The primary scientific objective of the High Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission selected by NASA is to investigate the physics of particle acceleration and energy release in solar flares. Observations will be made of x-rays and (gamma) rays from approximately 3 keV to approximately 20 MeV with an unprecedented combination of high resolution imaging and spectroscopy. The HESSI instrument utilizes Fourier- transform imaging with 9 bi-grid rotating modulation collimators and cooled germanium detectors. The instrument is mounted on a Sun-pointed spin-stabilized spacecraft and placed into a 600 km-altitude, 38 degrees inclination orbit.It will provide the first imaging spectroscopy in hard x-rays, with approximately 2 arcsecond angular resolution, time resolution down to tens of ms, and approximately 1 keV energy resolution; the first solar (gamma) ray line spectroscopy with approximately 1-5 keV energy resolution; and the first solar (gamma) -ray line and continuum imaging,with approximately 36-arcsecond angular resolution. HESSI is planned for launch in July 2000, in time to detect the thousands of flares expected during the next solar maximum.

Both instrument and spacecraft innovations are necessary to develop a mission to four solar radii called the Solar Probe. One of the key observable is the solar wind and its characteristics. To observe the solar wind and the entire plasma distribution function near the sun, two different plasma instruments have been incorporated in the current concept. One instrument can take advantage of velocity aberration to observe the solar wind. This plasma instrument uses an innovative pixelated APS-like plasma detector to view this aberrative solar wind and to allow the sampling of a complete plasma distribution function in 10^-2 sec. Another instrument innovation is the nadir viewing plasma spectrometer which will observe solar wind species in the nadir direction that have high velocities and little or no velocity aberration relative to the spacecraft. A high temperature system of electrostatic mirrors with its own solar instruments are another class of instrument innovations on the Solar Probe. Optical observations of the solar disc will be accomplished with filled aperture tubers which will contribute to the reduction of the 3000 suns solar flux to a few suns at the instrument aperture. The tubes will be fabricated form a carbon-carbon material using a process that optimizes its optical properties which reduces its temperature and its mass loss. Its parabolic shape allows the dual function of shield and antenna at the extreme perihelion temperatures of over 2000 K. he high temperature solar arrays will function near perihelion because of thee characteristics: a high temperature photovoltaic material, feathering of the solar arrays to high incidence angles, and the self occultation of the solar arrays near perihelion.

HIREX is a suite of three complementary solar-pointed instruments that is being proposed to NASA under the NASA MIDEX announcement of opportunity. The main instrument is a 0.6m clear aperture, 240m effective focal length normal incidence XUV telescope operated at 171 angstrom, with a spatial resolution of 0.01 inch. This main telescope is complemented by two other instruments: 1) a 0.3 m context telescope that images in a wavelength range that covers the UV and XUV spectral regime, based on the TRACE design. This context telescope places the high magnification, limited field of view images created by the high resolution telescope in both spatial and temperature context. 2) A spectrometer covering the spectral range from 170-220 angstrom, based on the SERTS design.

THe potential quality increase of the scientific results obtainable with stereographic observations on a Solar- Terrestrial Relations Observatory (STEREO) mission are examined in terms of the science objectives, observables, instrument performance, spacecraft vantage points and stereographic image reconstruction.

Studies of the 3D structure and dynamics of the solar corona have been severely limited by the constraint of single viewpoint observations. The Stereo X-Ray Coronal Imager (SXCI) mission will send a single instrument, an X-ray telescope, into deep space expressly to record stereoscopic images of the solar corona. The SXCI spacecraft will be inserted into an approximately 1 ZAU heliocentric orbit leading Earth by approximately 25 degrees at the end of nine months. The SXCI x-ray telescope forms one element of a stereo pair, the second element being an identical x-ray telescope in Earth orbit placed there as part of the NOAA GOES program. X-ray emission is a powerful diagnostic of the corona and its magnetic fields, and 3D information on the coronal magnetic structure would be obtained by combining the data from the two x-ray telescopes. This information can be used to address the major solar physics questions of (1) what causes explosive coronal events such as coronal mass ejections, eruptive flares and prominence eruptions and (2) what causes the transient heating of coronal loops. Stereoscopic views of the optically thin corona will resolve some ambiguities inherent in single line-of-sight observations. Triangulation gives 3D solar coordinates of features which can be seen in the simultaneous images form both telescopes. As part of this study, tools were developed for determining the 3D geometry of coronal features using triangulation. Advanced technologies for visualization and analysis of stereo images were tested. Results of mission and spacecraft studies are also reported.

The Solar Polar Sail Mission uses solar-sail propulsion to place a spacecraft in a circular orbit 0.48 Au from the Sun with an inclination of 90 degrees. The spacecraft's orbit around the Sun is in 3:1 resonance with Earth phased such that the Earth-Sun-spacecraft angle range from 30 degrees to 150 degrees. The polar view will further our understanding of: (1) the global structure and evolution of the corona, (2) the initiation, evolution, and propagation of coronal mass ejections; (3) the acceleration of the solar wind; (4) the interactions of rotation, magnetic fields, and convection within the Sun; (5) the acceleration and propagation of energetic particles; and (6) the rate of angular momentum loss by the Sun. Candidate imaging instruments are a coronagraph, an all-sky imager for following mass ejections and interaction regions from the Sun to 1 AU, and a disk imager. A lightweight package of fields and particle instruments is included. A mission using a 158 m square sail with an effective areal density of 6 g/m² would cost approximately $LR 250-300M for all mission phases, including the launch vehicle. This mission depends on the successful development and demonstration of solar-sail propulsion.

Emerging techniques allow instruments to view very large sky areas, a hemisphere or more, in visible light. In space, such wide-angle coverage enables observation of heliospheric features form close to the Sun to well beyond Earth. Observations from deep-space missions such as Solar Probe, Stereo, and Solar Polar Sail, coupled with observations near Earth, permit 3D reconstruction of solar mass ejections and co-rotating structures, discovery and study of new comets and asteroids, and detailed measurements of brightness variations in the zodiacal cloud. Typical heliospheric features have 1 percent or less of ambient brightness, so visible-light cameras must deliver < 0.1 percent photometry and be well protected from stray background light. When more than a hemisphere of viewing area is free of bright background-light sources, we have shown that corral-like structures with several vane-like walls reduces background light illuminating to wide-angle optical system by up to ten orders of magnitude. The optical system itself typically provides another five orders of surface-brightness reduction. With CCDs as the light-detection device, images of point-like sources must cover typically 100 pixels to average down sub-pixel response gradients and provide the above 0.1 percent photometry. With present-day CCDs this requires images of order 1 degree in angular size. Tolerating such large images in turn enables wide-angle sky coverage using simple reflecting and refracting optical systems such as convex spherical reflectors, toroids and thick lenses. We show that combining these with light- reducing corrals yields practical, light-weight instruments suitable for inclusion on deep-space probes.

All-sky cameras for viewing the heliosphere in white light are included in the design of several future spacecraft missions. The first of these to ge put in Earth-orbit will be the solar mass ejection imager, a joint project of the US Air FOrce, NASA, and the University of Birmingham, UK. Other missions, including an all-sky imager in their current design, are STEREO, Solar Probe and Solar Probe Sail. The white-light signal includes Thomson-scattered light from heliospheric electrons, which can be used to study the structure and evolution of large-scale heliospheric features. These studies are the principal reason for putting all-sky cameras in Earth-orbit or deep space. We discuss a tomographic technique, which uses the 2D information in the all-sky images provided by these cameras to reconstruct the heliospheric density structure in 3D. We present preliminary results of this tomographic technique applied to Thomson scattering data from the photometers onboard the two HELIOS spacecraft.

The study of high energy, transient astrophysical phenomena requires new instrumentation capable of simultaneously performing high spatial, temporal and spectral observations. Currently, there are no elements such as lenses or mirrors capable of reflecting or refracting X- and gamma-rays. Shadow-casting techniques must be employed to image such sources. These techniques rely on the total absorption of X- and gamma-rays to indirectly give images of the sources. We describe here a design for an x-ray telescope based on dual Fresnel Zone Plate (FZP) coders suitable for small satellites. Most shadow-casters requires an image plane detector with a spatial resolution comparable to the smallest features cut into the coder for the best angular resolution. The image plane detector for a telescope based on dual FZPs does not have such a requirement because the coders measure almost the exact spatial Fourier transform of the source distribution. We present here the results of laboratory tests and simulations that demonstrate the feasibility of constructing such a telescope and its ability to produce images of x-ray sources.

We present result from laboratory testing of a compact Energetic Particle Detector capable of making particle measurements in a variety of heliospheric and planetary environments. This ion composition telescope utilizes a novel electron-optics design and newly developed microcircuits to achieve combined directional particle flux measurements and high resolution mass spectroscopy in an extremely lightweight and low power package. The detector design provides for a high geometric factor and reasonable directional capability while maintaining excellent resistance to background radiation due to its small size and specialized coincidence logic. The prototype device, with an integrated time-of-flight custom integrated circuit, has been extensively characterized with electronic pulsers, radioactive sources in a vacuum chamber, and accelerator particle beams, revealing performance which substantially meets all requirements for mass resolution, bandwidth, power consumption, and weight allocation. A flight version would provide comprehensive measurements of the energetic particle environment on a mission where payload mass is tightly constrained, such as solar probe or a discovery-class mission.

The Extreme UV Imaging Telescope (EIT) instrument is operating on-board the SOHO spacecraft since January 1996. EIT is providing EUV observations of the solar corona in four narrow channels: 171, 195, 284 and 304 angstrom. Due to continuous exposure to the EUV solar irradiation, the instrument performance is continuously evolving. The backside thinned detector is showing important changes in its overall response and local damage of EUV highly exposed areas. These performance modifications can be characterized through several observation analyses that are discussed in this paper. Two major effects are identified: contamination on the detector surface and charge mobility changes in the CCD produced by the EUV irradiation. To restore the instrument response, bakeouts are regularly planned as well as specific observation sequences that are used to characterize the detector damages. An overview of the instrument response behavior is presented in this paper.

This paper describes the conceptual design of a soft x-ray telescope, super-x, which we will propose for the Japan/US/UK Solar-B mission. Super-X will break new ground in both angular resolution and solar coronal temperature discrimination. The telescope design is based upon the successful transition region and coronal explorer instrument. It features four XUV spectral channels spanning the 0.3 to 20 MK temperature range with an angular resolution of approximately 0.27 seconds of arc. We will describe considerations affecting spectral line selection and some details of the characteristics of the instrument.

The LASCO coronagraph consists of three CCD equipped telescopes taking images of up to 1024 X 1024 pixels in size. For approximately the last year a wavelet image compression has been in use for images from LASCO. The testing and performance of this compression will be discussed within the constraints of the spacecraft computer, hardware and telemetry. Effects of the image compression on the data taking, data analysis and reduction will be discussed.

Various configurations of Solar Pointing Control Systems have been used for NASA sounding rockets since an initial flight in December of 1967. Until now, these attitude control systems have used an analog controller. The demand for a more advanced attitude control system with better performance and flexibility leads to the testing of a digital control system. Computer aided design was used to develop the control equations and an embedded controller is used to implement these equations. The analog control system pointing performance was degraded by electrical noise and offsets getting into the sensor signals. The solution to this problem was to isolate the sun sensor from payload electrical nose and ground loops. To accomplish this the sun sensor output was digitized and the data was sent to the control system using a fiber optical cable. This control system was flown on Naval Research Laboratories rocket 36.140 and had less than 0.5 arc-second peak-to-peak jitter during the flight. With further refinements the digital system is expected to attain jitter of less than 0.2 arc- seconds peak-to-peak.

The design of a digital version of the solar pointing attitude rocket control system (SPARCS) posed the challenge of designing a new test platform. This platform should be capable of providing a broad evaluation of the control system in a cost effective manner. Previously, evaluating a control system consisted of performing a long and tedious procedure that tested the response of the system to individual excitations of sensor. The SPARCS attitude control system evaluator is designed to provide dynamic simulation of a rigid free body with controlled thrusters, and rate and attitude sensors. It interfaces directly with the digital control system and its design allows rapid, inexpensive and comprehensive evaluation of the attitude control system electronics, control equations and flight parameters. This evaluation consists mainly of assuring target acquisition, verifying critically damped pitch, yaw and roll payload maneuvers, and validating control system stability margins. The system yields results that are very close to those obtained in actual flights and is helping to improve performance and add new capabilities to SPARCS.

The solar EUV experiment (SEE) selected for the NASA Thermosphere, Ionosphere, and Mesosphere Energetics and Dynamics mission will measure the solar vacuum UV (VUV) spectral irradiance from 0.1 to 200 nm. To cover this wide spectral range two different types of instruments are used: grating spectrograph for spectra above 25 nm and a set of silicon soft x-ray (XUV) photodiodes with thin film filters for below 30 nm. Redundant channels of the spectrograph and XUV photodiodes provide in-flight calibration checks on the time scale of a week, and annual rocket underflight measurements provide absolute calibration checks traceable to radiometric standards. Both types of instrument have been developed and flight proven as part of a NASA solar EUV irradiance rocket experiment.

The solar x-ray imager (SXI) is a grazing incidence Wolter Type I X-ray Telescope to be fabricated and flown on the National Oceanic and Atmospheric Administration's (NOAA) Geo-Stationary Operational Environmental Satellite. The SXI will image the full sun at wavelengths between approximately 6 and 60 angstroms with a detector having 5 arc sec pixels. The goal for SXI is to forecast 'space weather', i.e., effects of charged particles that are produced at the Sun as they interact at the earth. Major contributors to space weather include: variations in the Sun's solar wind, solar flares, and solar mass ejections. Effects of space weather include: radiation damage and particle events in high- inclination orbit spacecraft, disruption of various kinds of communications equipment, degradation of navigational tools such as GPS, potential health hazard during space walks, and power blackouts the SXI and the resulting optical design for the grazing incidence telescope. Parametric performance predictions of image quality as degraded by diffraction, geometrical aberrations, and surface scattering effects due to residual optical fabrication errors are presented that establish realistic optical fabrication tolerances necessary to satisfy top-level image quality requirements.

Helioseismic and precise solar photometric measurements reveal that the Sun varies globally as a start during the source of an 11 year solar cycle. To understand the physical mechanisms of the magnetic cycle in the solar interior we must learn how to measure the tiny changes in the Sun's global properties, like its radius, internal temperature distribution and surface luminosity. The SoHO/MDI experimental has proven that exceedingly small solar shape fluctuations are measurable from outside our atmosphere. We describe here an instrument which will not only measure limb shape oscillations with unprecedented accuracy, but it will also detect solar radius changes with heretofore unachieved accuracy and precision. Variations in these parameters are caused by physical changes, both in the photosphere and the deep solar interior. Solar radius and shape observations will teach us how the Sun's convective envelope responds to emergent energy fluctuations. The determination of this outer boundary condition is essential to understand the solar total irradiance and luminosity variations.

We are presently developing a solar imager with spectrally uniform photometric response over all wavelengths between the UV and IR. Such a Solar Bolometric Imager (SBI) will be capable of accurately measuring heat flow inhomogeneities at the sun's photosphere and will provide an innovative new tool for identifying mechanisms of long-term solar luminosity variation. Our work builds on recent advances in uncooled, relatively high-definition thermal arrays. We have shown that the spectral absorptance of these arrays can be modified by deposition of gold blacks, to provide spectrally uniform response over at least the wavelength range between about 0.3(mu) and 2.5(mu) containing over 95 percent of the total solar irradiance. Our ongoing work is intended to show that quantitative photometry of the solar disc can be performed with such a modified array. We are constructing a breadboard SBI for immediate use with an 8-bit ferro- electric camera, developing a 12-bit camera to make full use of the ferro-electric array's capabilities, and optimizing our process of gold-blacking the TI arrays. Much of the science potential of the SBI could be realized in a balloon experiment. The combination of the SBI and a cavity radiometer would also constitute an excellent SMEX experiment to address a key challenge identified in the Sun- Earth Connection Roadmap recently issued by NASA/OSS.

High energy interplanetary proton events can jeopardize vital military and civilian spacecraft by disrupting logical circuits and by actually damaging spacecraft electronic components. Studies of solar hard x-rays indicate that high-energy proton events observed near Earth are highly associated with an uncommon type of solar flare exhibiting temporal progressively hardening hard x-ray spectra. A hard x-ray spectrometer is being developed by the Czech Astronomical Institute to provide a test bed for evaluating this phenomenon as a possible proton-storm prediction method. The instrument is designed to measure hard x-ray spectra in a high fluence, high-energy particle background environment such as that found at geosynchronous altitude. This experiment has been selected for space flight by the DoD Space Test Program and will fly aboard the Department of Energy satellite, Multi-spectral thermal Imager, scheduled for a three year mission, beginning in late 1999. The timing of this mission, fortuitously, coincides with the experiment are: 1) to evaluate the efficacy of this type of solar instrument in predicting interplanetary proton storms; 2) to study the high-energy physics of solar flares in concert with the premier flight of the NOAA soft x-ray imaging telescope, SXI, on the GOES 12 weather satellite and other solar mission. If the first goal is demonstrated by this mission, continuous monitoring of the Sun for proton events could become operational from geo-synchronous orbit during solar cycle 24.

In the proposed Mercury-Messenger mission, a satellite will approach the Sun to a distance of around 0.3 AU. A plasma instrument to be flown on this satellite provides a unique possibility to probe the inner heliosphere in a distance range which has previously only been investigated by the Helios missions. In addition, in situ observations of the low-energy ions in the Mercury magnetosphere can be performed for the first time. In some phase of the orbit pick-up ions from Mercury are also expected to be detected. Because of the tight mass constraints on this mission, a new low-weight plasma instrument FIPS was developed which is particularly suited for this near-solar plasma environment. It is a combination of an electrostatic deflection system and a linear time-of-flight system. Using numerical simulations we demonstrate the properties of this design and discuss possible applications.

We designed a low-cost flight instrument that images the full solar disk through two narrow band filters at the red nd blue 'wings' of the solar potassium absorption line. The images are produced on a 1024 X 1024 charge-coupled device with a resolution of 2 arcsec per pixel. Four filtergrams taken in a very short time at both wings in the left and right states of circular polarization are used to yield a Dopplergram and a magnetogram simultaneously. The noise-equivalent velocity associated with each pixel is less than 3 m/s. The measured signal is linearly proportional to the velocity in the range +/- 4000 m/s. The range of magnetic fields is from 3 to 3000 Gauss. The optical system of the instrument is simple and easily aligned. With a pixel size of 12 micrometers , the effective focal length is 126 cm. A Raleigh resolution limit of 4 arcsec is achieved with a 5-cm entrance apertures, providing an f/25 focal ratio. The foreoptic is a two-component telephoto lens serving to limit the overall optical length to 89 cm or less. The mass of the instrument is 14 kg. the power required is less than 30 Watts. The Compact Doppler Magnetograph can be used in space mission with severe mass and power requirements. It can also be effectively used for ground-based observations: large telescope, dome or other observatory facilities are not required.