviations in Space; Gaseous Nitrogen Orifice Mass Flow Calculator; Validation of Proposed Metrics for Two-Body Abrasion Scratch Test Analysis Standards; Rover Low Gain Antenna Qualification for Deep Space Thermal Environments; Automated, Ultra-Sterile Solid Sample Handling and Analysis on a Chip; Measuring and Estimating Normalized Contrast in Infrared Flash Thermography; Spectrally and Radiometrically Stable, Wideband, Onboard Calibration Source; High-Reliability Waveguide Vacuum/Pressure Window; Methods of Fabricating Scintillators With Radioisotopes for Beta Battery Applications; Magnetic Shield for Adiabatic Demagnetization Refrigerators (ADR); CMOS-Compatible SOI MESFETS for Radiation-Hardened DC-to-DC Converters; Silicon Heat Pipe Array; Adaptive Phase Delay Generator; High-Temperature, Lightweight, Self-Healing Ceramic Composites for Aircraft Engine Applications; Treatment to Control Adhesion of Silicone-Based Elastomers; High-Temperature Adhesives for Thermally Stable Aero-Assist Technologies; Rockballer Sample Acquisition Tool; Rock Gripper for Sampling, Mobility, Anchoring, and Manipulation; Advanced Magnetic Materials Methods and Numerical Models for Fluidization in Microgravity and Hypogravity; Data Transfer for Multiple Sensor Networks Over a Broad Temperature Range; Using Combustion Synthesis to Reinforce Berms and Other Regolith Structures; Visible-Infrared Hyperspectral Image Projector; Three-Axis Attitude Estimation With a High-Bandwidth Angular Rate Sensor Change_Detection.m; AGATE: Adversarial Game Analysis for Tactical Evaluation; Ionospheric Simulation System for Satellite Observations and Global Assimilative; Modeling Experiments (ISOGAME); An Extensible, User- Modifiable Framework for Planning Activities; Mission Operations Center (MOC) - Precipitation Processing System (PPS) Interface Software System (MPISS); Automated 3D Damaged Cavity Model Builder for Lower Surface Acreage Tile on Orbiter; Mixed Linear/Square-Root Encoded Single-Slope Ramp Provides Low-Noise ADC with High Linearity for Focal Plane Arrays; RUSHMAPS: Real-Time Uploadable Spherical Harmonic Moment Analysis for Particle Spectrometers; Powered Descent Guidance with General Thrust-Pointing Constraints; X-Ray Detection and Processing Models for Spacecraft Navigation and Timing; and Extreme Ionizing-Radiation-Resistant Bacterium

Source record↗
NASA NTRS1999

An L/X Dual-Band Dual-Polarized Shared-Aperture Array for Spaceborne SAR

Spaceborne synthetic aperture radar antennas have many special electrical requirements, such as operation at multiple frequencies with multiple polarization ability, with fairly wideband operation being required at these frequencies. They are also required to be electrically large, giving rise to issues such as low mass, easy and reliable deployability, and low cost. This paper describes the results of a prototype SAR array developed with these considerations in mind. The prototype antenna is a dual-frequency array operating at L and X bands, with dual linear polarization capability at both bands. This array shares the same radiating aperture for both bands and both polarizations, as was done in [l]. The prototype antenna described can also be used as a single module in a much larger array.

Targonski, Stephen D.↗
DOE OSTI2015

Magnetic antenna excitation of whistler modes. IV. Receiving antennas and reciprocity

Antenna radiation patterns are an important property of antennas. Reciprocity holds in free space and the radiation patterns for exciting and receiving antennas are the same. In anisotropic plasmas, radiation patterns are complicated by the fact that group and phase velocities differ and certain wave properties like helicity depend on the direction of wave propagation with respect to the background magnetic field B{sub 0}. Interference and wave focusing effects are different than in free space. Reciprocity does not necessarily hold in a magnetized plasma. The present work considers the properties of various magnetic antennas used for receiving whistler modes. It is based on experimental data from exciting low frequency whistler modes in a large uniform laboratory plasma. By superposition of linear waves from different antennas, the radiation patterns of antenna arrays are derived. Plane waves are generated and used to determine receiving radiation patterns of different receiving antennas. Antenna arrays have radiation patterns with narrow lobes, whose angular position can be varied by physical rotation or electronic phase shifting. Reciprocity applies to broadside antenna arrays but not to end fire arrays which can have asymmetric lobes with respect to B{sub 0}. The effect of a relative motion between an antenna and the plasma has been modeled by the propagation of a short wave packet moving along a linear antenna array. An antenna moving across B{sub 0} has a radiation pattern characterized by an oscillatory “whistler wing.” A receiving antenna in motion can detect any plane wave within the group velocity resonance cone. The radiation pattern also depends on loop size relative to the wavelength. Motional effects prevent reciprocity. The concept of the radiation pattern loses its significance for wave packets since the received signal does not only depend on the antenna but also on the properties of the wave packet. The present results are of fundamental interest and of relevance to loop antennas in space.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

On infrasound generated by wind farms and its propagation in low-altitude tropospheric waveguides

Infrasound from a 60-turbine wind farm was found to propagate to distances up to 90 km under nighttime atmospheric conditions. Four infrasound sensor arrays were deployed in central New Mexico in February 2014; three of these arrays captured infrasound from a large wind farm. The arrays were in a linear configuration oriented southeast with 13, 54, 90, and 126 km radial distances and azimuths of 166°, 119°, 113°, and 111° from the 60 1.6 MW turbine Red Mesa Wind Farm, Laguna Pueblo, New Mexico, USA. Peaks at a fundamental frequency slightly below 0.9 Hz and its harmonics characterize the spectrum of the detected infrasound. The generation of this signal is linked to the interaction of the blades, flow gradients, and the supporting tower. The production of wind-farm sound, its propagation, and detection at long distances can be related to the characteristics of the atmospheric boundary layer. First, under stable conditions, mostly occurring at night, winds are highly stratified, which enhances the production of thickness sound and the modulation of other higher-frequency wind turbine sounds. Second, nocturnal atmospheric conditions can create low-altitude waveguides (with altitudes on the order of hundreds of meters) allowing long-distance propagation. Third, night and early morning hours are characterized by reduced background atmospheric noise that enhances signal detectability. This paper describes the characteristics of the infrasound from a quasi-continuous source with the potential for long-range propagation that could be used to monitor the lower part of the atmospheric boundary layer.

17 WIND ENERGY↗
DOE OSTI2018

Magnetothermodynamics: measurements of the thermodynamic properties in a relaxed magnetohydrodynamic plasma

We have investigated the thermodynamics of compressed magnetized plasmas in laboratory experiments and we call these studies ‘magnetothermodynamics’. The experiments are carried out in the Swarthmore Spheromak eXperiment device. Here, a magnetized plasma source is located at one end and at the other end, a closed conducting can is installed. We generate parcels of magnetized plasma and observe their compression against the end wall of the conducting cylinder. The plasma parameters such as plasma density, temperature and magnetic field are measured during compression using HeNe laser interferometry, ion Doppler spectroscopy and a linear ${\dot{B}}$ probe array, respectively. To identify the instances of ion heating during compression, a PV diagram is constructed using measured density, temperature and a proxy for the volume of the magnetized plasma. Different equations of state are analysed to evaluate the adiabatic nature of the compressed plasma. A three-dimensional resistive magnetohydrodynamic code (NIMROD) is employed to simulate the twisted Taylor states and shows stagnation against the end wall of the closed conducting can. The simulation findings are consistent to what we observe in our experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Mechanism of Mesoscale Woodpile Development via Photoelectrochemical Deposition of Se–Te

A combination of experiments and optical modeling provided insight into the mechanism of mesoscale woodpile formation in response to an orthogonal shift in polarization during photoelectrochemical deposition of Se–Te. Cathodic deposition of semiconducting Se–Te using spatially uniform, linearly polarized illumination produced arrays of lamellae that were aligned parallel to the optical E-field oscillation. Continued deposition in conjunction with an orthogonal shift in the polarization direction then produced aligned bridging features that spanned the void space between, and were orthogonal to, the preexisting lamellae. The height and pitch, respectively, in each layer of the woodpile were a function of the charge density and illumination wavelength during deposition. A Monte Carlo model, in which material addition was scaled by the absorption magnitude obtained from electromagnetic simulations, produced morphologies that were nominally identical to those observed experimentally. Here, the formation of mesoscale woodpiles is consistent with a mechanism that involves a series of spontaneously initiated, concerted light–matter interactions during the photoelectrochemical deposition process.

absorption↗
DOE OSTI2016

Comparison of four MPPT techniques for PV systems

The working behavior of a module / PV array is non-linear and highly dependent on working conditions. As a given condition, there is only one point at which the level of available power at its output is maximum. This point varies with time, enlightenment and temperature. To ensure optimum operation, the use of MPPT control allows us to extract the maximum power. This paper presents a comparative study of four widely-adopted MPPT algorithms, such as Perturb and Observe, Incremental Conductance, Measurements of the variation of the open circuit voltage or of the short-circuit current. Their performance is evaluated using, for all these techniques. In particular, this study compares the behaviors of each technique in presence of solar irradiation variations and temperature fluctuations. These MPPT techniques will be compared using the Matlab / Simulink tool.

14 SOLAR ENERGY↗
DOE OSTI2023

Integration of Ag-CBRAM crossbars and Mott ReLU neurons for efficient implementation of deep neural networks in hardware

In-memory computing with emerging non-volatile memory devices (eNVMs) has shown promising results in accelerating matrix-vector multiplications. However, activation function calculations are still being implemented with general processors or large and complex neuron peripheral circuits. Here, we present the integration of Ag-based conductive bridge random access memory (Ag-CBRAM) crossbar arrays with Mott rectified linear unit (ReLU) activation neurons for scalable, energy and area-efficient hardware (HW) implementation of deep neural networks. We develop Ag-CBRAM devices that can achieve a high ON/OFF ratio and multi-level programmability. Compact and energy-efficient Mott ReLU neuron devices implementing ReLU activation function are directly connected to the columns of Ag-CBRAM crossbars to compute the output from the weighted sum current. We implement convolution filters and activations for VGG-16 using our integrated HW and demonstrate the successful generation of feature maps for CIFAR-10 images in HW. Our approach paves a new way toward building a highly compact and energy-efficient eNVMs-based in-memory computing system.

Mott insulators↗
DOE OSTI2019

Development of Tunable, Precision Permanent Magnet Phase Shifter for X-Ray FEL

In an X-ray free-electron laser (XFEL), a high quality relativistic electron beam from a linear accelerator traverses an array of undulator magnets to generate a high power X-ray beam by allowing the FEL to reach saturation. In many XFELs, phase shifters haven’t been commonly used or included in their designs. The importance of the phase shifter was recognized only recently. A phase shifter can be constructed using either an electromagnet (EM) or a permanent magnet (PM). EM phase shifters are heavy, complex to operate, and take more valuable longitudinal space between the undulators. Compared to EMs, the PMs have several outstanding advantages, including compactness, lighter weights, simple operation, and low maintenance. This project has developed an innovative design of fixed-gap, tunable PM-based compact phase shifter for the XFEL. While keeping the poles and gap fixed, the new design incorporates proven features from the prior work, along with novel field tuning methods, which allow the field strength to be adjusted within a reasonably large range to achieve the desired range of the phase delay control. The PM design without electric currents and power electronics associated with EMs improves reliability by reducing the number of possible points of failure and lowers the operation cost. The compactness makes it possible to use PM phase shifters in the existing XFELs with small gaps between adjacent undulators, as well as in the new XFELs without significantly increasing the FEL length. In Phase I we have accomplished the followings; 1) determined the optimum physics parameters for both weak and strong phase shifters; 2) conducted magnetic design with concrete magnetic and geometrical parameters of actual produced PM materials; 3) provided solutions to achieve precision fine tuning of the magnetic fields with unique methods; 4) elaborated a design based upon ease of manufacturing and cost analysis; and 5) produced rare-earth PM materials, which will be used for prototyping in Phase II. The phase I results provide a foundation for the research and development in the Phase II project, which will aim at building two full-size PM phase shifters and testing at SLAC National Accelerator Laboratory. This project will significantly provide the potential to bring a paradigm change in accelerator technology, replacing conventional electromagnets for many existing accelerator applications and allowing the development of new applications in completely new areas. For example, the PM FEL phase shifters can help significantly improve coherence and power of an XFEL. The proposed use of PM phase shifters will also bring significant cost savings to the construction, operation and maintenance of FEL facilities.

43 PARTICLE ACCELERATORS↗
NASA NTRS1981

Imaging spectroscopy; Proceedings of the Seminar, Los Angeles, CA, February 10, 11, 1981

The seminar concentrated on the science applications of spectral imaging, multispectral imaging sensors, and spectral filtering. Papers are presented on spectroscopic remote sensing for geological applications, spectral mapping of Jupiter, system design of an interferometer imaging sounder and a common module imaging spectral radiometer. Other topics discussed include the performance and application of an intensified linear self-scanned array instrument, tunable optical filtering using an interferometer for selective modulation, design studies for a spectrally agile staring sensor system, and acousto-optic tunable filters for high-resolution spectral analysis.

Norris, D. D.↗
NASA NTRS1999

Control of a Thrust Alignment Table for Modeling the Coning Dynamics of a Spinning Spacecraft with a Follower Force

This document presents a system controlling the motion of a spherical air bearing used in the modeling of spacecraft dynamics and controls in a laboratory environment. The system is part of the Spinning Rocket Simulator (SRS), used to simulate the coning of spacecraft during a thrusting stage. The reaction force at the spherical air bearing supporting the spacecraft model must coincide with the thrust axis of the model for proper simulation. Therefore, the bearing is translated in a circular path to introduce a centrifugal force. This horizontal force along with the gravitational reaction force at the bearing combines to simulate the direction of the spacecraft's thrust force. The control system receives attitude information from the spacecraft model via a laser beam embedded in the model that impinges on a photosensitive array. The non-linear system is controlled using high-speed lookup tables and digital techniques. A vector-controlled motor and a stepper motor are given the necessary signals to accurately control the turntable and platform supporting the air bearing. Preliminary performance data is presented. Mechanical elements of the table and platform are described in detail. A wireless (RF) data path for all devices on the spacecraft model to an off-table command computer is also described.

Halsmer, Dominic↗
NASA NTRS2011

Multi-Segment Radius Measurement Using an Absolute Distance Meter Through a Null Assembly

This system was one of the test methods considered for measuring the radius of curvature of one or more of the 18 segmented mirrors that form the 6.5 m diameter primary mirror (PM) of the James Webb Space Telescope (JWST). The assembled telescope will be tested at cryogenic temperatures in a 17-m diameter by 27-m high vacuum chamber at the Johnson Space Center. This system uses a Leica Absolute Distance Meter (ADM), at a wavelength of 780 nm, combined with beam-steering and beam-shaping optics to make a differential distance measurement between a ring mirror on the reflective null assembly and individual PM segments. The ADM is located inside the same Pressure-Tight Enclosure (PTE) that houses the test interferometer. The PTE maintains the ADM and interferometer at ambient temperature and pressure so that they are not directly exposed to the telescope s harsh cryogenic and vacuum environment. This system takes advantage of the existing achromatic objective and reflective null assembly used by the test interferometer to direct four ADM beamlets to four PM segments through an optical path that is coincident with the interferometer beam. A mask, positioned on a linear slide, contains an array of 1.25 mm diameter circular subapertures that map to each of the 18 PM segments as well as six positions around the ring mirror. A down-collimated 4 mm ADM beam simultaneously covers 4 adjacent PM segment beamlets and one ring mirror beamlet. The radius, or spacing, of all 18 segments can be measured with the addition of two orthogonally-oriented scanning pentaprisms used to steer the ADM beam to any one of six different sub-aperture configurations at the plane of the ring mirror. The interferometer beam, at a wavelength of 687 nm, and the ADM beamlets, at a wavelength of 780 nm, pass through the objective and null so that the rays are normally incident on the parabolic PM surface. After reflecting off the PM, both the ADM and interferometer beams return to their respective instruments on nearly the same path. A fifth beamlet, acting as a differential reference, reflects off a ring mirror attached to the objective and null and returns to the ADM. The spacings between the ring mirror, objective, and null are known through manufacturing tolerances as well as through an in situ null wavefront alignment of the interferometer test beam with a reflective hologram located near the caustic of the null. Since total path length between the ring mirror and PM segments is highly deterministic, any ADM-measured departures from the predicted path length can be attributed to either spacing error or radius error in the PM. It is estimated that the path length measurement between the ring mirror and a PM segment is accurate to better than 100 m. The unique features of this invention include the differential distance measuring capability and its integration into an existing cryogenic and vacuum compatible interferometric optical test.

Merle, Cormic↗