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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS2018

ICESat-2 Photon Classification: Finding Signal Photons in the ATL03 Geolocated Photon Data Product

ICESat-2 carries NASA's next-generation laser altimeter, ATLAS, (Advanced Topographic Laser Altimeter System), designed to measure changes in ice sheet height, sea ice freeboard, and vegetation canopy height. ATLAS contains a photon-counting lidar which transmits green (532-nm) pulses at 10kHz. Each pulse is split into 3 pairs of beams (one strong and one weak). Approximately 1014 photons per pulse travel from ATLAS through the atmosphere to reflect off the Earth's surface. Some return back into the ATLAS telescope where they are recorded. Photons from sunlight and instrument noise at the same wavelength are also recorded. The flight software time tags all photons within a 500m to 6 km range window and generates histograms. Using the histograms, it selects a telemetry window which varies from 20m over flat surfaces to hundreds of meters over rougher terrain. ATL03 contains the time, height (relative to the WGS-84 ellipsoid), latitude and longitude of every photon within the telemetry window. The basic challenge is to determine which of these photons were reflected off the surface. We have developed an algorithm that identifies these signal photons and assigns a confidence level (low, medium, or high) to each signal photon based on the signal to noise ratio. We present an overview of the signal identification algorithm and show the results on actual ICESat-2 data over ice sheet, sea ice, vegetated, and water surfaces. Higher level ATLAS products work with aggregations of the photons in order to determine the ellipsoidal height of the Earth, canopy height and structure, and other quantities of geophysical interest.

Remote Sensing↗
NASA NTRS2012

Observations of GEO Debris with the Magellan 6.5-m Telescopes

Optical observations of geosynchronous orbit (GEO) debris are important to address two questions: 1. What is the distribution function of objects at GEO as a function of brightness? With some assumptions, this can be used to infer a size distribution. 2. Can we determine what the likely composition of individual GEO debris pieces is from studies of the spectral reflectance of these objects? In this paper we report on optical observations with the 6.5-m Magellan telescopes at Las Campanas Observatory in Chile that attempt to answer both questions. Imaging observations over a 0.5 degree diameter field-of-view have detected a significant population of optically faint debris candidates with R > 19th magnitude, corresponding to a size smaller than 20 cm assuming an albedo of 0.175. Many of these objects show brightness variations larger than a factor of 2, suggesting either irregular shapes or albedo variations or both. The object detection rate (per square degree per hour) shows an increase over the rate measured in the 0.6-m MODEST observations, implying an increase in the population at optically fainter levels. Assuming that the albedo distribution is the same for both samples, this corresponds to an increase in the population of smaller size debris. To study the second issue, calibrated reflectance spectroscopy has been obtained of a sample of GEO and near GEO objects with orbits in the public U.S. Space Surveillance Network catalog. With a 6.5-m telescope, the exposures times are short (30 seconds or less), and provide simultaneous wavelength coverage from 4500 to 8000 Angstroms. If the observed objects are tumbling, then simultaneous coverage and short exposure times are essential for a realistic assessment of the object fs spectral signature. We will compare the calibrated spectra with lab-based measurements of simple spacecraft surfaces composed of a single material.

Seitzer, Patrick↗
NASA NTRS1978

Three-mirror space telescope, volume 2

The anastigmatic three mirror telescope for STARSAT, a space astronomy facility, was redesigned and reoptimized in order to improve the optical system and increase its versatility. A more compact system was obtained by decreasing the primary focal ratio from 2.2 to 2.0. A high performance Rowland spectrograph that uses only a total of three reflections and does not interfere with the imaging process, was successfully incorporated into the telescope so that it could be a permanent part of the system. The usefulness of this telescope concept as a high resolution coronagraph is examined.

Korsch, D.↗
NASA NTRS2005

Near-Infrared (0.67-4.7 microns) Optical Constants Estimated for Montmorillonite

Various models of the reflectance from particulate surfaces are used for interpretation of remote sensing data of solar system objects. These models rely upon the real (n) and imaginary (k) refractive indices of the materials. Such values are limited for commonly encountered silicates at visual and near-infrared wavelengths (lambda, 0.4-5 microns). Availability of optical constants for candidate materials allows more thorough modeling of the observations obtained by Earth-based telescopes and spacecraft. Two approaches for determining the absorption coefficient (alpha=2pik/lambda) from reflectance measurements of particulates have been described; one relies upon Kubelka-Munk theory and the other Hapke theory. Both have been applied to estimate alpha and k for various materials. Neither enables determination of the wavelength dependence of n, n=f(lambda). Thus, a mechanism providing this ability is desirable. Using Hapke-theory to estimate k from reflectance measurements requires two additional quantities be known or assumed: 1) n=f(lambda) and 2) d, the sample particle diameter. Typically n is assumed constant (c) or modestly varying with lambda; referred to here as n(sub 0). Assuming n(sub 0), at each lambda an estimate of k is used to calculate the reflectance and is iteratively adjusted until the difference between the model and measured reflectance is minimized. The estimated k's (k(sub 1)) are the final results, and this concludes the typical analysis.

Roush, T. L.↗
NASA NTRS2021

Reflection Modeling of the Black Hole Binary 4U 1630–47: The Disk Density and Returning Radiation

We present the analysis of X-ray observations of the black hole binary 4U 1630−47 using relativistic reflection spectroscopy. We use archival data from the Rossi X-ray Timing Explorer, Neil Gehrels Swift Observatory, and Nuclear Spectroscopic Telescope Array observatories, taken during different outbursts of the source between 1998 and 2015. Our modeling includes two relatively new advances in modern reflection codes: high-density disks, and returning thermal disk radiation. Accretion disks around stellar-mass black holes are expected to have densities well above the standard value assumed in traditional reflection models (i.e., n(e) ∼ 10^(15) per cu. cm). New high-density reflection models have important implications in the determination of disk truncation (i.e., the disk inner radius). This is because one must retain self-consistency in the irradiating flux and corresponding disk ionization state, which is a function of disk density and system geometry. We find that the disk density is n(e) ≥ 10^(20) per cu. cm across all spectral states. This density, combined with our constraints on the ionization state of the material, implies an irradiating flux impinging on the disk that is consistent with the expected theoretical estimates. Returning thermal disk radiation—the fraction of disk photons that bend back to the disk producing additional reflection components—is expected predominantly in the soft state. We show that returning radiation models indeed provide a better fit to the soft-state data, reinforcing previous results that show that in the soft state, the irradiating continuum may be blackbody emission from the disk itself.

Riley M. T. Connors↗
DOE OSTI2020

Updated design of the CMB polarization experiment satellite LiteBIRD

Recent developments of transition-edge sensors (TESs), based on extensive experience in ground-based experiments, have been making the sensor techniques mature enough for their application on future satellite CMB polarization experiments. LiteBIRD is in the most advanced phase among such future satellites, targeting its launch in Japanese Fiscal Year 2027 (2027FY) with JAXA's H3 rocket. It will accommodate more than 4000 TESs in focal planes of reflective low-frequency and refractive medium-and-high-frequency telescopes in order to detect a signature imprinted on the cosmic microwave background (CMB) by the primordial gravitational waves predicted in cosmic inflation. The total wide frequency coverage between 34GHz and 448GHz enables us to extract such weak spiral polarization patterns through the precise subtraction of our Galaxy's foreground emission by using spectral differences among CMB and foreground signals. Telescopes are cooled down to 5Kelvin for suppressing thermal noise and contain polarization modulators with transmissive half-wave plates at individual apertures for separating sky polarization signals from artificial polarization and for mitigating from instrumental 1/f noise. Passive cooling by using V-grooves supports active cooling with mechanical coolers as well as adiabatic demagnetization refrigerators. Sky observations from the second Sun-Earth Lagrangian point, L2, are planned for three years. An international collaboration between Japan, USA, Canada, and Europe is sharing various roles. In May 2019, the Institute of Space and Astronautical Science (ISAS), JAXA selected LiteBIRD as the strategic large mission No. 2.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS1974

Light scattering indicatrices of lunar surface material returned by Luna 16 automatic station

Measurements are made of the indicatrix of scattering of lunar surface material with an indicatometer that has a spread of the illuminating beam of less than 0.5 deg and of the detected beam of about 1.5 deg. The results are compared with the indicatrices for the lunar mean obtained by terrestrial telescopic measurements. It is concluded that the main features of the reflection of light by the moon) (rapid rise in brightness with approach to the full moon) are accounted for by the microrelief caused principally by grains smaller than a millimeter.

Barabashov, N. P.↗
NASA NTRS1975

Development of an airborne laser bathymeter

An airborne laser depth sounding system was built and taken through a complete series of field tests. Two green laser sources were tried: a pulsed neon laser at 540 nm and a frequency-doubled Nd:YAG transmitter at 532 nm. To obtain a depth resolution of better than 20 cm, the pulses had a duration of 5 to 7 nanoseconds and could be fired up to at rates of 50 pulses per second. In the receiver, the signal was detected by a photomultiplier tube connected to a 28 cm diameter Cassegrainian telescope that was aimed vertically downward. Oscilloscopic traces of the signal reflected from the sea surface and the ocean floor could either be recorded by a movie camera on 35 mm film or digitized into 500 discrete channels of information and stored on magnetic tape, from which depth information could be extracted. An aerial color movie camera recorded the geographic footprint while a boat crew of oceanographers measured depth and other relevant water parameters. About two hundred hours of flight time on the NASA C-54 airplane in the area of Chincoteague, Virginia, the Chesapeake Bay, and in Key West, Florida, have yielded information on the actual operating conditions of such a system and helped to optimize the design. One can predict the maximum depth attainable in a mission by measuring the effective attenuation coefficient in flight. This quantity is four times smaller than the usual narrow beam attenuation coefficient. Several square miles of a varied underwater landscape were also mapped.

Kim, H., H.↗
NASA NTRS1982

Copernicus crater central peak - Lunar mountain of unique composition

Olivine is identified as the major mafic mineral in a central peak of Copernicus crater. Information on the mineral assemblages of such unsampled lunar surface material is provided by near infrared reflectance spectra (0.7 to 2.5 micrometers) obtained with earth-based telescopes. The composition of the deep-seated material comprising the Copernicus central peak is unique among measured areas. Other lunar terra areas and the wall of Copernicus exhibit spectral characteristics of mineral assemblages comparable to the feldspathic breccias returned by the Apollo missions, with low-calcium orthopyroxene being the major mafic mineral.

Pieters, C. M.↗
NASA NTRS1994

Laser altimetry simulator. Version 3.0: User's guide

A numerical simulator of a pulsed, direct detection laser altimeter has been developed to investigate the performance of space-based laser altimeters operating over surfaces with various height profiles. The simulator calculates the laser's optical intensity waveform as it propagates to and is reflected from the terrain surface and is collected by the receiver telescope. It also calculates the signal and noise waveforms output from the receiver's optical detector and waveform digitizer. Both avalanche photodiode and photomultiplier detectors may be selected. Parameters of the detected signal, including energy, the 50 percent rise-time point, the mean timing point, and the centroid, can be collected into histograms and statistics calculated after a number of laser firings. The laser altimeter can be selected to be fixed over the terrain at any altitude. Alternatively, it can move between laser shots to simulate the terrain profile measured with the laser altimeter.

Abshire, James B.↗
NASA NTRS2004

Solar C: Scatter-Free Observatory for Limb Active Regions and Coronae

The SOLARC observatory is a 0.5m off-axis reflecting coronagraph. NASA SRT funding has allowed the completion of this telescope facility, which we believe is now the world's largest operational coronagraph. We have achieved our proposal goals both in the development of the new technology for this facility and in the demonstration of its scientific and educational potential: 1) The SOLARC engineering development was successful and has spawned other similar instruments. The off-axis design of ATST (a new 4m telescope) has benefited from the SOLARC development. The off-axis 1.6m NST facility at BBSO is also now under construction and will be the world's largest solar disk observing telescope until ATST is completed. Both of these telescope designs are derivatives of the SOLARC 0.5m off-axis. Some of this technical development is described in the publications. 2) The most important scientific goal of SOLARC has been to demonstrate that coronal magnetic fields can be measured using infrared spectropolarimetry techniques. With the completion of the optical fiber-bundle imaging spectropolarimeter we have measured the coronal field strength with a sensitivity to the line-of-sight field component of 2G at 150,OOOkm above the limb. We believe this capability opens new opportunities for space weather, and coronal physics research. In particular we have demonstrated a new tool for understanding the effect of the Sun on the terrestrial space environment. A paper reporting these results has recently been submitted to the Astrophysical Journal Letters.

Source record↗
NASA NTRS2002

Microaltimeter

A microaltimeter measures altitude or range highly accurately from an orbiting vehicle. The microaltimeter has a low power solid state laser that is pulsed at a rate above 1 kilohertz. The pulses are delivered to a small telescope which sends them to a planetary surface and receives return reflections. A high efficiency photon detector measures received photons and supplies received photon signals to a process or which makes a time-based bin-wise comparison to find the time of flight and hence the range.

Degnan, John J↗
NASA NTRS1991

X-ray/EUV optics for astronomy, microscopy, polarimetry, and projection lithography; Proceedings of the Meeting, San Diego, CA, July 9-13, 1990

Topics discussed in this issue include the fabrication of multilayer X-ray/EUV coatings; the design, characterization, and test of multilayer X-ray/EUV coatings; multilayer X-ray/EUV monochromators and imaging microscopes; X-ray/EUV telescopes; the test and calibration performance of X-ray/EUV instruments; XUV/soft X-ray projection lithography; X-ray/EUV space observatories and missions; X-ray/EUV telescopes for solar research; X-ray/EUV polarimetry; X-ray/EUV spectrographs; and X-ray/EUV filters and gratings. Papers are presented on the deposition-controlled uniformity of multilayer mirrors, interfaces in Mo/Si multilayers, the design and analysis of an aspherical multilayer imaging X-ray microscope, recent developments in the production of thin X-ray reflecting foils, and the ultraprecise scanning technology. Consideration is also given to an active sun telescope array, the fabrication and performance at 1.33 nm of a 0.24-micron-period multilayer grating, a cylindrical proportional counter for X-ray polarimetry, and the design and analysis of the reflection grating arrays for the X-Ray Multi-Mirror Mission.

Hoover, Richard B.↗
NASA NTRS1975

Scanning Kirkpatrick-Baez X-ray telescope to maximize effective area and eliminate spurious images - Design

We consider the design of a Kirkpatrick-Baez grazing-incidence X-ray telescope to be used in a scan of the sky and analyze the distribution of both properly reflected rays and spurious images over the field of view. To obtain maximum effective area over the field of view, it is necessary to increase the spacing between plates for a scanning telescope as compared to a pointing telescope. Spurious images are necessarily present in this type of lens, but they can be eliminated from the field of view by adding properly located baffles or collimators. Results of a computer design are presented.

Kast, J. W.↗
NASA NTRS2006

"The Reflection Grating Spectrometer on Constellation-X"

The Constellation-X Reflection Grating Spectrometer (RGS) is designed to provide high-throughput, high-resolution spectra in the long wavelength band of 6 to 50 angstrom. In the nominal design an array of reflection gratings is mounted at the exit of the Spectroscopy X-ray Telescope (SXT) mirror module. The gratings intercept and disperse light to a designated array of CCD detectors. To achieve the throughput (A_eff > 1000 cm2 below 0.6 keV) and resolution (R > 300 below 0.6 keV) requirements for the instrument we are investigating two possible grating designs. The first design uses in-plane gratings in a classical configuration that is very similar to the XMM-Newton RGS. The second design uses off-plane gratings in a conical configuration. The off-plane design has the advantage of providing higher reflectivity and potentially, a higher spectral resolution than the in-plane configuration. In our presentation we will describe the performance requirements and the current status of the technology development.

Cottam, J.↗
DOE OSTI2021

Standoff Detection of Oil and Powder Mixtures at 12 Meters Using a Tunable Quantum Cascade Laser-Based System with a Close Focus Telescope and Uncooled Infrared Detector

We have designed and demonstrated a quantum cascade laser (QCL) based standoff system that utilizes an uncooled mercury cadmium telluride (MCT) detector with lock-in signal processing for chemical identification at a distance of 12.5 meters in indoor ambient light conditions. In the system, a tunable quad-QCL operating (1 MHz) in quasi-continuous wave mode between 8.45 and 10.03 μm (~1182 to 1000 cm –1 ) serves as the active mid-infrared source for remotely interrogating mineral, powder, and thin film oil samples including powder mixtures (6, 12.5, 25, and 50%) of crystalline quartz (SiO 2 ) in KBr. Light as reflected from a given sample is collected using a 10-inch (25.4 cm) Dall Kirkham telescope and coupled with ZnSe optics to an uncooled MCT detector. Furthermore, the mixture dependence of the highly transparent KBr and strongly absorbing quartz was found to fit a modified version of the Schatz reflectance model for compacted powder mixtures. All reflectance spectra reported are relative to an Au-coated diffuse reflector. A NIST traceable polystyrene standard reflector was also used to determine the QCL wavelength tuning range and calibration.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1977

The asteroid albedo scale. I - Laboratory polarimetry of meteorites

Laboratory reflection polarimetry is presented for crushed rocks, lunar soils, and 24 meteoritic samples. The polarization-phase curves found for the meteorites closely resemble telescopic results for minor planets, but only if the samples are crushed finely enough to coat the larger particles with dust. The results generally confirm the identifications previously made from reflection spectroscopy for the meteoritic analogues of various types of asteroids; the more metal-rich ordinary chondrites, for example, match telescopic observations of the S-type minor planets well enough, as do the basaltic achondrites for Vesta. No meteorite measured, however, has a geometric albedo less than 0.05 as required for many of the C asteroids. A calibration of the polarimetric slope-albedo law, now determined strictly for meteoritic samples, confirms the asteroid albedo and diameter scale derived by the independent method of thermal radiometry.

Zellner, B.↗
NASA NTRS1977

A refracting radio telescope

Observations of extraterrestrial radio sources at the lower end of the radio frequency spectrum are limited by reflection of waves from the topside ionosphere and by the large size of antenna apertures necessary for the realization of narrow beamwidths. The use of the ionosphere as a lens is considered. The lens is formed by the release of chemicals such as H2 and H2O at the F2-layer peak. These chemicals promote dissociative recombination of O(+) in the ionosphere resulting in a local reduction in plasma density. Gradients in electron density in the vicinity of the gas release tend to focus rays propagating through the depleted region. Preliminary calculations indicate that a lens capable of focusing cosmic radio waves in the 1 to 10 MHz frequency range may be produced by the release of 100 kg of H2 at the peak of the nighttime F layer. The beamwidth of a refracting radio telescope using this lens may be less than 1/5 degree.

Bernhardt, P.↗