New experimental results are presented on measurements of the degree of linear polarization for ionization-excitation of He following electron and proton impact on a He gas target. These measurements have been performed using an optically characterized molybdenum/silicon (Mo/Si) multilayer mirror (MLM) polarimeter whose reflection and polarization characteristics have been optimized for the 304 angstrom wavelength region. In particular, the polarization of the photon emission from the decay of HeII (2p) ²P⁰ yields (1s) ²S with a wavelength of 304 Angstrom (Lyman (alpha) of HeII) has been measured for electron impact energies ranging from threshold to 1500 eV. These results are compared with polarization measurements of HeII Lyman (alpha) following proton impact on He for 187 to 460 keV beam energy corresponding to 2.7 a.u. less than v less than 4.3 a.u. projectile velocities. The integral alignment parameter A₀ is also provided for negatively and positively charged particle impact to elucidate similarities and differences in the collision dynamics of two electron targets such as helium. Such results are of importance for astrophysical and laboratory plasma diagnostics.

A molybdenum/silicon (Mo/Si) multilayer mirror (MLM) polarimeter has been constructed and used to analyze the extreme ultraviolet (EUV) emission from excited HEI and HeII states following electron impact on helium for wavelengths ranging from approximately 256 angstrom to 584 angstrom. A ratio of reflectivities for s and p polarized light, R_s/R_p approximately equals 10, and a resolving power of (lambda) /(Delta) (lambda) approximately equals 6 at 304 angstrom were obtained. These characteristics and the use of a VYNS²⁶ spectral filter were sufficient to allow a detailed polarization study of the first two members of the Lyman series of He⁺ at wavelengths of 304 angstrom (HeII 2p yields 1s) and 256 angstrom (HeII 3p yields 1s) for impact electron energies ranging from threshold to 1500 eV. The MLM has also been used as a single flat surface mirror polarimeter for the analysis of longer wavelength radiation (517 angstrom to 584 angstrom) from the HeI (1snp) ¹P^o yields (1s²) ¹S series of neutral helium with R_s/R_p approximately equals 3. While MLM polarimeters have previously been used for EUV measurements with bright photon sources such as those provided by synchrotron facilities, the results from this work clearly demonstrate the feasibility of such devices with lower intensity electron and ion impact sources. The compact design of the apparatus makes it suitable as a portable measurement and calibration device.

A calibration facility of laboratory size and modular in design is described for absolute spectral reflectivity and spatial resolution in image-forming devices. It is based on an optical table concept where all optical elements can be modularly positioned on remotely controlled translation/rotation stages. XUV photons are provided by a minifocus conventional source with replaceable anode. K-, L-, and M- band transition radiation from different metals is used. This source can be positioned at variable distances (up to 5 m) from the measurement chamber for imaging purposes. Spectral control of the radiation used in the tests can be achieved with transmission filters, with grazing incidence reflections or with a grazing incidence monochromator. Channeltron detectors in photon counting mode monitor incident and reflected signals. This instrument is now being used for characterizing grazing incidence flat mirrors as well as multilayer-coated optical surfaces.

A compact multidetector Compton polarimeter has been developed and calibrated to be used for energy resolved linear polarization analysis of X-rays in the energy range from about 3 to 20 keV. The polarization sensitivity of the polarimeter employing 4 thermoelectrically cooled silicon PIN photodiodes has been calibrated with monochromatized synchrotron radiation of known linear polarization. The scattering yields determine the polarization vector in one single measurement with an analyzing power of 76% and an uncertainty of the orientation of the polarization plane of less than 1 degree. The polarimeter is presently being used for linear polarization analysis of Parametric X-radiation (PXR) of about 11 keV emitted by 60.5 MeV electrons at an observation angle of 20 degrees. A second polarimeter with much improved spectroscopic performance and rate capabilities will employ X-ray detectors based on the principle of the semiconductor drift chamber. At a temperature of 260 K and a signal shaping time constant of 500 nsec an electronic noise contribution of 10 electrons (rms) has been measured. The drift detector polarimeter will be used to analyze PXR at backward observation angles.

We present results on novel image intensifier tubes for single photon detection. We have adopted an image charge coupling technique that allows a read-out of image intensifiers with good imaging properties and much superior time resolution than obtainable with the standard phosphor screen read-out. Although combinations of sealed microchannel plate detector tubes with position and time sensitive anode structures have already been reported, our method has the advantage that the superficial electrode array has not be implemented inside the tube. We couple the image charge from a high-resistive anode layer through the vacuum housing to a wedge-and-strip or delay-line pattern that can be attached from outside. We show results on single photon imaging with special intensifiers produced by Proxitronic GmbH in the visible and UV for an active diameter of 25 mm. The variability of the system, especially a version with a solar-blind UV-cathode and 40 mm active diameter, should open great opportunities for detection task in various fields like astronomy, reconnaissance, bioluminescence, atomic physics, and material research, particularly when both good imaging and timing performance are required.

We report on the development of a Compton scatter polarimeter for measuring the linear polarization of hard X-rays (50 - 300 keV) from solar flares. Such measurements would be useful for studying the directivity (or beaming) of the electrons that are accelerated in solar flares. We initially used a simple prototype polarimeter to successfully demonstrate the reliability of our Monte Carlo simulation code and to demonstrate our ability to generate a polarized photon source in the lab. We have recently fabricated a science model based on a modular design concept that places a self-contained polarimeter module on the front-end of a 5-inch position- sensitive PMT (PSPMT). The PSPMT is used to determine the Compton interaction location within an annular array of small plastic scintillator elements. Some of the photons that scatter within the plastic scintillator array are subsequently absorbed by a small centrally-located array of CsI(Tl) crystals that is read out by an independent multi-anode PMT. The independence of the two PMT readout schemes provides appropriate timing information for event triggering. We are currently testing this new polarimeter design in the laboratory to evaluate the performance characteristics of this design. Here we present the initial results from these laboratory tests. The modular nature of this design lends itself toward its accommodation on a balloon or spacecraft platform. A small array of such modules can provide a minimum detectable polarization (MDP) of less than 1% in the integrated 50 - 300 keV energy range for X-class solar flares.

A new multichannel x-ray/extreme ultra violet (EUV) spectrometer is developed for monitoring the time history of x-ray/EUV spectral line intensities from a hot plasma to estimate an electron density and temperature of plasmas. Each independently controlled channel includes a crystal (or multilayer mirror) and a fast x-ray diode. At the same time, an imaging transmission grating is applied to study a spatial distribution of spectral line intensities in a wide spectral region with time gated resolution. The multichannel spectrometer can be applied for measurements of polarization- dependent spectra which will be used for diagnostics of electron beams and measuring a magnetic field in z-pinch plasmas.

The optical design of an EUV spectrometer/monochromator for diagnostic of high-order harmonics generated by an ultra-short (less than 30 fs) pulsed laser focused onto a gas jet is presented. A grazing-incidence flat-field spectrometer for the 3 - 75 nm spectral region has been designed: it adopts a stigmatic toroidal mirror and a varied line-space flat grating mounted in converging light. The almost flat stigmatic spectrum is acquired by a 40 mm diameter MCP intensifier; the whole detector can be moved by means of an x-y linear drive and a rotator to scan various portions of the spectrum. The resolution is limited by the pixel size of the detector. Different gratings can be accommodated to work in medium- resolution or high-resolution mode. The same optical scheme can be applied to the design of a constant deviation angle EUV monochromator by substituting the detector block with an exit slit: the rotation of the grating gives the spectral scan. By using the grating at two different subtended angles a medium- resolution monochromator for the 3 - 50 nm spectral region is realized.

An ultra-high vacuum test facility to characterize optics, coatings and detectors in the EUV and soft X-ray (38 eV - 10 keV) spectral region is presented. The system consists of a 30 1 test chamber which can be coupled with two different beamlines: it accommodates two rotators in the 0 - 20 configuration for reflectivity and diffraction efficiency measurements on optics at grazing incidence up to 6 degrees. The low-energy beamline consists of a grazing incidence spherical grating monochromator optimized for the 38 - 1000 eV spectral range. The design is based on the principle to scan the external spectrum by rotating the grating and to maintain the spectral focusing on the exit slit by changing the length of the entrance arm. Two configurations are presented for the 38 - 200 eV (160 degree deviation angle) and the 180 - 1000 eV (172 degree deviation angle) region. The high-energy beamline is optimized for the 1.5 - 10 keV range: it consists of a facility mounting the soft X-ray source and a thin filter of the same material as the emitting anode. The results of the characterization in the whole spectral region are presented. Examples of measurement of the detection efficiency of EUV and soft X-ray detectors are discussed.

Silicon photodiode detectors with multilayer coatings were characterized using synchrotron radiation. The coatings were composed of thin layers of metals and other materials and were designed to provide wavelength bandpasses in the 17 - 150 angstrom wavelength region. The measured transmittances of the multilayer coatings are in good agreement with the calculated transmittances. The modeling accounts for the transmittance of the multilayer coating and the deposition of the radiation energy in the underlying silicon photodiode. Detectors with the following layer materials (and wavelength bandpasses were characterized: Fe/Al (17 - 30 angstrom), Mn/Al (19 - 30 Angstrom), V/Al (24 - 35 angstrom), Ti/C (27 - 40 angstrom), Pd/Ti (27 - 50 angstrom), Ti/Zr/Al (27 - 50 angstrom), Ag/CaF₂/Al (36 - 50 angstrom), and Ti/Mo/C (50 - 150 angstrom).

The new territory of ultraviolet imaging polarimetry is being explored at the University of Wisconsin Space Astronomy Lab by a sounding rocket payload, the Wide-Field Imaging Survey Polarimeter ('WISP'). WISP uses an 8-inch F/1.9 all-reflective Schmidt telescope, a large rotatable stressed CaF₂ waveplate, and a fixed polarizing mirror coated with an opaque monolayer of ZrO₂, illuminated at the Brewster angle. The payload has flown successfully four times, targeting the Pleiades Reflection Nebula, the Large Magellanic Cloud, comet Hale-Bopp, and the diffuse light of our Galaxy. An improved payload in the concept stage is Cosmic Ultraviolet Polarimetric Imaging Device ('CUPID'), which would use a 20- inch Gregorian Paul-Baker telescope, WISP-style polarimetric optics, and multilayer reflective coatings to achieve wide- field imaging polarimetry with more than 15 times the sensitivity of WISP. It is to be used to separate the Galactic and Cosmological components of the ultraviolet diffuse background using their polarimetric properties. High resolution stellar spectropolarimetry in the far ultraviolet is to be pioneered by a sounding rocket payload in the design stage, the Far-Ultraviolet Spectropolarimeter ('FUSP'). This instrument is to have a spectral resolution of 0.07 nm and a spectral coverage from 105 to 145 nm. It uses a 20-inch telescope with polarimetric optics at the prime focus, and a far ultraviolet spectrometer using an aberration-corrected holographic grating. The polarimetric analyzer will be a thin LiF stressed waveplate, followed by a Brewster-angle polarizer of natural diamond. Scientific goals include diagnosis of the geometry and magnetic fields in stellar envelopes via resonance scattering and the Hanle Effect.

The Advanced Solar Coronal Explorer (ASCE) is a mid-explorer (MidEx) mission selected, together with other five, for the a Phase A Concept Study in the 1999 round of MidEX proposal. ASCE's spacecraft bus is a SPARTAN 400 reusable carrier deployed in low Earth orbit by the Space Shuttle. ASCE's payload comprises two instrument modules, the Spectroscopic and Polarimetric Coronagraph (SPC) and the Extreme Ultraviolet Imager (EUVI). The scientific objective of the mission is the investigation, through spectroscopic and polarimetric techniques, of the physics of the coronal heating and of the solar wind acceleration. A critical physical parameter of the corona is the magnetic field. Polarimetric measurements of UV coronal radiation and their interpretation through the Hanle effect can be used for coronal magnetic field diagnostics. One of the SPC spectrometers, the Spectroscopy/Polarimetry channel (SPCH), includes a reflecting Brewster-angle polarimeter for measurements of the linear polarization of the HI Lyman series lines (i.e., Ly-(alpha) , -(beta) , and (gamma) ) and of the O VI 1032 Angstrom line. In this paper, the optical design of the SPCH polarimeter is described. A relevant element of this design is the external occulter (EXO) that is supported on a boom, which is extended 10 m beyond the instrument aperture, once the instrument is in station. The analysis of the stray- light reduction provided by this occulting system is described in this paper. The principal source of stray light is solar disk light that is diffracted from the edge of the EXO and scattered from the telescope mirror. The analysis shows that the stray-light is less than 10^-2 the coronal signal. This level of stray-light rejection minimizes the polarized stray light that may be introduced by the EXO's straight edge. The most appropriate material for the polarizer has been found to be CaF₂. The material selection criteria are described. Finally, the paper illustrates with an example that if the linear polarization can be measured better than 1%, then the instrumental sensitivity to magnetic fields may reach a few gauss (greater than 2 gauss), in coronal active regions.

I summarize the ST-2010 mission concept: a large-aperture (4 - 8 m) ultraviolet/optical successor to the Hubble Space Telescope, which is scheduled for de-orbit in the year 2010. A complete scientific and technological study is given in the report of the Ultraviolet-Optical Working Group (UVOWG), commissioned by NASA to study the scientific rationale for new missions in ultraviolet/optical space astronomy. ST-2010 will provide a major facility for many key projects: (1) Mapping dark baryons and large-scale structure; (2) detecting unseen matter through weak gravitational lensing; (3) Studying feedback from star formation; (4) Determining the origin of star, planets, and the elements. With an emphasis on high- throughput UV spectroscopy and wide-field imaging, ST-2010 will require significant advances in technology: UV detectors, gratings, mirrors, spectrographs and imagers. To achieve our science goals, the ST-2010 spectrographs will need to deliver 10 - 100-fold increases in throughput and multiplex efficiency over Hubble. The ST-2010 imagers should achieve similar gains in field of view (10' X 10') and efficiency.

Superconducting tunnel junctions (STJ) have now reached a state of maturity where small arrays have for the first time been used in practical applications in ground based astronomy. Future generations of devices are now under construction to enhance the current capability. For example larger format arrays, higher readout speeds, and improved spectral resolution are all issues being addressed. In this paper we present specifically the performance at ultraviolet wavelengths from 100 - 500 nm of the current generation of STJ arrays based on tantalum. Recent results on such tantalum based photon counting superconducting tunnel junctions (STJ) which are now suitable for use as broad-band low resolution spectrometers for ultraviolet astronomy are presented. These STJs, operated at a temperature of approximately 0.3 K, have demonstrated a limiting resolution of approximately 8 nm at 200 nm. These devices can be designed to be extremely linear in response with photon energy while measuring the individual photon wavelength and arrival time. The quantum efficiency for single photons is over approximately 50% at approximately 200 nm. Such an STJ has been packaged into a small prototype 6 X 6 array and shown to have good uniformity of response across all pixels. Larger arrays are under development which could contribute significantly to many fields of ultraviolet astronomy being able to provide efficiently and simultaneously the broad band spectrum and photon arrival time history of every single object in the field over a very wide dynamic range. With lower energy gap superconducting materials the resolution should become higher and possibly for hafnium based devices as high as approximately 1 nm at 200 nm. Is such devices can be developed then imaging spectrometers which can simultaneously record the image and spectra of objects in a large field and therefore provide an object's red-shift may become a reality.

Our detectors are superconducting transition edge sensors (TESs) optimized for the wide band detection of individual photons from the mid infrared, through the optical, and into the near ultraviolet. We typically achieve an energy resolution of 0.15 eV FWHM over this range with timing resolution of 100 ns. We have measured photon events with sub- microsecond rise times and 3 microsecond fall times allowing count rates as high as 30 kHz without significant degradation in energy resolution. Such characteristics along with the predicted high quantum efficiency (10% in IR to 50% in optical-UV) make our TES detectors very appealing for low-flux applications which have energy and timing requirements, such as fast spectrophotometry for observational astronomy. We present results from our recent observation of the Crab Pulsar (PSR BO531 + 21) which demonstrate the ability of our sensors to extract wide band phase-resolved spectroscopic information of the pulsar using the student-class 24 inch telescope on the campus of Stanford University. We present a description of the optical system and an analysis of the single pixel energy response.

A wide variety of ultraviolet detectors are used aboard the Solar and Heliospheric Observatory (SOHO) spacecraft. The ultraviolet instrument package aboard SOHO includes one full disk EUV flux monitor (SEM: 30.4 nm), one full sky mapper (SWAN: 121.6 nm), one full-Sun imager (EIT: 17.1 - 30.4 nm), and three spectrometers (CDS: 15.1 - 78.5 nm; SUMER: 66.0 - 161.0 nm; UVCS: 93.7 - 136.1 nm). All wavelengths are first order. In total, there are fifteen UV detectors aboard SOHO with six distinctly different designs. These range from photodiodes, through backside-thin CCDs, to two-dimensional microchannel-plate detectors. Some instruments measure an analog signal (such as the charge deposited in a CCD well), while others measure single photon events. The intense brightness of the Sun imposes unique challenges on these astronomical detectors. After almost three years of continuous observation in space, a large body of data has been gathered on their performance. How well each detector system has performed over this period is examined in turn.

The Space Telescope Imaging Spectrograph (STIS) is a versatile HST instrument covering the 115 - 1000 nm wavelength range in a variety of spectroscopic and imaging modes. Coverage of the ultraviolet range (115 - 310 nm) is provided by two Multi- Anode Microchannel Array (MAMA) detectors built by Ball Aerospace. The FUV MAMA covers the 115 - 170 nm range using an opaque CsI photocathode on the microchannel plate; the NUV MAMA covers the 165 - 310 nm range using a semi-transparent Cs₂Te photocathode on the detector window. Both MAMAS utilize a 1024 X 1024 anode format, but detected photon events are positioned to half the spacing of the anode lines, leading to a 2048 X 2048 format for the final readout. The active area of each detector is 25.6 X 25.6 mm. Since the installation of STIS onto the Hubble Space Telescope (HST) in February 1997, the MAMAs have carried out a varied program of astronomical observing and in-flight calibration. The detectors have performed extremely well. In this report, we briefly describe the design of the STIS MAMA detectors, provide illustrative examples of their scientific use on HST, and summarize their technical performance in orbit, in such areas as sensitivity, resolution, flat-field uniformity and stability, signal-to-noise capability, dynamic range, and background.

We have developed a video signal processing system to detect and store photoevents recorded by a magnetically focused, electron-bombarded CCD (EBCCD) image sensor of the type that flew on the Interstellar Medium Absorption Profile Spectrograph (IMAPS). The EBCCD represents a good alternative to detectors that use microchannel plates: the opaque photocathode is on a solid substrate which leads to a very uniform response with a high quantum efficiency. Our experience with sounding rocket missions has demonstrated that this image sensor can perform well in poor vacuum conditions. Previous uses of the EBCCD on IMAPS missions have worked with video signals that were accumulated in an analog fashion. To make the EBCCD more useful for viewing very faint images in the future, we have recently developed new circuitry to detect photoevents in real time and store them discretely. This photon-counting signal processing mode overcomes the CCD readout noise penalty that accumulates when many successive analog frames are added together. The new circuits also provide for an automatic, accurate subtraction of the CCD dark current pattern.

At NASA GSFC we are developing a high resolution solar-blind photon counting detector system for UV space based astronomy. The detector comprises a high gain MCP intensifier fiber- optically coupled to a charge injection device (CID). The detector system utilizes an FPGA based centroiding system to locate the center of photon events from the intensifier to high accuracy. The photon event addresses are passed via a PCI interface with a GPS derived time stamp inserted per frame to an integrating memory. Here we present imaging performance data which show resolution of MCP tube pore structure at an MCP pore diameter of 8 micrometer. This data validates the ICID concept for intensified photon counting readout. We also discuss correction techniques used in the removal of fixed pattern noise effects inherent in the centroiding algorithms used and present data which shows the local dynamic range of the device. Progress towards development of a true random access CID (RACID 810) is also discussed and astronomical data taken with the ICID detector system demonstrating the photon event time-tagging mode of the system is also presented.

We have been testing the vacuum ultraviolet (UV) response of several types of detectors, supporting investigators developing photodiodes made of GaN, near UV photocathodes, and Electron-Bombarded CCDs (EBCCDs). We are currently supporting 4 independent research groups developing GaN most of which have produced devices with significant sensitivity down to 1200 Angstroms. Over the past year, we have also tested bare CCDs with coatings to enhance ultraviolet response. Detectors based on microchannel plate (MCP) have been used extensively for a wide variety of NASA mission and continue to be the standard to beat. Two particularly promising detector technologies are (1) EBCCDs which offer an immediate factor of 3 - 4 improvement in sensitivity and (2) devices made of GaN or GaAlN which may eventually offer factors of 6 - 8 increased sensitivity, both compared to MCPs for wavelengths between 1200 to 3000 Angstroms. We present latest results and plans to expand our vacuum UV testing.

We have successfully developed a prototype 256 X 256 photoconductive GaN ultraviolet (UV) imaging array. The array, with its pixels (30 X 30 micrometer²) indium bump bonded to a Lockheed Martin Fairchild Systems LT9601 readout integrated circuit, is highly sensitive to ultraviolet light below 365 nm with a sharp reduction in response to visible and infrared light. The array was installed into a custom designed UV camera utilizing a Nikon UV lens with all the off-chip electronics interfaced to an automatic computer controlled system. To the best of our knowledge, this is the first reported UV array camera based on the nitride materials.

This paper describes the performance of an advanced high- resolution full-frame architecture CCD imaging device for use in scientific, medical, and other high performance monochromatic digital still imaging applications. Of particular interest is the replacement of the polysilicon second gate electrode with that of a more spectrally transparent material, thereby dramatically improving device sensitivity. This has been achieved without compromising performance in other areas such as dark current, noise, transfer efficiency and, most importantly, yield. Devices have also been produced with and without antiblooming protection depending on an application's primary need for sensitivity or control of over-exposure conditions.

BATC was contracted by Langley Research Center (LaRC) to design and fabricate a gallium arsenide photomultiplier tube (GaAs PMT) for LIDAR applications. This particular GaAs PMT uses a high strip current ceramic channel electron multiplier (CEM), manufactured by K&M electronics, capable of operating at a gain of 10⁴ to 10⁷. The GaAs photocathode, processed by Litton Electro-Optical Systems, is used in reflection mode so that light passes through a faceplate, strikes the photocathode and generates photoelectrons, which are collected and multiplied by the CEM. Key issues during the development of this device were; (1) increasing the CEM strip current, (2) improvements in the CEM collection efficiency, (3) processing of an opaque extended blue GaAs photocathode, (4) CEM and photocathode lifetime and (5) electron optic modeling. This paper will discuss the test results from the three functional GaAs PMTs fabricated during this effort as well as development and fabrications issues.

Semitransparent Mo/Si multilayer films with a completely free- standing active area have been developed for use as optical elements in the soft x-ray region. They work not only as a beam splitter, but also as a transmissive polarizer and quarter-wave plate. To achieve a flat, smooth free-standing reflecting surface with a high reflectivity, the following fabrication problems were investigated: stress control of Mo/Si multilayer films, the surface roughness of the initial membrane, and the removal of the initial membrane. A flatness of 1 nm (rms) in the active area was obtained for a fabricated 10-mm-square semitransparent multilayer film. A multilayer film consisting of 50 free-standing pairs of semitransparent Mo/Si were fabricated for transmissive polarizer, and a soft x-ray ellipsometer was developed based on them. The fabricated multilayer polarizer was found to have good polarization performance. Placing two transmissive polarizers in the polarizer/compensator-sample-analyzer configuration enabled full control of the polarization of the probe beam. The modified polarization of light reflected from a sample was analyzed by the rotating-analyzer ellipsometry method. This system was used to measure a multilayer mirror. We verified that the soft X-ray ellipsometer is a very promising tool for the structural evaluation of multilayer films, providing a sensitivity in the sub-angstrom range.

The Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO) comprises two telescopes and two spectrometer channels for spatially resolved ultraviolet spectral diagnostics of the solar corona. The principal lines for which the two channels are optimized are the H I 'Lyman-(alpha) ' line at 121.5 nm and the O VI (O⁵⁺) doublet at 103.2 and 103.7 nm. An 'in-flight' method, using observations of stars and scattered solar disk light, has been devised to determine the flat field function, i.e., the relative detection efficiency of the detector pixels. We present the details and results of this process. Local pixel-to-pixel efficiency variation is found to be, typically, about plus or minus 9% to plus or minus 17% (1 (sigma) ) for the H I Lyman-(alpha) channel and plus or minus 9% for the O VI channel.

The Advanced Solar Coronal Explorer (ASCE) is one of five missions selected for a Phase A Concept Study in the current round of proposed MIDEX missions. ASCE's instrument complement is supported by a SPARTAN 400 reusable carrier. The spacecraft is carried into orbit and deployed by the Space Shuttle; at mission's end, nominally 2 years later, it is retrieved and returned to earth for post-flight calibration. ASCE comprises two instrument modules, the Spectroscopic and Polarimetric Coronagraph (SPC) and the Extreme Ultraviolet Imager (EUVI). The external occulter for the coronagraph is supported on a boom, which is extended 10 meters beyond the instrument apertures once the spacecraft is on station. Large aperture optics can therefore be used, and this, in combination with improvements in optical and photon detection efficiencies, will provide spectroscopy of the extended solar corona with unprecedented sensitivity and spatial resolution, routine measurements of the electron temperature, and polarimetry of the H I Lyman lines. SPC also extends the short wavelength limit to 28 nm. As a consequence, SPC will be able to perform the first He II 30.4 nm and He I 58.4 nm spectroscopy of the extended corona. In the visible part of the spectrum (450 - 600 nm), SPC's Large Aperture Spectroscopic Coronagraph (LASCO) channel will provide polarimetric images with 1.8 arc second resolution elements, which will allow the determination of polarized brightness of the coronal plasma. In a separate parallel channel LASCO will also provide images at single minor ion line wavelengths from which can be determined the shapes and Doppler shifts of those lines. The distant external occulter provides for major improvement in stray light suppression. The EUVI instrument will take high cadence images of the full disk and low corona at four selectable wavelengths with 0.9 arc second resolution elements. A description of the instrument design and performance capabilities is presented.

The optical configurations studied for the EUV channels on board the Advanced Solar Coronal Explorer (ASCE) mission are presented. ASCE has been proposed to study fundamental physical processes as heating of coronal holes and streamers, driving of solar wind and coronal mass ejections. It comprises three EUV channels: the HEII channel for high-resolution spectroscopic measurements in the 57.3 - 64.3 nm region, the Te channel for the determination of the electron temperature by measuring the shapes of the electron-scattered wings of the coronal HI Ly(alpha) (121.6 nm) and the S/P channel for spectroscopic measurements in the regions near HI Ly(alpha) and O VI (103.2 nm) lines and for polarimetric measurements of the HI Lyman series lines, that will explore the possibility of measuring coronal magnetic fields. The possibility of using toroidal gratings or spherical varied line-space ones has been analyzed. The results of the ray-tracing simulations on the three channels will be presented in details.

Among the University of Firenze's XUVLab interests is the study of vacuum UV (VUV) polarimetry. The XUVLab is developing a reflecting VUV polarization analyzer with improved sensitivity. The analyzer is obtained by optimizing the instrumental configuration (best compromise between polarizing efficiency and throughput), the detector choice, and the selection of the reflecting material. Here, a preliminary theoretical study of material properties is described in order to select the best candidates for experimental characterization activity in the spectral region of the HI Lyman series lines (90 - 130 nm). The experimental setup for the characterization of the reflecting materials is also described. The whole study is related to the development of a UV polarimeter for the MIDEX/ASCE NASA project.