The transmission of ultraviolet radiation in the middle ultraviolet region of the spectrum (2000-3000Å is investigated taking into consideration 02 and 03 absorption, Rayleigh scattering and aerosol scattering and absorption. The computer code LOWTRAN 6 with O2 absorption added was used to perform the calculations. The major portion of the calculations deals with transmission to space from a given altitude in the earth's atmosphere.
The Naval Research Laboratory's Far Ultraviolet Cameras experiment (NRL-803) is planned for flight on the space shuttle as part of a Space Test Program mission, AFP-675. The experiment objectives are imagery and photometry of natural and artificial sources of far-ultraviolet radiation, with emphasis on faint, diffuse sources such as night airglow, diffuse nebulae, and galactic background. The instrument package consists of two electrographic Schmidt cameras, covering the wavelength ranges 1050-1600 and 1230-2000 Å, a dedicated pointing platform, an image fine stabilization system for long exposures on celestial targets, and a low light level television camera for pointing verification.
Recent observations of visible and near-ultraviolet glows extending from ram-directed surfaces of low earth-orbiting spacecraft (such as Shuttle Orbiter and Atmosphere Explorer) have aroused interest in the mechanism responsible and the potential for similar foreground emissions at heretofore unmonitored wavelengths. The spectral and spatial distribution of the glow in the visible indicates that it is predominantly due to recombination of nitric oxide and atomic oxygen catalyzed by the surface exposed to the airflow. Incident ambient 0 and N atoms are also expected to be recombined into oxygen, nitrogen, and nitric oxide molecules in electronically excited states, many of which radiate in the near- and vacuum ultraviolet. We predict the emission wavelengths and off-surface spatial extents of these UV glows from existing laboratory data on surface-catalyzed and homogeneous gas-phase recombination radiations. Among the features expected are the 02 Herzberg I bands, the NO a bands (whose presence is in fact suggested by past spacecraft measurements), and the N2 Lyman-Birge-Hopfield and Vegard-Kaplan bands. The observed variation of glow brightnesses with surface material properties could serve as the basis for remote optical discrimination among exo-reentry space objects.
The introduction of the flash gate has made possible the fabrication of backside-illuminated CCDs with high sensitiv4y and stability throughout a wide range of ultraviolet and visible wavelengths (100-5000 Å). It has been determined that the characteristics of the oxide layer beneath the gate are critical to the ultimate performance that can be achieved. By creating an improved oxide layer in conjunction with the flash gate, we are now able to consistently produce CCDs with near-ideal UV performance. In the interest of transferring flash technology to industry, we present in this paper recent results and related background theory that optimize the flash gate specifically for application in the UV.
Measurement of photoconductivity spectral response over the wavelength range of 220 to 950 nm, linearity with intensity over the optical power range of 10 nW to 4 μW, characteristics, response time, and CV curves were made on Zn3P2 thin-film samples. The results include slow response time (on the crder of a few seconds), maximum photoconductivity response at 400 nm wavelength, and non-linear characteristics. Changes in device characteristics, due to vacuum operation and heat treatment, indicate that surface effects and intergrain boundaries can play important roles in determining the properties of thin film Zn3P2, primarily as regards the surface conductivity and a depletion layer which penetrates into the bulk material.
The ability to remotely sense ionospheric conditions for improved operation of communications and radar systems has been a long-term goal of some DOD programs. This capability now appears to be possible through improvements in computer models of the ionosphere and in UV remote sensing methods. The approach is to use passive ultraviolet optical measurements and in-situ ion density measurements as inputs to a comprehensive ionospheric model which will calculate the electron density profile . A novel feature of this approach is the use of naturally occurring airglow and auroral ultraviolet radiation. This method can be used for the midlatitude day ionosphere (90 to 800 km) and the night auroral E layer. Eventually, extensions of the technique will cover the night mid-latitude as well. The remote sensing measurement can also be used to locate regions of ionospheric irregularity, and hence probable phase scintillation, in both equatorial and polar cap regions and to locate the realtime position of the auroral oval particle precipitation.
This paper addresses the problem of using satellite ultraviolet measurements to deduce the ionospheric electron density profile (EDP). The ionospheric processes that produce the ultraviolet emissions differ from region to region, so it is necessary to consider separate approaches for the various ionospheric subregions. We will discuss approaches suitable for (1) the midlatitude daytime ionosphere, (2) the midlatitude nighttime ionosphere, and (3) the undisturbed auroral E-layer.
Fourier transform spectroscopy is a long-established technique for basic research and analytical spectroscopy in the infrared and visible wavelength regions. However, its many advantages have not been put to use at vacuum ultraviolet wavelengths. This paper addresses applications and advantages of Fourier techniques for spectroscopy at VUV wavelengths and presents a report of progress toward a VUV Fourier transform spectrometer (FTS) capable of operating throughout the vacuum ultraviolet, as well as in the ultraviolet and visible. In addition, the stigmatic property of the FTS is discussed. This feature could be exploited in spatially-resolved spectroscopy for remote sensing, spectral diagnostics of plasmas and flames, space astronomy, and national defense.
The design of a multi-mode instrument known as the Auroral Ionospheric Remote Sensor, AIRS, is described. The design criteria are enumerated. The goal of the AIRS instrument is to produce data on the global imaging of the auroral display in both dark and sunlit hemispheres with the remote sensing of ionospheric airglows to deduce ionospheric parameters such as electron density profiles and atmospheric background emissions. The AIRS will fly on the POLAR BEAR spacecraft in a near polar circular orbit at an altitude of 1,000 km with a scheduled launch in the fall of 1986. The AIRS instrument is designed as a multi-mode system with four (4) channels of data to yield simultaneous operation in the vacuum ultraviolet (VUV), near ultraviolet (UV) and visible spectral bands. Two of the data channels are designed to operate in the VUV with 30A windows having a 240Å separation. These two channels utilize an Ebert-Fastie spectrometer which can provide total coverage for each of these channels from 1150Å to 1800Å. The other two channels utilize a filter selector system to provide preselected, 10Å bandwidth spectral channels at 3371Å, 3914Å and 6300Å, and a 200Å wide channel centered at 2250Å. These spectral bands are paired to provide simultaneous pair coverage of 2250Å and 3371Å and simultaneous pair coverage of 3914Å and 6300Å. All four channels view the auroral scene of the north polar cap via appropriate optics and a scan mirror system. In effect a line scan image of the auroral scene is produced via the scan mirror operating in the orbit cross plane with the longitudinal direction provided by the forward motion of the spacecraft. All four channels can also operate in the photometer mode by locking of the scan mirror in the nadir viewing position. The two VUV channels can also operate in a spectrometer mode with the scan mirror locked in the nadir viewing position and the Ebert-Fastie spectrometer performing a spectral scan. The basic ground level spatial resolution at nadir of the four (4) spectral channels is 6.5 km x 26.7 km for the VUV channels and 26 km x 39.26 km for the near UV, visible channels. All four channels are spatially aligned to view the same scene pixel area simultaneously. The instrument sensitivity is designed to result in 50 Rayleigh detection levels for the VUV channels and approximately 1K Rayleigh detection levels for the near UV, visible channels. The total system operates at a maximum average power level of 9.3 watts in the imaging mode with a data rate of 0.884 K bits per second per channel. The total instrument weight is 23.0 lbs.
High-gain microchannel plates (MCPs) which utilize curvature of the channel to inhibit ion feedback (C-plate MCPs) have demonstrated excellent performance characteristics. However, C-plate MCPs are at present costly to fabricate, and the shearing process used to curve the channels produces a low device yield. We describe here a totally new type of high-gain MCP structure in which each channel has an axially symmetric curvature. Initial tests of proof-of-concept units of these MCPs with 75-micron-diameter channels (macroplates) suggest that their performance characteristics have the potential to be equal to those of a C-plate MCP while the fabrication process is no more complex than that of a conventional straight-channel MCP.
Multi-Anode Microchannel Array (MAMA) detector systems with formats as large as 256 x 1024 pixels are currently under evaluation at ultraviolet wavelengths. Sealed MAMA detector tubes with semi-transparent Cs2Te photocathodes are being used at wavelengths between 1800 and 3000 A, and sealed and open-structure MAMA detector tubes with opaque CsI photocathodes are being used at wavelengths from 1800 A to below 10 A. These detectors provide a high-resolution imaging capability with pixel dimensions of 25 x 25 microns 2 and have the unique capability to determine the arrival time of the detected photon to an accuracy of 100 ns or better. Very large format MAMA detectors with CsI and Cs2Te photocathodes and active areas of 52 x 52 mm2 (2048 x 2048 pixels) have been selected for use as the ultraviolet "solar blind" detectors for the NASA Goddard Space Flight Center's Space Telescope Imaging Spectrograph (STIS). This second-generation instrument will be installed in the Hubble Space Telescope by shuttle astronauts at some time between 1992 and 1995. The MAMA detectors have also been baselined for use in the prime 900 to 1200 A spectrograph on the Lyman far-ultraviolet spectroscopic explorer mission. This paper will review the performance characteristics of the current range of ultraviolet detector systems and will describe the program for the development of the (1024 x 1024) and (2048 x 2048)-pixel imaging detector systems.
We present measurements of the quantum efficiency of opaque CsI photocathodes applied directly onto the surface of microchannel plates in open face detectors and sealed tube devices. The quantum efficiency as a function of the radiation input angle (0° to 40°) and wavelength (44Å to 1800Å) has been studied with and without the use of a photoelectron collection field. These measurements provide confirmation of recent published results of high quantum efficiencies (>60%) at 100Å. We also present new results for the region 250Å to 1800Å which show quantum efficiences higher (>30%) than have been achieved to date. The contribution to the quantum efficiency of a photoelectron collection field is shown to be significant, particularly where the photon absorption coefficient is high. The fabrication of CsI photocathodes is described and the quantum efficiency is discussed in relation to the reflection of incident radiation, cathode absorption coefficient and photoelectron escape probability. Assessments of the spatial uniformity of the quantum efficiency are also presented.
This paper describes the development of backside illuminated, deep-depletion charge coupled devices (CCD's) optimized for imaging in the ultraviolet. A model is presented to describe the UV detection process at the CCD backside, and the factors controlling the UV quantum efficiency. A 32 x 32-pixel area imager was obtained with excellent operating characteristics, such as high transport efficiency and low readout rwise. By appropriate treatments of the CCD backside, a quantum efficiency of 20% at 1216 Å was obtained on the best devices. Good resolution was obtained in the visible and UV regions of the spectrum.
A TV camera system has been developed for imaging ultraviolet sources in the middle ultraviolet region of the spectrum. The imager consists of 200 mm focal length ultraviolet optics with a focal ratio of f/2.7, has a resolution of better than 1/3 milliradian, a 25 mm image intensifier, a filter wheel with filter wavelengths from 2150 to 3250 Angstroms and a CCD focal plane array of 380x488 elements. The performance characteristics of this imager will be given and examples of images of ultraviolet sources that have been obtained will be shown.
The Remote Atmospheric and Ionospheric Detection System (RAIDS) experiment, to fly on a TIROS spacecraft in the late 1980's, consists of a comprehensive set of one limb imaging and seven limb scanning optical sensors. These eight instruments span the spectral range from the extreme ultraviolet to the near infrared, allowing simultaneous observations of the neutral and ion composition on the day and night side as well as in the auroral region. The primary objective of RAIDS is to demonstrate a system for remote sensing of the ionosphere to produce global maps of the electron density, peak altitude and critical frequency.
A photon-counting microchannel plate (MCP) photomultiplier tube (PMT) has been developed having a 28 mm active optical input diameter and ultraviolet solar-blind sensitivity. Its quantum yield is high in the uv, eg 10% at 250 nm, while the unwanted long wavelength response is greatly reduced, eg only 1.7E-6% at 600 nm. By using MCPs, this new PMT is one of the smallest to be developed to date for uv solar-blind applications. A potted assembly containing this tube and a high-voltage power supply is only 16 mm long, it has an outside diameter of 52 mm, and its mass is only 50 g. Prototype units have passed MIL-E-5400 environmental tests for class-1B equipment. Several other types of uv optical detectors are discussed which allow sub-nanosecond events to be recorded, which detect the arrival rates and locations of single photons within a 25 mm active diameter, and which have increased quantum efficiency in the far uv.