The various demands on funding agencies make it difficult to sustain the level of expenditure required to provide the
broad range of space astronomy missions that the research community would like to have available. Multi-billion
pound/dollar observatories such Chandra, XMM-Newton and HST have been enormously successful, but JWST has been
delayed and plans for an equivalent large X-ray mission seem to be on-hold. Furthermore, the medium size ESA and
NASA missions provide only a small number of opportunities over the next decade. Much exciting and important
science, by default, will not be done. If satellite mission costs could be reduced significantly, by a factor of 5-10, we
would open up a new parameter space of opportunity that is not currently offered by any agency. Significant
improvement in instrument technology coupled with simplification of optical systems and the development of efficient,
high performance small satellite platforms and ground systems has led to the prospect of the development of some low-cost
opportunities. In this paper, we outline one such possible mission, based on a successful sounding rocket-borne
payload. This comprises a high throughput normal incidence extreme ultraviolet spectrometer, with the design adapted
for accommodation on the SSTL 300 platform. We make use of a segmented diffraction grating to provide an overall
wavelength coverage from ~170-260Å by tuning the multi-layers of the individual elements to different, overlapping
ranges. We outline the capability and science goals of the mission, and how they influence the design and operation of
the satellite platform. We conclude with a discussion of how missions of this type operating both as constellations and
as formation flying sparse apertures, could offer a scientifically viable alternative to monolothic 'great observatory'
missions in the future.
The EUV waveband includes critical spectral features containing diagnostic information often not available at other
wavelengths, and the bulk of radiation from million degree plasmas is emitted in the EUV. Such plasmas are ubiquitous,
and examples include white dwarf photospheres; accretion phenomena in young stars, CVs and AGN; stellar coronae;
and the ISM of our galaxy and of others. However, sensitive high-resolution spectroscopy is required to resolve and
identify source and ISM spectral features unambiguously, and to measure line profiles and Doppler shifts. This allows
exploitation of the full range of plasma diagnostic techniques developed in laboratory and solar physics. The J-PEX
high-resolution EUV spectrometer has made a breakthrough in capability with an effective area of 7 cm2 (220-250 Å)
and resolving power of 4000, which exceed EUVE by factors of 7 and 20 respectively, and cover a range beyond the
170-Å cutoff of the Chandra LETG. J-PEX has flown successfully twice on NASA sounding rockets, but NASA has
approved no new orbital EUV mission. It is time for one. Here we describe the scientific case for high-resolution EUV
spectroscopy, summarize the technology that makes practical such measurements, and present concepts for a ~3-month
orbital mission and for larger missions.
As astronomical observations are pushed to cosmological distances (z>3) the spectral energy distributions of X-ray
objects, AGN for example, will be redshifted into the EUV waveband. Consequently, a wealth of critical spectral
diagnostics, provided by, for example, the Fe L-shell complex and the O VII/VIII lines, will be lost to future planned X-ray
missions (e.g., IXO, Gen-X) if operated at traditional X-ray energies. This opens up a critical gap in performance
located at short EUV wavelengths, where critical X-ray spectral transitions occur in high-z objects. However, normal-incidence
multilayer-grating technology, which performs best precisely at such wavelengths, together with advanced
nanolaminate replication techniques have been developed and are now mature to the point where advanced EUV
instrument designs with performance complementary to IXO and Gen-X are practical. Such EUV instruments could be
flown either independently or as secondary instruments on these X-ray missions. We present here a critical examination
of the limits placed on extragalactic EUV measurements by ISM absorption, the range where high-z measurements are
practical, and the requirements this imposes on next-generation instrument designs. We conclude with a discussion of a
breakthrough technology, nanolaminate replication, which enables such instruments.
A 4 mm diameter zone plate was calibrated in the 7 nm to 18.5 nm wavelength range using synchrotron radiation. The efficiency in the focused 1st order was measured using the scanning monochromator at the Naval Research Laboratory beamline X24C at the National Synchrotron Light Source. The measured efficiencies were compared to efficiencies calculated by accounting for the partial transmittance through the molybdenum zone plate rings and the resulting phase enhancement of the efficiencies. Accurate absolute efficiency calibrations enable the use of zone plates in EUV solar
irradiance monitors having excellent stability against contamination and oxidation.
Since hot 100,000-1,000,000K gas in stars radiates predominantly at EUV and soft X-ray wavelengths, observations in
these bands provide important diagnostics of the physical conditions in hot photospheres, stellar coronae and stellar
winds. However, such studies are only able to examine the bulk of material, without being able to separate out the
several gas components present. Radial velocity diagnostics have been used frequently in the UV and visible bands to
distinguish different emission or absorption components in stellar spectra. Now, developments in grating and instrument
technology provide a first opportunity to extend this technique into the EUV. Based on capabilities of the improved JPEX
spectrometer (reported elsewhere in this volume), this paper reports on the key science that might be carried out
with such an instrument; both as a sounding rocket payload and longer duration mission.
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.
We have fabricated five new holographic ion-etched polymer-coated gratings for a reflight on a sounding rocket of the J-PEX high-resolution EUV spectrometer. The gratings are parabolic (nominal 2000-mm focal length), large (160 mm x 90 mm), and have a blazed groove profile of high density (3600 grooves/mm at the vertex). They have been coated with a high-reflectance multilayer of Mo/Si/C. Using an atomic force microscope, we examined grating topography before multilayer coating. The surface roughness is 2 angstrom rms and the blaze angle is near the target value of 2.4°. Using synchrotron radiation, we completed an efficiency calibration map of each multilayer-coated grating over the wavelength range 220-245 angstrom. At an angle of incidence of 5°, the average efficiency in the first inside order peaks near 234 angstrom. The average peak efficiency is 12.3 ± 1.0% for Grating 1, 12.6 ± 2.4% for Grating 2, 12.6 ± 1.8% for Grating 3, 14.1 ± 3.0% for Grating 4, and 13.0 ± 1.0% for Grating 5. The derived groove efficiency averaged over all gratings is approximately
50%, which meets our goals. Refined models of the multilayer gratings are required to resolve remaining issues.
We have measured the topography and near-normal incidence EUV efficiency of five flat multilayer-coated polymer-overcoated blazed ion-etched holographic test gratings. Blaze angles were in the range 2.0-4.1°. All gratings had a surface roughness <3 Å rms (20-4000 Å). The best grating had a measured efficiency of 29.9% in the second order at 157.9 Å and a derived groove efficiency of 53.0%. At the shortest wavelength investigated (100.0 Å) another grating produced a measured efficiency in the first order of 12.9% and a derived groove efficiency of 33.6%. In third order another grating produced a measured efficiency at 137.8 Å of 13.4% and a derived groove efficiency of 21.8%. To the best of our knowledge these values exceed previous published results. Some issues remain that may be associated with modification of the groove profile by the multilayer coating.
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.
The measured efficiencies of two flight gratings and the reflectances of two flight mirrors developed for the Extreme-Ultraviolet Imaging Spectrometer (EIS) for the Japanese Solar-B mission are presented. Each optic has two sectors with Mo/Si multilayers that refelct the 17 - 21 nm and 25 - 29 nm wavebands at normal incidence. The efficiencies that were measured using monochromatic synchrotron radiation are in good agreement with the calculated efficiencies.
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%.
The Joint Astrophysical Plasmadynamic Experiment is a high- resolution extreme UV spectrometer which operates at near- normal incidence and incorporates the most recent developments in reflecting multilayer gratings. The spectrometer comprises the following principal elements: collimators which define the 1.2 degrees field of view, a spherical multilayer-coated grating which consists of 4 segments, each 8 cm by 16 cm in area and with a focal length of 2.2 m, and a microchannel plate imaging detector. The collimator is an adaptation of a previous design which was modified to minimize the input of scattered airglow radiation while maintaining the maximum effective area within the desired field of view. This is achieved by blackening the collimator surfaces with an EBONOL-C process. In this paper we will discuss the design requirements and manufacturing techniques for the collimator.
Multilayer coatings have been applied to holographic diffraction gratings that have ion-etched blazed groove profiles. For each grating, the multilayer coating was designed to match the blaze angle so that the grating had high efficiency near normal incidence and in a wavelength region of choice in the 125-300 angstrom range. The grating efficiencies as functions of wavelength and incidence angle were measured using synchrotron radiation and were compared to the expected results. Normal-incidence efficiencies up to 13 percent were achieved at a wavelength of 128 angstrom. The performance characteristics of multilayer holographic and ruled replica gratings were compared and were related to the shape and roughness of the groove profiles as determined by atomic force microscopy.
High-reflectance multilayer mirrors and gratings have been developed and implemented in the x-ray and XUV regions. The imaging and spectroscopic instruments have high throughput and can be positioned at a large distance from the radiation source where damage from the plasma debris and the radiation flux does not occur.
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.
A program is underway at the Naval Research Laboratory (NRL) to develop a high-resolution spectrometer for the study of astrophysical sources at EUV/soft X-ray wavelengths. The spectrometer design is simple in that the sole optic is a multilayer-coated spherical grating or mosaic of co-aligned gratings used at near-normal incidence, allowing large effective collecting area without the strict tolerance requirements of grazing incidence optics. Therefore, both high resolution and high throughput can be obtained over several selected narrow bandpasses. We present efficiency and resolving power measurements of spherical gratings which have parameters similar to that intended for our flight instrument. Two gratings were replicated from the same ruled master and then coated with a multilayer of molybdenum and silicon. A third sister grating was used as a control and overcoated with gold.
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