Space and aircraft based sensors working in the ultraviolet spectral region may offer many advantages over longer wavelength systems designed to monitor, identify and track rockets. However, the ultraviolet signatures of various rockets are known to display great sensitivity to operational parameters such as propellant type, oxidizer/fuel ratio, and combustor and nozzle design, as well as trajectory parameters including altitude and degree of solar illumination. This paper will review the basic chemical physics which govern both the emitted and scattered ultraviolet rocket plume signatures for selected rocket types in both low and high altitude regimes. Key chemiluminescent and thermal emission processes will be discussed along with important solar fluorescence and Mie scattering processes. Important emitting and scattering species in the near ultraviolet (400-300 nm), mid ultraviolet (300-200 nm) and far ultraviolet (200-100 nm) spectral regions will be identified and their impact on both spectral signatures and spatial images will be discussed. The ultraviolet absorption properties of the atmosphere and their effects on the utility of the various ultraviolet spectral regions for aircraft and satellite borne sensors will also be presented. Major areas which require further laboratory or theoretical research will be identified.
This paper addresses the problem of determining accurate, real-time ionospheric electron density profiles (EDP) using passive UV and other sensor measurements from satellites. This is done by using real-time satellite data to constrain the geophysical parameters that appear in an ab initio theoretical daytime midlatitude ionospheric model, creating what we call the AFGL constrained EDP model. In February and March 1987 a series of coincident measurements were made by the Millstone Hill incoherent scatter radar and the Polar BEAR satellite multispectral UV imager. These observations of electron density profiles and daytime UV airglow give us an excellent opportunity to test the AFGL constrained EDP model with observational data. Using the satellite UV data and the radar-determined plasma data at one altitude (in-situ satellite plasma measurements were not available), we apply the constrained AFGL model to compute the EDP from about 90 to 600 km. By comparing the model results to the EDP measured by the radar, we show that the AFGL constrained EDP model predicts the EDP more accurately than can empirical models, such as the International Reference Ionosphere (IRI), which contain no real-time data.
The AIRS instrument (Auroral Ionospheric Remote Sensor) was launched into a 1000 km polar orbit aboard the Polar BEAR satellite on 13 November 1986. Optical data from AIRS are obtained at two vacuum ultraviolet wavelengths at the output of a grating monochromator and from two ultraviolet-visible wavelengths at the output of fixed filter photometers. Normally data are transmitted when the satellite is above 45 degrees North latitude; the primary receiving sites are permanent stations at Tromso, Norway and Sondrestrom, Greenland and a portable station often sited near Seattle, Washington, USA. During its first year of operation AIRS has provided imaging data at fixed wavelengths or nadir looking spectrometer data from the vacuum ultraviolet and simultaneous imaging or photometric data from the fixed filters.
The brightness of the reflected sunlight from the Earth has been measured in the UV by the Solar Mesosphere Explorer (SME)1 and by the Solar Backscatter UltraViolet experiment (SBUV)2. The observed spectral radiance, Iλ, divided by the solar spectral irradiance, Fλ, is the albedo, Aλ, shown in Fig. 1 for nadir observations. The logarithm of the albedo is reasonably well fitted by a quadratic in wavelength (Ångstroms) and solar zenith angle, 0, (degrees) as shown in the figure and given by:
One of the instruments operated during the successful Swedish Viking satellite mission of 1986 was an Ultraviolet Imager contributed by Canada. The camera provided global images of the aurora with exposure times of about one second and repetition rates of one to three per minute. This time resolution was achieved using the combination of a fast (f/1) camera, an image intensifier and a coupled CCD detector operated in the time-delay-integration (TDI) mode to "de-spin" the instrument. Interactive control and real-time analysis of data were integral parts of the instrument design. The instrument provided approximately 45,000 images. Based upon the Viking experience, a new UV imager is being developed for the Auroral Probe satellite of the Soviet Union's multi-satellite Interball program.
The Naval Research Laboratory's Middle Atmosphere High Resolution Spectrograph (MAHRS) is included in the experiment complement for the Air Force Space Test Program's Spartan-282 shuttle payload. The major science objectives of the MAHRS experiment are to determine the abundances of nitric oxide (NO) and the hydroxyl radical (OH) between the altitudes of 50 and 150 km, and to measure the diurnal and global variation of these species. The MAHRS instrument is a Czerny-Turner spectrograph fed by a spherical telescope. It's primary measurement objectives are the OH (A E-4" - X2H) 3090 A band and the NO gamma band at 2150 A. The spectral coverage is 1980 to 3091 A at 0.1 A spectral resolution and the field of view is 0.01Â° X 1.15Â°. The focal length is 750 mm and the optical speed is f/7.5. The instrument was designed to produce a flat image at the surface of a 15 mm high X 25 mm wide microchannel plate intensified 2-d array detector with a cesium telluride photocathode and 0.025 mm spatial resolution. In this configuration, for every setting of the grating drive, a passband of about 35 A is imaged on the detector. Recovery of the highest spectral resolution is accomplished by image processing the data.
The Spartan-202 space shuttle mission will include, as its primary scientific instrument, the Naval Research Laboratory's Mark II Far Ultraviolet Camera. The primary scientific objectives of this mission are imaging and photometry, over large areas of the sky, of hot stars, diffuse nebulae, and nearby external galaxies. It is intended to produce a more sensitive and higher resolution far-UV sky survey than previous, similar investigations. The observations provide information on the temperatures and total luminosities of hot stars, and on the far-UV extinction and reflection by interstellar dust, which cannot be obtaihed with ground-based instrumentation or as efficiehtly with other space-based instruments. The measurements complement and support those to be made with the Hubble Space Telescope and the Astro Ultraviolet Imaging Telescope, which have higher angular resolutions and point-source sensitivities but smaller fields of view and lower diffuse-source sensitivities than the Mark II Far UV Camera. The instrument is an electrographic Schmidt camera, similar to but larger (and having higher resolution and point-source sensitivity) than similar cameras we have used in numerous other space flight experiments. The principles, design, and sounding rocket flight history of the Mark II camera are discussed, as well as modifications required to adapt the instrument for the Spartan flight. Target selection criteria, tentative observing plans, and growth potential in the Space Station era are also discussed. Data reduction and analysis plans are described.
The Naval Research Laboratory's Far Ultraviolet Imaging Spectrograph (FUVIS) is currently under development for flight as a Spartan shuttle payload. The science objectives are concerned with spectroscopy of diffuse sources in the far-UV (1000-2000 A) with very high sensitivity and with moderate spatial and spectral resolution. Sources of interest include diffuse nebulae, the gener-al galactic background radiation, and artificially induced radiations associated with the space shuttle vehicle (e.g., surface glow and RCS thruster firings). The measurements complement and support other investigations, such as with the Hubble Space Telescope and the Astro shuttle instruments, which have higher spectral and spatial resolutions but lower diffuse-source sensitivities and smaller fields of view. The principles, design, and sounding-rocket flight experience of FUVIS are discussed, as well as the adaptations required for flight in the Spartan mission. The instrument can be adapted for future, missions by use Of an electron-bombarded CCD array in place of electrographic film recording. The data reduction and analysis plans for the Spartan-281 FUVIS results are presented.
Multi-anode Microchannel Array (MAMA) detector systems with formats of 2048 x 2048 pixels and pixel dimensions of 25 x 25 pm2 are being developed for use in the NASA Goddard Hubble Space Telescope Imaging Spectrograph (STIS). This paper describes the current state of development of these detector systems.
In the next few years the Remote Atmospheric and Ionospheric Detector System (RAIDS) package will be flown on a TIROS spacecraft. The Extreme Ultraviolet Spectrometer (EUVS) experiment contains a position-sensitive detector based on wedge and strip anode tech-nology. A University of California Berkeley detector design has been implemented in brazed alumina and kovar to provide a rugged, bakeable housing and anode. A stack of three 80:1 microchannel plates is operated at 3500-4100 V. to achieve a gain of - 107. The top MCP is to be coated with MgF for increased quantum efficiency in the range of 500-1150 Å. Fabrication of the wedge and strip anode on brazed alumina has presented some challenging problems. Below we present a summary of fabrication techniques and detector performance characteristics.
The microchannel plate (MCP) is a semiconducting glass plate containing an array of open channels that can provide photo-electron amplification for imaging applications. Conventional MCP channel geometries are designed to inhibit ion-feedback that occurs in straight channel plates at high applied potentials. Ion-feedback must be reduced as much as possible since its presence degrades the performance of the plate by adding noise to the plate's output. The performance characteristics of a new geometrical design for a high-gain MCP, that employs channels that spiral downward in a helical fashion to inhibit ion-feedback, have been evaluated in the laboratory. A Multi-Anode Microchannel Array (MAMA) detector was used to test the new MCP. The "helical-channel" MCPs tested to date have channel diameters of 50 microns and have demonstrated stable operation at gains above 106 electrons/pulse. The goal of the program is to develop "helical-channel" MCPs with channel diameters as small as 12 microns within two years. Detailed performance data including the dynamic range, pulse-height distribution, and gain characteristics of this new MCP will be presented.
The spatial frequency response of proximity focused image tubes is inversely proportional to the photoelectron emission energy which is, in turn, dependent on the incident photon energy and the photocathode work function. While the relationship is understood, it is difficult to calculate the spatial frequency response because of the uncertainty in the emission characteristics of a given photocathode. This paper compares the results of measuring the MTF vs. wavelength of three microchannel plate image tubes having S-20, K2CsSb and Cs2Te photocathodes deposited on quartz faceplates. Emphasis is on the wavelength range from 200 to 400 nm where the photon energy significantly exceeds the photocathode band gap. When the photon energy is high enough photoelectron pair production begins to occur there is a decrease in the photoelectron emission energy and corresponding improvement in spatial frequency response.
Currently under fabrication at Optovac, Inc. are two prototype test windows (MgF2 and LiF) to be used on the 75 mm UV MAMA detector tubes for the Hubble Space Telescope Imaging Spectrograph (STIS). The spatial and optical constraints of this instrument dictate that the thickness of the window materials be no greater than 2 to 3 mm to achieve a minimum 50 percent transmission at Hydrogen Lyman alpha (121.6 nm), and that the window must be domed to minimize optical aberrations and provide structural strength. The detector window has an input diameter of about 100 mm with a radius-of-curvature of 70 mm. The manufacturing processes involved in the fabrication of these windows will be discussed, as well as test programs (optical and structural) to be performed at Goddard Space Flight Center (GSFC).
Ultraviolet filters of improved resolution quality have been developed for the Air Force Geophysics Laboratory (AFGL), to enable the acquisition of high-resolution images of missile plumes and similar phenomena. The objectives were to provide a specified passband in the region 2000-4000A, and reject all visible radiation to better than six orders of magnitude, without degrading the transmitted image resolution by more than 5 line-pairs per mm. The resulting filter set has peak transmittance better than 25 percent, with rejection in the visible better than eight orders of magnitude. Inherent image definition capability is estimated better than 40 line-pairs per mm. The imaging performance of the intended camera system (estimated 15 to 20 line-pairs per mm) should not be perceptibly degraded. Based on these results, a more general design and production capability for high-resolution UV filters is considered feasible.
Silicon photodiodes have been developed by defect-free phosphorus diffusion having practically no carrier recombination at the Si-SiO2 interface or in the front diffused region. The quantum efficiency of these photodiodes was found to be around 120% at 100 nm. Unlike the previously tested silicon photodiodes, the developed photodiodes exhibit extremely stable quantum efficiency over extended periods of time. Currently, we are investigating the possibility of using these photodiodes as vacuum ultraviolet detector standards.
We have obtained two mechanically ruled replica gratings from Hyperfine Inc. and one holographically ion-etched grating from Ferranti Astron, Ltd. The gratings, made on spheroid cervit blank, are 86.3 x 86.3 x 12mm and have a groove density of 1710 grooves per mm with a platinum overcoating. The grating manufacturers were given identical specifications (size, groove density reflective coating, etc.). Both gratings were blazed to reflect into the first inside order for a wavelength of ≈ 700 Å. We have evaluated the two gratings at the extreme ultraviolet (EUV) calibration facility at the University of California, Berkeley, Space Sciences Laboratory to determine the suitability for adoption in an EUV spectrometer, the primary instrument of the Remote Atmospheric and Ionospheric Detection System (RAIDS) to be flown aboard a TIROS satellite in 1991. The grating efficiency was measured at five EUV wavelengths using a pencil beam at five positions on the grating. Our measurements show that the efficiencies of the mechanically ruled grating were higher than the holographically ruled grating for the operating order. However, the efficiencies of the holographic grating into the other orders were unexpectedly higher than the ruled ones. The holographically ruled grating displayed about an order of magnitude less scattered light than the mechanically ruled grating.
Optical backgrounds observed from space are highly variable from extreme ultraviolet to the infrared wavelengths. Variations with solar activity can be more than an order of magnitude. Viewing geometry as well as season, latitude, longitude and time of day produce variations of several orders of magnitude. A group of models is described which includes these effects for all of the important optical emissions observed between 500 A and 10,000 A. Solar and particle sources are considered for atomic, molecular, and Rayleigh and aerosol radiation.
Ultraviolet spectra over the southern hemisphere nightside auroral oval were obtained from the AFGL spectral/photometric experiment on board the polar-orbiting S3-4 satellite at 270 km between mid-May and June 1978. Spectra with 30-Å resolution from seven auroral oval crossings were selected to analyze the dif-fuse auroral region under various magnetic activities (Κρ, = 0 to 7 + ). The observed spectra were compared with synthetic spectra and model calculations of the LBH and VK band systems for var-ious studies. The large range of wavelengths (1100-2900 Å) mea-sured allowed the analysis of band and line intensities such as the LBH and VK band systems and the NI (1744-Å), NII (2143-Å), and OI (1356-Å) lines. Certain wavelengths can be used to determine the energy and flux characteristics of the auroral electron precipitation over the auroral oval and provide quantitatively important information on atmospheric parameters. Of the latter, emphasis here is on the atomic oxygen quenching coefficient for the VK band (ν' = 0), the atomic oxygen column density, the NII (2143- Å) line emission cross section, the impact of 02 SR absorption on specific emissions, and the presence and importance of heavy particle precipitation. This study illustrates the range of information that can be obtained by the synthesis of atmospheric emissions model calculations and laboratory measurements.
The Naval Research Laboratory's Remote Atmospheric and Ionospheric Detection System (RAIDS) will fly onboard a TIROS spacecraft in 1990-1991. The orbiting observatory contains, among its complement of instruments, an EUV spectrograph that will obtain vertical profiles from 75 to 750 km of ionospheric emissions over the 500 to 1100 A band. RAIDS will provide a proof-of-concept opportunity for validating passive UV remote sensing of ionospheric weather on a global basis. As part of this effort, predicted ionospheric airglow signatures are being investigated. A promising technique involves the use of observations of the limb at the 0+ 834 Å emission line to deduce the F2 electron density profile during the day. The sensitivity of observations to changes in the ionosphere are investigated theoretically by examining the airglow emissions predicted using model ionospheres. Comparisons among the predicted 834 Å signals demonstrate the technique's sensitivity to temporal, geographic, and geophysical variation. Typical ionospheric profiles are investigated and it is found that observations from RAIDS altitudes are ideally suited to monitoring ionospheric weather.
Rocket plume images photographed with the AFGL UV imager on a video system have been analyzed with a video digitizer and an MS-DOS* computer. The analysis consists of distortion-free rotation of the images to align the plume axis with the horizontal pixel direction, smoothing the images with appropriate image filters, plotting contours and brightness slices of the images, measuring the size from knowledge of the rocket's ephemeris, and correcting for atmospheric absorption. Similar analysis techniques will be presented for laboratory images showing the diffusion of an optically absorbing gas in air.
We have incorporated contemporary grating and detector technologies into the design of a new sounding rocket payload which consists of a slitless, objective grating spectrograph with no transmission elements in the opti-cal train (or detector). Built several years ago, this instrument has an effective collecting area of about 4 cm2 and can record spectra of point-like sources at a wavelength resolution of 0.004Å (profile FWITM) and with a sample interval (pixel width) of 0.002Å. It is designed to give continuous coverage over the wavelength range 950 < X < 1150A, a region of immense importance to our research on the interstellar medium. The instrument, called the Interstellar Medium Absorption Profile Spectrograph (IMAPS), has flown on a Black Brant rocket and obtained a spectrum of superb quality on an early-type star, π Scorpii. Data obtained on this flight also demonstrated the excellent response of our imaging detector, which uses an electron-bombarded CCD to register individual photoevents.
An intensified UV imaging system covering the wavelength range 120 to 330 nm has been built and tested for space flight. A six-position fil-ter wheel divides the range into four narrow bands of interest, provides an ND1 filter for increased dynamic range, and provides a calcium fluo-ride window for maximum sensitivity and good rejection of Lyman al-pha. The optical path includes a reflective telescope, a cesium telluride image intensifier, a fiber-optic reducer, and a 380 by 488 pixel CCD sen-sor. The overall field of view is approximately 2.4° x 1.8°, with an angular spatial resolution exceeding 125 μrad. The system includes digitiz-ing electronics, associated memory, and a microprocessor-controlled AGC system. The maximum sensitivity of the system to a point source ex-ceeds 20 photons/cm2-s in the photon counting mode; with the use of the AGC and ND1 filter, the dynamic range exceeds 107.
We report on the results of laboratory and telescopic measurements of the position sensitivity of a visible MAMA. detector utilizing a "coarse-fine" array. We also report on the photometric accuracy of this detector under the condi-tion of point source illumination. The detector measured uses a bialkalai photocathode and has 25 μm2 pixels in a 256 x 1024 format. It was illuminated with a 3.5 pm spot of light (FWHM) which was moved across the detector in 1 μm increments. It is found that computed centroid positions are accurate across the entire array to within 0.04 pixels. For the extreme case of illumination with a delta function source, this detector experienced photometric errors of 10-15%. Both the position sensitivity and the photometric accuracy with point source illumination are limited in this detector by errors in the flat field corrections caused by the presence of cross-coupling between adjacent pixels. Initial measurements with a MAMA detector utilizing a "fine-fine" array indicate that this cross-coupling has been eliminated in the most recent versions of the detector.
Real-time compensation for platform jitter and target motion has been demonstrated at ultraviolet wavelengths using a photon-counting detector. The detector, a multi-anode microchannel array (MAMA), is a rectangular array of pixels in which the (x,y) position of each received photon is recorded as a function of time. This method of recording photon events permits shifting the pixel address in a contolled fashion when each photon count is added to longterm memory. This shift in pixel address was used to compensate for target motion in the observation of a Scout rocket at 265 nm, thereby permitting measurements of the rocket plume size at this wavelength. The same technique was used in recent laboratory experiments, with the same 256x1024 MAMA array, to provide real-time compensation for platform jitter.
Narrow bandpass UV radiometers are used in a variety of high-temperature measurement applications. These instruments are traditionally calibrated using spectrally flat deuterium lamps. Accurate calibration of these instruments requires detailed knowledge of the filter transmission function and the spectral sensitivity of the photodetector, as well as assumptions about the spectral intensity distribution of the source itself. Significant systematic errors, in the form of an apparent wavelength shift in the system response curve may be introduced when interpreting data obtained from spectrally nonuniform sources (e.g. gray-or blackbody sources). Theoretical calculations, using transmission curves from commercially available narrow bandpass filters, show that the apparent shift in the system spectral response is a function of temperature for a blackbody source. A brief comparison between the theoretical analysis and experimental data is presented.
The temporal and spatial dependence of the transmission of radiation in the near ultraviolet (3000-4000Å) is investigated. Using the LOWTRAN 6 computer code, the transmittance of radiation from several natural and man-made sources through the earth's atmosphere is calculated and compared to observation. Sources are chosen which represent a wide variety of slant paths to space. As a result, transmission of ultraviolet radiation from points near the horizon may be compared to that from sources at smaller zenith angles.
The UV/Ozone cleaning process has been studied as both a method for preventing contamin-ant films from forming on optical surfaces of a space sensor during storage, and for removing them from these surfaces after formation. Using mercury resonance lines at 253.7 and 184.9 nm and 02 pressures in the range of 8 x 10-5 to 4 x 10-4 torr, removal efficiencies from 1.8 x 10-26 to 4.4 x 10-26 cm3/photon were measured. The UV/Ozone process has been shown to be an effective method for preventing contaminant buildup during the storage of a sensor. If the removal efficiencies can be improved, as expected, by using higher energy photons, the UV/Ozone process should also prove to be a viable method for cleaning contaminant films from optical surfaces in space.
We have assembled an ultraviplet-sensitive intensified camera for observing hydrogen combustion by imaging the OH, A2Σ+ - x2II bandhead emissions near 309 nm. The camera consists of a quartz and CaF achromat lense-coupled to an ultraviolet image intensifier which is in turn fiber-coupled to a focus projection scan (FPS) vidicon. The emission band is selected with interference filters which serve to discriminate against background. The camera provides optical gain of 100 to 1000 and is capable of being shuttered at nanosecond speeds and of being framed at over 600 frames per second. We present data from observations of test flames in air at standard RS-170 video rates with varying background conditions. Enhanced images using background subtraction are presented. Finally, we discuss the use of polarization effects for further discrimination against sky background. This work began as a feasibility study to investigate ultraviolet technology to detect hydrogen fires for the NASA space program.
The Multi-Anode Microchannel Array (MAMA) is a photon counting detector which decodes the position of an event through coincidence discrimination. The decoding algorithm which associates a given event with the appropriate pixel is determined by the geometry of the array. The decoding process can be divided into two tasks: anode encoding and pixel decoding. Techniques for performing both tasks are described and contrasted in light of application requirements. Decode times of 75 nanoseconds have been achieved, and considerably faster decoders are possible given new architectures and technologies.
Window and photocathode combinations are used to produce detectors having broadband spectral sensitivities. Using synthetic fused silica input windows and multialkali photocathodes, useful sensitivities are produced in the 160 - 900 nm spectral range. Interesting and strongly wavelength dependent sensitivity and photoelectron emission energy effects are observed in the uv and near-uv spectral regions. A physical model is proposed to describe the observations, based upon wavelength dependent photon absorption, single and multiple photoelectron emission, and thin-film interference effects. These window/photocathode combinations are used in microchannel plate (MCP) photomultiplier tubes (PMTS), MCP image tubes, and position-sensing-PMTS. A near-uv transmissive fiber-optic window/cathode assembly is used on a streak-tube, and its characteristics are also discussed.
The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) flown on Spacelab 2 (SL2) and a second, advanced instrument being prepared for flight on the Upper Atmospheric Research Satellite (UARS) are calibrated using a number of absolute UV irradiance standards in order to meet the stringent calibration goals of the SUSIM program. The SUSIM instruments, the methods of calibration and the error analyses are described.
A high resolution spectrograph has been developed to study thermospheric, ionospheric and auroral airglow processes in the extreme-and far-ultraviolet. The instrument, called the High Resolution Airglow/Aurora Spectrograph (HIRAAS), has been designed to measure the entire airglow spectrum from 500 R to 1550 X at 0.5 X resolution. This spectral resolution is adequate to resolve the fine structure of several atomic and ionic lines and provide partial resolution of the rotational structure of molecular nitrogen bands. The spectrograph employs dual gratings in a modified Rowland circle mount and uses a microchannel plate intensified electrographic detector. The use of electrography provides a large data format, high resolution and high sensitivity. The HIRAAS instrument was designed for sounding rocket flights and has been selected as part of the shuttle deployed Spartan-282 mission. Future plans for HIRAAS include the replacement of the electrographic detector with an electron bombarded charge coupled device (EBCCD) to allow flight on a long-duration satellite mission.
This paper presents the first results obtained at Thomson-CSF on thinned CCDs (576 x 384 pixels) developed for ultraviolet imaging. The process involves chemical thinning of the CCD down to about 10 microns, followed by a shallow implantation (p+ for backside accumulation) activated by laser annealing. UV quantum efficiencies as high as 20 % are measured at 2540 A despite an absorption length in silicon of 55 A at this wavelength. Measurements under 5.9 keV (Fe55) X-ray irradiation have shown 295 eV FWHM energy resolution. After optimization of various parameters in the backside treatment, this technology will in course be applied to several sensors : 1024 x 1024 and buttable devices.