The Joint astrophysical Plasmadynamic EXperiment (J-PEX) is a high-resolution extreme ultraviolet (EUV)
spectrometer (220-245 Å) used for the study of white dwarf atmospheres. Significant improvements have been achieved
in both the normal-incidence gratings and the focal-plane detector since its first successful sounding rocket flight in
2001. The spherical laminar gratings have been replaced by paraboloidal gratings. The substrates of the new gratings
have measured slope errors less than 0.35 arcsec. The gratings were recorded holographically and the rulings transferred
into the silica substrates by ion etching. This procedure was followed by polymer overcoat to reduce the blaze angle of
the groove profile. The detector uses microchannel plates with 6 μm pores and a cross-strip anode, providing 17.9 μm
resolution in the dispersion direction. The detector employs a KBr photocathode with a projected efficiency of 0.24 at
256 Å. Using ray tracing simulations, we predict the resolving power expected from the spectrometer during upcoming
EUV calibrations with a He II discharge source.
APEX is a proposed mission for a Small Explorer (SMEX) satellite. The instrument is a suite of 8 near-normal incidence EUV spectrometers and is the outgrowth of 17 years of research at NRL on multilayer coatings and holographic ion-etched diffraction gratings. A prototype spectrometer has been flown successfully on a sounding rocket. We have examined different multilayer and gratings designs and produced a configuration optimized for the proposed science. APEX will achieve a peak effective area of at least 30-50 cm2 in the range 90-275 Å with resolution ~10,000, significant improvements on Chandra and EUVE.
APEX is a proposed mission for a Small Explorer (SMEX) satellite. APEX will investigate the density, temperature, composition, magnetic field, structure, and dynamics of hot astrophysical plasmas (log T = ~5-7), which emit the bulk of their radiation at EUV wavelengths and produce critical spectral diagnostics not found at other wavelengths. APEX addresses basic questions of stellar evolution and galactic structure through high-resolution spectroscopy of white dwarf stars, cataclysmic variables, the local interstellar medium, and stellar coronae. Thus APEX complements the Chandra, Newton-XMM, FUSE, and CHIPS missions. The instrument is a suite of 8 near-normal incidence spectrometers (~90-275 Angstroms, resolving power ~10,000, effective area 30-50 cm2) each of which employs a multilayer-coated ion-etched blazed diffraction grating and a microchannel plate detector of high quantum efficiency and high spatial resolution. The instrument is mounted on a 3-axis stabilized commercial spacecraft bus with a precision pointing system. The spacecraft is launched by a Taurus vehicle, and payload size and weight fit comfortably within limits for the 2210 fairing. Of order 100 targets will be observed over the baseline mission of 2 years. These are selected carefully to maximize scientific return, and all were detected in the EUVE and the ROSAT WFC surveys.
EUVE and the ROSAT WFC have left a tremendous legacy in astrophysics at EUV wavelengths. More recently, Chandra and XMM-Newton have demonstrated at X-ray wavelengths the power of high-resolution astronomical spectroscopy, which allows the identification of weak emission lines, the measurement of Doppler shifts and line profiles, and the detection of narrow absorption features. This leads to a thorough understanding of the density, temperature, abundance, magnetic, and dynamic structure of astrophysical plasmas. However, the termination of the EUVE mission has left a gap in spectral coverage at crucial EUV wavelengths (~100-300 Å), where hot (105 - 108 K) plasmas radiate most strongly and produce critical spectral diagnostics. CHIPS will fill this hole only partially as it is optimized for diffuse emission and has only moderate resolution (R~150). For discrete sources, we have successfully flown a follow-on instrument to the EUVE spectrometer (Aeff ~ 1 cm2, R ~ 400), the high-resolution spectrometer J-PEX (Aeff ~ 3 cm2, R ~ 3000). Here we build on the J-PEX prototype and present a strawman design for an orbiting spectroscopic observatory, APEX, a SMEX-class instrument containing a suite of 8 spectrometers that together achieve both high effective area (Aeff > 10 cm2) and high spectral resolution (R ~ 10,000) over the range 100-300 Å. We also discuss alternate configurations for shorter and longer wavelengths.
We report on the successful sounding rocket flight of the high resolution (R=3000-4000) J-PEX EUV spectrometer. J-PEX is a novel normal incidence instrument, which combines the focusing and dispersive elements of the spectrometer into a single optical element, a multilayer-coated grating. The high spectral resolution achieved has had to be matched by unprecedented high spatial resolution in the imaging microchannel plate detector used to record the data. We illustrate the performance of the complete instrument through an analysis of the 220-245Å spectrum of the white dwarf G191-B2B obtained with a 300 second exposure. The high resolution allows us to detect a low-density ionized helium component along the line of sight to the star and individual absorption lines from heavier elements in the photosphere.
We have fabricated the four flight gratings for a sounding rocket high-resolution spectrometer using a holographic ion-etching technique. The gratings are spherical (4000-mm radius of curvature), large (160 mm x 90 mm), and have a laminar groove profile of high density (3600 grooves/mm). They have been coated with a high-reflectance multilayer of Mo5C/Si/Mo2Si. Using an atomic force microscope, we examined the surface characteristics before and after multilayer coating. The average roughness is approximately 2-3A rms after coating, somewhat smoothened by the multilayer. Using synchrotron radiation, we completed an efficiency calibration map of each grating over the wavelength range 225-245A. At an angle of incidence of 5 degree(s) and a wavelength of 234A, the average efficiency peaks in the first inside order at 10.3+/- 0.6% for Grating 1, 7.3+/- 0.9% for Grating 6, 7.2+/- 1.2% for Grating 3, and 9.0+/- 1.5% for Grating 4. These values exceed all previously published results for a high density grating. The first order groove efficiency for Grating 1 is 34.4+/- 1.9%, in good agreement with the best value obtained from similar test gratings and close to the theoretical limit of 40.5%.
A feasibility study of megapixel microcalorimeter arrays, based on thermoelectric energy to voltage conversion and digital superconducting readout, is presented. The design concept originated from the philosophy of employing the simplest principles at the single-pixel level to enable large arrays without sacrificing energy resolution, fast operation speed, and quantum efficiency. Initial experimental tests confirm the basic predictions of theory, and show no major obstacle in achieving the desired characteristics.
Kent Wood, Gilbert Fritz, Paul Hertz, W. Neil Johnson, Michael Kowalski, Michael Lovellette, Michael Wolff, Daryl Yentis, Edward Bloom, L. Cominsky, K. Fairfield, G. Godfrey, J. Hanson, A. Lee, Peter Michelson, R. Taylor, H. Wen
The Unconventional Stellar Aspect (USA) experiment on the Advanced Research and Global Observations Satellite (ARGOS) of the Space Test Program is a low-cost, quick, yet scientifically ambitious X-ray timing experiment. The research program emphasizes observing neutron stars and black holes for extended periods with sub-millisecond timing resolution. The scientific program is achieved using hardware whose key features are large collecting area (2000 cm2), energy response extending as low as 1 keV, long accumulated observing times (1 month), high time resolution (1 microsecond(s) ), absolute time-tagging (also to 1 microsecond(s) ) achieved using a GPS receiver, sustained high data rates (40 kbps at all times supplemented by 128 kbps as needed), and flexibility in data handling by using a solid state recorder and a radiation- hardened 20 MIPS 32-bit onboard computer to provide onboard processing.
Robert McCoy, Robert Meier, Kenneth Wolfram, J. Picone, Stefan Thonnard, Gilbert Fritz, Jeff Morrill, David Hardin, Andrew Christensen, David Kayser, James Pranke, Paul Straus
The Remote Atmospheric and Ionospheric Detection System (RAIDS) experiment is an optical remote sensing platform consisting of eight sensors, (spectrographs, spectrometers, and photometers) covering the wavelength range 550 to 8744 Å. RAIDS employs a mechanical scan platform to view the Earth's limb and measure line-of-sight column emission from tangent altitudes from 50 to 750 km. These measurements provide vertical profiles of atmospheric dayglow and nightglow from the mesosphere to the upper regions of the F-region ionosphere. RAIDS will be flown on the National Oceanographic and Atmospheric Administration (NOAA) J weather satellite through the auspices of the U.S. Air Force Space Test Program. The RAIDS wavelength and altitude coverage allows remote sensing of the major and many minor constituents in the thermosphere and ionosphere. These measurements will be used as part of a proof of concept for remote sensing of ionospheric and neutral density profiles. The RAIDS database will be used to study composition, thermal structure, and couplings between the mesosphere, thermosphere, and ionosphere. RAIDS is a joint venture of the Naval Research Laboratory (NRL) and The Aerospace Corporation. We describe the subset of RAIDS instruments developed at NRL covering the far to near UV regions (1300 to 4000 Å).
The Naval Research Laboratory is developing a series of far- and extreme-ultraviolet spectrographs (800 to 1700 Å) to measure altitude profiles of the ionospheric and thermospheric airglow from the U.S. Air Force Defense Meteorological Satellite Program's Block 5D3 satellites. These spectrographs, which comprise the Special Sensor Ultraviolet Limb Imager (SSULI), use a near-Wadsworth optical configuration with a mechanical grid collimator, concave grating, and linear array detector. To image the limb, SSULI employs a rotating planar SiC mirror that sweeps the field of view perpendicular to the limb of the Earth. In the primary operating mode, the mirror sweeps the instrument field of view through 17 deg to view tangent heights from about 50 to 750 km. The SSULI detectors use microchannel plate intensification and wedge-and-strip decoding anodes to resolve 256 pixels in wavelength dispersion. The detector is windowless and uses an o-ring sealed door to protect the Csl photocathode from exposure prior to insertion in orbit. The altitude distributions of the airglow measured by the SSULI sensors will be used to infer the altitude distributions of electrons and neutral species. At night, electron densities will be determined by measurement of ion recombination nightglow. Daytime electron densities will be obtained from measurements of multiple resonant scattering of O+ 834-Å radiation produced primarily by photoionization excitation of atomic oxygen. Dayside neutral densities and temperatures will be inferred from the measurement of dayglow emissions from N2 and O produced by photoelectron impact excitation.
Gregory Saulnier, Robert Zacher, Deborah Van Vechten, Craig Boyer, Michael Lovellette, Gilbert Fritz, Robert Soulen, Joonhee Kang, Mark Blamire, Eugenie Kirk, Robert Somekh
We report on our ongoing work using Nb/Al/AIOx!Nb junctions for the detection ofx-rays. Detectors based on superconducting tunneling junctions offer the prospect of resolution over an order of magnitude higher than is obtainable with the current generation of semiconductor-based detectors. Unlike Sn junctions, these Nb-based devices are not degraded by repeated thermal cycling and are known to be exceptionally "radiation hard". We present results on measurements taken at 1.85 K, a temperature achievable with current space flight technology. These measurements include the current-voltage (1-V) curve, subgap current vs. temperature, the dependence of the superconducting current on the applied magnetic field (Fraunhofer pattern), x-ray pulses, and the spectra from a 6 keV x-ray source which gave an intrinsic device resolution of approximately 700 eV. A comparison of the x-ray spectrum peak with a known injected pulse of 105 electrons indicates the collection of more than 105electrons per 6 keV photon.
Robert McCoy, Robert Meier, Kenneth Wolfram, J. Picone, Stefan Thonnard, Gilbert Fritz, Jeff Morrill, David Hardin, Andrew Christensen, David Kayser, James Pranke, Paul Straus
The RAIDS experiment is an optical remote sensing platform consisting of eight sensors (spectrographs, spectrometers and photometers) covering the wavelength range 550 A to 8744 A. RAIDS employs a mechanical scan platform to view the Earth's limb and measure vertical profiles of atmospheric dayglow and nightglow from the mesosphere to the upper regions of the F region ionosphere (75 -750 km). RAIDS will be flown on the NOAA J weather satellite through the auspices of the Air Force Space Test Program (STP). The RAIDS wavelength and altitude coverage allows remote sensing of the major, and many minor constituents in the thermosphere and ionosphere. These measurements will be used as part of a proof-of-concept for remote sensing of ionospheric and neutral density profiles. The RAIDS database will be used to study composition, thermal structure and couplings between the mesosphere, thermosphere and ionosphere. RAIDS is a joint venture of the Naval Research Laboratory (NRL) and The Aerospace Corporation. This paper describes the subset of RAIDS instruments developed at NRL covering the far to near ultraviolet (1300 A - 4000 A). A companion paper describes the balance of the experiment complement.
The Naval Research Laboratory is developing a limb imaging far- and extreme-ultraviolet (FUV/EUV) spectrograph (800-1700 A) to measure vertical profiles of the ionospheric and thermospheric airglow from DMSP Block 5D3 satellites. The spectrograph, called the Special Sensor Ultraviolet Limb Imager (SSULI), uses a near-Wadsworth optical configuration with a mechanical grid collimator, concave grating and linear array detector. Measured airglow profiles from the SSULI sensors will be used to infer vertical profiles of electron density and neutral density. At night, electron densities will be determined by measurement of ion recombination nightglow. Daytime electron densities will be obtained from measurements of multiple resonant scattering of O(+) 834 A radiation produced primarily by photoionization excitation. Dayside neutral densities and temperatures will be inferred from measurement of dayglow emissions from N2 and O produced by photoelectron impact excitation.
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