A silicon photodiode detector, type AXUV-100G, was modeled by calculating the electromagnetic field strength in each region of the photodiode. The reflectance and transmittance at each boundary and the absorption of energy in each region of the device were calculated. By using an empirical carrier collection efficiency in each region, the calculated responsivity was in agreement with the experimental responsivity in the 1 nm to 25 nm wavelength range. In the 2.7 nm to 80 nm wavelength range, the model was used to simulate the experimentally observed decrease in responsivity of a photodiode that had undergone radiation damage.

The high QE and large variety of formats make modern back illuminated Charge-coupled devices (CCDs) nearly ideal detectors for most scientific imaging applications. In the ultraviolet (UV), however, quantum efficiency (QE) instability with temperature and with environmental conditions has limited their widespread use, especially for space applications. We have developed several techniques to achieve stable and high QE in the 200 - 300 nm wavelength range with back illuminated CCDs fabricated by various manufacturers. In this paper we report peak QE of over 90% at 240 nm (uncorrected from quantum yield). We describe a series of tests which demonstrate stability of these devices with temperature, humidity, and UV illumination. These results are all based in the chemisorption backside coating processes developed at the Steward Observatory CCD Laboratory.

Polycrystalline diamond films grown by chemical vapor deposition have been considered recently for a number of UV detection applications. Negative electron affinity, chemical and mechanical stability and relative ease of fabrication make such films attractive candidates for effective and stable UV photoconverters. In this paper we present our study of the absolute quantum efficiency of a thin film diamond reflective photocathode in the spectral range of 25 - 200 nm. Modification of the surface by microwave hydrogen plasma etching resulted in a substantial increase of the photocathode sensitivity. The quantum efficiency of the photocathode at approximately 40 nm was as high as 37 percent and the sensitivity cut off was found to be about 200 nm. We also verified that the photocathode is relatively stable under air exposure. The relative QE degradation in the spectral range studied did not exceed 15 percent after the sample was left in ambient air for 18 hours. In addition, the diamond photocathode appeared to be chemically stable and mechanically robust: alcohol and water ultrasonic cleaning, followed by the same surface activation in hydrogen plasma, did not result in any degradation of the sample UV sensitivity. The photoyield from the diamond film at 256 angstrom exhibited an increase with the angle of radiation incidence, which is in agreement with the results of our calculations.

The Far Ultraviolet (FUV) detector for the Cosmic Origins Spectrograph (COS), scheduled to be installed in the Hubble Space Telescope in June 2003, is currently being built by the Experimental Astrophysics Group at The University of California, Berkeley. The COS FUV detector system is based on the detectors flown on the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite with changes to take advantage of technological improvements since the development of those detectors. The COS FUV detector is a dual segmented, cylindrical input face, MCP detector with cross delay line (XDL) readouts. Each segment is a Z-stack of MCPs with an active area 85 mm by 10 mm. The segments are abutted end to end to form a total active area approximately 180 mm by 10 mm (with a gap in the middle). Detector spatial resolution in the long (spectral) dimension is better than 25 microns and in the short dimension (cross-dispersion) is better than 50 microns. The MCPs are coated with a CsI photocathode to achieve the optimal quantum detection efficiency (QDE) in the 1150 - 1750 angstrom bandpass. Improvements in the understanding of the processing required to produce higher QDE MCPs has lead to significant improvements in the FUV QDE relative to previous missions. This paper presents the basic design parameters and performance characteristics of the COS FUV detector.

Many current and future space missions use microchannel plate (MCP) detectors with delay line anode readouts (e.g. FUSE, GALEX and the FUV detector on the upcoming Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope). Delay line anodes are used to measure the position of the centroid of the charge clouds that exit MCP detectors. This is accomplished by measuring the time difference between the arrival of the pulse at both ends of the delay line. The spatial resolution of this position determination is dependent on the accuracy of the temporal measurement. These high frequency pulses (approximately 100 MHz) are usually amplified and directed to constant fraction discriminators (CFDs) whose output pulses start and stop a time to amplitude converter (TAC). This paper reviews the optimization of these circuit elements. It includes the characteristics of various delay line types (serpentine and helical) and their effect on pulse shape. The choice of amplifier filter bandpass and optimum fraction and delay for the CFDs is also presented. Examples are taken from the MCP detectors on the missions mentioned above.

A new photon counting detector for UV Astronomy is presented. The SD2000 detector consists of a single MCP, coated with a suitable photocathode, closely coupled to a Silicon Drift Detector (SDD). A good spatial resolution, of the order of 20 micrometer in both directions, can be achieved with a relatively small number of readout channels (10 divided by 100). The maximum allowable rate, proportional to the drift length, is about 10⁵ Hz for a 20 mm length focal plane. A new type of SDD, without metal deposition on the cathodes, has been developed and tested for this particular application. A single MCP has been coupled also to a metal anode and a silicon diode in order to characterize it and study the interaction between the incoming (low-energy) electron cloud and a silicon p-n junction similar to those present in the drift chamber.

The optical design of a stigmatic spectroscopic system for diagnostic of laser-produced plasmas in the 2.5 - 40 nm region is presented. The system consists of a grazing-incidence toroidal mirror which focuses the radiation emitted by a laser-produced plasma on the entrance slit of a spectrograph. The latter has a spherical variable-line-spaced grating with flat-field properties coupled to a spherical focusing mirror that compensates for the astigmatism. The mirror is crossed with respect to the grating, i.e. it is mounted with its tangential plane coincident with the equatorial plane of the grating. The spectrum is acquired by an EUV-enhanced CCD detector with high quantum efficiency. This stigmatic design has also spectral and spatial resolution capability for extended sources: the spectral resolution is independent from the size of the source while the spatial resolution decreases for sources far from the optical axis. The expected performances are presented and compared with those of a conventional stigmatic design with a plane variable-line- spaced grating.

We present the design of a pinhole lamp recently flown aboard two NASA/JHU sounding rocket missions as a wavelength standard for a far-UV spectrograph with a 900 - 1400 Angstrom bandpass. Lamp configuration, spectral output, gas supplies, payload accommodation and operation procedures are discussed. This lamp could easily be incorporated into future far-UV spectroscopic orbital missions and would benefit science return. We also discuss the use of Bayard-Alpert tubes (ionization gauges) as far-UV sources, which have the advantage of not requiring an external gas supply. At pressures between 10^-5 and 10^-7 Torr these tubes produce a strong emission line spectra, caused by electron impact with residual gas atoms in the vacuum. Below 10^-7 Torr the residual gas line intensities have weaken enough to reveal the long wavelength tail of a 150 eV bremsstrahlung spectrum produced by electron impact onto tungsten grid. The use of ionization gauges in flat field and end-to-end calibration experiments is described. We show how an ionization gauge and spectrograph can be used as a real- time residual gas analyzer sensitive to atomic and molecular gas species that emit within the bandpass. Such a device could be useful in material processing and contamination control environments.

A detailed investigation of excitation of He (1s²) ¹S to He (1snp) ¹P degree(s) (n equals 2 - 5) states and ionization-excitation of He (1s²) ¹S to He (2p) ²P degree(s) and He (3p) ²P degree(s) states in e^- + He and H⁺ + He collision systems is presented for a wide range of projectile velocities (2.2 a.u. < v < 6.9 a.u.). Specifically new experimental data are presented on measurements of the degree of linear polarization for excitation and excitation-ionization of He following proton impact in the extreme ultraviolet (EUV) wavelengths. These measurements have been performed using a characterized molybdenum/silicon multilayer mirror polarimeter (MLM) whose polarization characteristics have been optimized for EUV emission of He and He⁺. Furthermore, the proton experimental results are compared with theoretical polarization data using the first Born approximation and recent atomic orbital close coupling (AOCC) calculations for the excitation process. A comprehensive comparison of experimental data for negatively and positively charged projectiles at equal impact velocities is also given in order to elucidate differences in the collision mechanisms of two electron targets. It is important to note that these results are relevant for astrophysical diagnostics such as solar flares.

A far UV (FUV) reflectometer was developed at the Metal Optics Laboratory (Instituto de Fisica Aplicada, CIC, Madrid) for in situ reflectance as well as transmittance measurements of ultra high vacuum (UHV) deposited thin films. The spectral region covered by the reflectometer is 50 - 200 nm. The angle of incidence can be continuously changed from 3 degrees to 87 degrees. The sample holder is provided with two perpendicular rotation axes to perform reflectance measurements in two perpendicular planes of incidence. Thin films of the materials to be investigated can be deposited by evaporation in an adjacent chamber that is connected to the reflectometer through a gate valve and a long linear/rotary feedthrough. In this way, thin films are deposited and their reflectance is measured in UHV conditions without breaking vacuum. Two different deposition systems, including an electron gun and resistive evaporation sources, can be used for multilayer deposition. The instrument is furnished with a substrate heating system for deposition on a heated substrate, and/or for post-deposition sample annealing. A gas entrance system allows exposing the sample to controlled doses of different gases to analyze their effect over the sample reflectance. An atomic oxygen source is also installed in the reflectometer for aging simulations of in orbit operating optical instruments. The instrument is particularly useful to investigate the effect on the sample FUV reflectance of exposure to controlled atmospheres and other in situ treatments.

Extreme UV reflectance measurements of multilayer coatings based on IR and non-oxidized Al films are presented. Two kinds of multilayer coatings were prepared: bilayers of a thin non- oxidized Al film over an IR film, and multilayers IR/Al/IR and IR/Al/IR/Al/IR. The bilayers were aimed at enhancing pure Al reflectance below Al plasma wavelength (approximately 83 nm). Multilayers with IR as the outermost film were designed for highest reflectance at 53.6 nm. We deposited IR films by evaporation on heated and unheated substrates by means of an electron gun, and Al films by evaporation on room temperature substrates from a thermal source. Multilayer coatings were prepared under UHV conditions and their reflectance was measured in situ. The short-term multilayer aging was also investigated.

The Advanced Camera for Surveys (ACS) will fly on the Hubble Space Telescope (HST) Servicing Mission 3b in late-2001 and includes a Solar Blind Channel (SBC) comprising correcting/magnifying relay optics, a far ultraviolet (FUV) filter selection, and a 1K X 1K multi-anode microchannel array (MAMA) detector with cesium iodide photocathode. In order to characterize SBC's flat field response over its full spectral range and to radiometrically calibrate ACS at two FUV lines through as many SBC filters as possible, a sophisticated and automated STimulus for Ultraviolet Flat Fields (STUFF) was developed whose application extends to other vacuum ultraviolet optical instrumentation having similar characterization requirements. Challenges in STUFF's development and resulting design features are presented along with results from in vacuo characterizations carried out before and during thermal vacuum testing of ACS.

The stimulus for ultraviolet flat fields (STUFF) was developed to supply spatially flat, broadband, far ultraviolet irradiance in thermal vacuum testing of the Solar Blind Channel (SBC) of the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) which will fly on Servicing Mission 3b in mid-2001. Because the SBC's 1K X 1K multi-anode microchannel array (MAMA) detector has a global count rate limit of about 300,000 events/s, it takes a minimum of roughly 10 hours of expensive test time in thermal vacuum to collect a deep flat field having 10,000 signal counts in each pixel (1% certainty in Poisson statistics). As such, a diffuser with far ultraviolet (FUV) throughput substantially higher than conventional state-of-the-art Lambertian diffuser material was sought to insure that the length of flat field exposures could be minimized. An FUV diffuser with concentrating properties was conceived as the overcoating of the convex lens side of commercially available microlens array material with Goddard Space Flight Center's (GSFC) standard aluminum-magnesium fluoride coating optimized for reflectance at 120 nm. The concept for this diffuser, a geometric optical model of its performance, visible light measurements to test that model, and the diffuser's FUV performance in STUFF relative to conventional diffuser material is presented.

The Far Ultraviolet Spectroscopic Explorer (FUSE) satellite was launched on June 24, 1999. FUSE is designed to make high resolution ((lambda) /(Delta) (lambda) equals 20,000 - 25,000) observations of solar system, galactic, and extragalactic targets in the far ultraviolet wavelength region (905 - 1187 angstrom). Its high effective area, low background and planned three year life allow observations of objects which have been too faint for previous high resolution instruments in this wavelength range. FUSE has now been in orbit for one year. We discuss the accomplishments of the FUSE mission during this time, and look ahead to the future now that normal operations are under way.

The Far Ultraviolet Spectroscopic Explorer is a NASA astrophysics satellite which produces high-resolution spectra in the far-ultraviolet (90.5 - 118.7 nm bandpass) using a high effective area and low background detectors. The observatory was launched on its three-year mission from Cape Canaveral Air Station on 24 June 1999. The instrument contains four co- aligned, normal incidence, off-axis parabolic mirrors which illuminate separate Rowland circle spectrograph channels equipped with holographically ruled diffraction gratings and delay line microchannel plate detectors. The telescope mirrors have a 352 X 387 mm aperture and 2245 mm focal length and are attached to actuator assemblies, which provide on-orbit, tip, tilt, and focus control. Two mirrors are coated with silicon carbide (SiC) and two are coated with lithium fluoride over aluminum (Al:LiF). We describe mirror assembly in-flight optical and mechanical performance. On-orbit measurements of the far-ultraviolet point spread function associated with each mirror are compared to expectations based on pre-flight laboratory measurements and modeling using the Optical Surface Analysis Code and surface metrology data. On-orbit imaging data indicate that the mirrors meet their instrument-level requirement of 50% and 95% slit transmission for the high- and mid-resolution spectrograph entrance slits, respectively. The degradation of mirror reflectivity during satellite integration and test is also discussed. The FUV reflectivity of the SiC- and Al:LiF-coated mirrors decreased about 6% and 3%, respectively, between coating and launch. Each mirror is equipped with three actuators, which consist of a stepper motor driving a ball screw via a two-stage planetary gear train. We also discuss the mechanical performance of the mirror assemblies, including actuator performance and thermal effects.

The Far Ultraviolet Spectroscopic Explorer (FUSE) satellite was launched into orbit on June 24, 1999. FUSE is designed to make high resolution ((lambda) /(Delta) (lambda) equals 20,000 - 25,000) observations of solar system, galactic, and extragalactic targets in the far ultraviolet wavelength region (905 - 1187 Angstrom). Its high effective area, low background and planned three year life allow observations of objects which have been too faint for previous high resolution instruments in this wavelength range. The FUSE instrument includes two large format microchannel plate detectors. Each detector system consists of two microchannel plate segments in a Z-stack configuration with double delay line anodes and associated electronics. High detector spatial resolution was required in order to obtain scientific data with high spectral resolving power, and low detector background was necessary in order to observe faint objects. We describe the performance of the FUSE detectors during their first year on orbit, including the mechanical and thermal stability, throughput, background, and flat field of the detector system. We will also discuss the regular single event upsets of the detector electronics, and the strategy adopted in order to minimize their impact on mission efficiency.

The Far Ultraviolet Spectroscopic Explorer mission imposes stringent requirements on the satellite attitude control system. Target acquisition accuracy and target tracking stability must each be no greater than 0.5 arcseconds FWHM. The data required by the attitude control system to meet these requirements are provided by two redundant Fine Error Sensors. Each Fine Error Sensor operates as a slit-jaw camera that provides either complete images of the star-field around the line of sight of the telescope, or centroided positions of selected guide stars in the field of view. The satellite pointing requirements must be met over a wide dynamic range of target or guide star brightness, for both sparse and crowded starfields, and for targets that may be either point sources or extended objects. We will describe the operational characteristics of the FES and present data on its performance. We also discuss the optical, mechanical, thermal, and electronic design challenges encountered in meeting the mission requirements, and how they were addressed in the context of a very tight development schedule.

The FUSE satellite employs innovative techniques for autonomous target acquisitions and fine pointing control. One of two Fine Error Sensors, incorporated in the optical path of the science instrument, provide the Instrument Data System computer with images, for target identification, and field star centroids, for fine pointing information to the spacecraft attitude control system. A suite of 'toolbox' functions has been developed to locate stars, selected and track on 'unknown' guide stars from the image, identify the star field, track preselected 'known' guide stars, follow moving targets, and provide pointing optimizations to fine- tune the centering of a target. After a maneuver to a new field, initial attitude is determined by identifying stars found in a 20' X 20' image. Identification is done by matching stars with an uploaded table of up to 200 objects selected from the Hubble Space Telescope (HST) Guide Star Catalog (GSC), ranging from V equals 9 to 13.5 mag., and typically covering a one degree field around the target. During identification, tracking is performed on unidentified stars in the image to prevent the satellite from drifting. A corrective slew is then commanded to place the target at the desired position. Tracking is then resumed on preselected guide stars. If desired, further fine alignment of the science apertures is performed by a target peakup using the FUV detectors. We discuss the target acquisition process; end-to- end performance; and problems encountered due to the limitations of the small field of view of the FES, HST GSC errors, and stray light in the telescope baffles.

The Far Ultraviolet Spectroscopic Explorer (FUSE) is a NASA astrophysics satellite designed to produce high resolution spectra in the far-ultraviolet (90.5-118.7 nm bandpass) with a high effective area (20-70 cm²) and low background detector. It was launched on a three-year mission in June 1999 aboard a Boeing Delta II rocket. The satellite has been performing routine science observations since December 1999. FUSE contains four co-aligned, normal incidence, off-axis parabolic primary mirrors which illuminate separate Rowland circle spectrograph channels equipped with holographically ruled diffraction gratings and microchannel plate detectors. Fine error sensors (slit jaw cameras) operating in the visible on two of the channels are used for target acquisition and guiding. The FUSE mission was first proposed in the late 1980s, and experienced several major conceptual changes prior to fabrication, assembly, and testing, which lasted from 1996 through 1999. During the program, we realized both positive and negative aspects to our design and processes that may apply to other space missions using telescopes and spectrographs. The specific topics we address are requirements, design, component specification, integration, and verification. We also discuss on-orbit alignment and focus. These activities were complicated by unexpected levels of motion between the optical elements, and the logistical problems associated with limited ground contact passes in low Earth orbit. We have developed methods to characterize the motions and mitigate their resultant effects on the science data through a combination of observing techniques and modifications to the data reduction software.

A Mission into Hot Phenomena in the Universe is proposed by means of a small telescope of 50 cm aperture accommodated on the International Space Station. Two operating modes are envisaged: 3 angstrom dispersion imaging spectroscopy in the 90 - 320 nm range (1st priority) or wide field (1 degree) medium bandwidth imaging in the same range but Ly-(alpha) (2nd priority). It will use a pointing platform attached to an Express Pallet Adapter available to the Italian Space Agency (ASI) more than 4 - 6 months per year. During a life time of 6 yr focal plane instruments may be changed when on-ground refurbishment occurs. With reasonable exposure times hot thermal sources as faint as V equals 19 - 2 can be observed in the spectroscopy mode at 110 nm and active chromospheres on cool stars as faint as V equals 15 at 250 nm can be monitored. Assessment of FUV imaging is underway, possibly providing observations of hot sources as faint as V equals 21 - 22. Nominal uplift to ISS is set in Autumn 2005.

This report summarizes the conclusions of the study so far developed at CISAS 'G. Colombo' of Padua University about the optical configuration of the spectrometer to be installed on UVISS, a SiC far- and near-UV telescope for the International Space Station. This spectrometer has to cover the whole 91 - 320 nm spectral region, by providing a resolving power greater than 300 at 100 nm and around 600 at 200 nm; a spatial resolution of 4 arcsec on-axis and minimization of spatial aberrations over arcmin's long entrance slit is required. Several designs have been considered, from the simple on- Rowland toroidal grating to the more complex aberration corrected holographically ruled one. Due to room limitations in UVISS instrument bay and in order to minimize the number of optical elements because of throughput, a two in-flight interchangeable channel configuration has provisionally been selected, each one using a single dispersive element: the first covers the 91 - 130 nm region, the second the 130 - 320 nm one. Both channels use a spherical grating with parallel variable line spacing in the Harada mounting. The theoretical performance of the two channels is obtained by ray-tracing simulation.

We present a feasibility study of a wide field UV imaging camera for the UV Italian Sky Surveyor (UVISS) under study by the Italian Space Agency as possible payload externally attached on an Express Pallet to the International Space Station. The camera will operate in the range 950 - 2700 angstrom, it will have a 1 square degree field of view with an expected angular resolution of about 3' at best. Two layouts are considered: the first is a camera based on the use of transmittance filters to select the bandpass of interest while the second one is based on the use of multilayers mirrors to define the bandpasses of the filters. In both options the detectors used will be intensified CCDs with an appropriate choice of photocathodes. Finally, we present some simulations to evaluate the performances of the camera.

The characteristics of the fine pointing and tracking system of UVISS, UltraViolet Italian Space Surveyor, which is proposed to fly as an external payload attached to the ISS (International Space Station) Express Pallet is presented. The system is derived from the pointing system of the UVSTAR telescope that has flown three times on the Shuttle. The electro-optical architecture of the tracking telescope, including the ICCD detector and the computational unit based on a DSP board with direct high speed interface to the camera and the on-board stellar catalog is described. The simulations performed to determine the instrument expected tracking precision and performances considering the dynamics of the ISS are reported. Finally the utilization of a high frequency tracking error correction based on the detection of the spectrographs zero-th order signal is considered as an independent tracking error measurement which provides information and corrections of possible residual tracking errors and could be used for alignment check as well.

The Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) on the NASA Solar Terrestrial Relations Observatory (STEREO) mission is a suite of remote sensing instruments consisting of two white light coronagraphs, an extreme ultraviolet (EUV) imager, and a heliospheric imager. SECCHI will observe coronal mass ejections (CMEs) from their birth at the sun, through the corona to their impact at earth. SECCHI includes a coordinated effort to provide magnetohydrodynamic (MHD) models and visualization tools to interpret the images that will be obtained from two viewpoints and to extrapolate those imagery to in situ and radio emission measurements obtained from STE-REO. The resulting 3- dimensional analysis of CMEs will resolve some of the fundamental questions in solar physics.

The NASA Solar Terrestrial Relations Observatory (STEREO) mission will place two spacecraft into solar orbits with sufficient separation to provide remote sensing instruments with a stereoscopic view of the heliosphere extending from the lower solar corona to beyond one astronomical unit. Analysis of the stereographs returned from the two spacecraft will allow solar physicists to infer the three-dimensional structure of small and large components of the corona. The Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) suite of remote sensing instruments includes a Heliospheric Imager (HI) to view the heliosphere in the interval from 12 to 215 solar radii. The HI will obtain the first stereographic images of coronal mass ejections in interplanetary space. Of particular interest is the subset of coronal mass ejections that propagate through the heliosphere and ultimately impact the earth. This paper presents the design concept for this new wide field coronagraph.

The Extreme-ultraviolet Imaging Spectrometer combines, for the first time, high spectral, spatial and temporal resolution in a satellite based, solar extreme ultraviolet instrument. The instrument optical design consists of a multilayer-coated off- axis paraboloid mirror telescope followed by a toroidal grating spectrometer. The instrument includes thin film aluminum filters to reject longer wavelength solar radiation and employs CCD detectors at the focal plane. The telescope mirror is articulated to allow sampling of a large fraction of the solar surface from a single spacecraft pointing position. Monochromatic images are obtained either by rastering the solar image across the narrow entrance slit or by using a wide slit or slot in place of the slit. Monochromatic images of the region centered on the slot are obtained in a single exposure. Half of each optic is coated to maximize reflectance at 195 angstrom; the other half is coated to maximize reflectance at 270 angstrom. The two EUV wavelength bands were selected to optimize spectroscopic plasma diagnostic capabilities. Particular care was taken to choose wavelength ranges with relatively bright emission lines to obtain precision line of sight and turbulent bulk plasma velocity measurements from observed line profiles. The EIS spectral range contains emission lines formed over a temperature range from approximately 10⁵ - 10⁷ K. The wavelength coverage also includes several density sensitive emission line pairs. These line pairs provide spatial resolution independent density diagnostics at nominal coronal temperatures and densities. Each wavelength band is imaged onto a separate CCD detector. The main EIS instrument characteristics are: wavelength bands -- 180 - 204 angstrom and 250 - 290 angstrom; spectral resolution -- 0.0223 angstrom/pixel (23 - 34 km/second-pixel); slit dimensions -- 4 slits: 1 X 1024 arc- seconds and 50 X 1024 arc-seconds with two positions unspecified as of this writing; fine raster range -- >6 arc-minutes on the sun; coarse raster range -- > 1600 arc- seconds on the sun; largest spatial field of view in a single exposure -- 50 X 1024 arc-seconds; nominal time resolution for active region velocity studies -- 3.4s. The Solar-B satellite is scheduled for launch in August 2005 into a nominal 600 km sun-synchronous orbit.

The X-ray observations from the Yohkoh SXT provided the greatest step forward in our understanding of the solar corona in nearly two decades. We believe that the scientific objectives of the Solar-B mission can best be achieved with an X-ray telescope (XRT) similar to the SXT, but with significant improvements in spatial resolution and in temperature response that take into account the knowledge gained from Yohkoh. We present the scientific justification for this view, discuss the instrumental requirements that flow from the scientific objectives, and describe the instrumentation that will meet these requirements. XRT is a grazing-incidence (GI) modified Wolter I X-ray telescope, of 35 cm inner diameter and 2.7 m focal length. The 2048 X 2048 back-illuminated CCD has 13.5 (mu) pixels, corresponding to 1.0 arcsec and giving full Sun field of view. This will be the highest resolution GI X-ray telescope ever flown for Solar coronal studies, and it has been designed specifically to observe both the high and low temperature coronal plasma.

The ability to derive physical parameters of the Sun from observations by the Solar and Heliospheric Observatory (SOHO) Extreme Ultraviolet Imaging Telescope (EIT) greatly increases the scientific return of the mission. The absolute and time variable calibration of EIT therefore is of extreme interest. The NRL EIT Calibration Sounding Rocket (CalRoc) program was initiated to provide well calibrated, contemporaneous observations in support of SOHO EIT. These observations provide three benefits to the SOHO EIT data, absolute calibration points, temporal and spatial information of the EIT EUV response variability in flight via flat field information and clues to the physics of the degradation. Details of the bandpasses of the multilayered optics and the total telescope photometry are presented. Comparisons are shown with the contemporaneous images from SOHO EIT. Plans for the second CalRoc flight are discussed. Loss of reflectivity in the multilayer mirrors has been identified as a new component to the SOHO EIT and CalRoc degradation.

The Very high Angular Resolution ULtraviolet Telescope experiment was successfully launched on May 7, 1999 on a Black Brant sounding rocket vehicle from White Sands Missile Range. The instrument consists of a 30 cm UV diffraction limited telescope followed by a double grating spectroheliograph tuned to isolate the solar Lyman (alpha) emission line. During the flight, the instrument successfully obtained a series of images of the upper chromosphere with a limiting resolution of approximately 0.33 arc-seconds. The resulting observations are the highest resolution images of the solar atmosphere obtained from space to date. The flight demonstrated that subarc-second ultraviolet images of the solar atmosphere are achievable with a high quality, moderate aperture space telescope and associated optics. Herein, we describe the payload and its in- flight performance.

Traditional magnetographs measure the solar magnetic field at the visible 'surface' of the Sun, the photosphere. The Solar Ultraviolet Magnetograph Investigation (SUMI) is a hardware development study for an instrument to measure the solar magnetic field higher in the atmosphere, in the upper chromosphere and in the transition region at the base of the corona. The magnetic pressure at these levels is much stronger than the gas pressure (in contrast to the situation at the photosphere), so the field controls the structure and dynamics of the atmosphere. Rapid changes in the magnetic structure of the atmosphere become possible at this height, with the release of energy. Measurements of the vector magnetic field in this region will significantly improve our understanding of the physical processes heating the Sun's upper atmosphere and driving transient phenomena such as flares and coronal mass ejections. The instrument will incorporate new technologies to achieve the polarization efficiencies required to measure the magnetic splitting of lines in the VUV an UV (C_IV at 1550 angstrom and Mg_II at 2800 angstrom). We describe the scientific goals, the optical components that are being developed for a sounding rocket program, and the SUMI baseline design.

We describe a ground-based eclipse instrument for measuring solar coronal polarization brightness and intensity, and the calibration procedures for this instrument. We present coronal measurements from the February 26, 1998 total solar eclipse observed at Curacao, N.A.. The instrument employs a liquid crystal variable retarder for analysis of coronal broad band linear polarization and collects data on an array detector spanning a 6.5 solar radius field of view. Polarization calibration of the liquid crystal variable retarder utilizes the tangential orientation of coronal polarization to calculate retardance values.

The Ultraviolet and Visible-light Coronagraph (UVC) is one of the solar remote-sensing instruments proposed for the model payload of the Solar Orbiter mission. The Solar Orbiter is one of the two 'Flexible' missions selected in September 2000 by the European Space Agency (ESA) for the definition study phase. A novel orbital design takes the orbiter as close as 0.21 astronomical units (AU) to the Sun, with heliographic latitudes as high as 38 degrees for observations of the solar polar regions at very high spatial resolution. From this vantage point, the UVC can, at the same time, image the visible and ultraviolet coronal emissions and diagnose, with unprecedented temporal and spatial resolution (down to 1200 km) the full solar corona. The UVC's optical design, presented here, consists of an externally occulted, off-axis Gregorian with multilayer-coated optics. The UVC can obtain monochromatic images in the neutral hydrogen HI Lyman (alpha) , (lambda) 121.6 nm, and single-ionized helium HEII Lyman (alpha) , (lambda) 30.4 nm, lines and measure the polarized brightness (pB) of the visible K-corona. The ultraviolet Lyman (alpha) lines are separated with two multilayer coatings mirror and an extreme-ultraviolet transmission filter. The mirrors' coating optimized for 30.4 nm still has a good reflectivity at 121.6 nm and visible. The optical performances, resulting from ray-tracing calculations, are presented here, along with the expected system response to the coronal signal.

Tunable UV Fabry-Perot etalons have the potential of providing images, dopplergrams, and density maps of the Sun's upper chromosphere and transition region. To study the feasibility of this approach, we developed and built tunable laboratory etalons for the 135 - 141 and 120 - 123 nm wavelength ranges. At 140 nm we achieved a finesse of 10.5 and a peak transmission of 3 percent. This performance is sufficient to observe the density sensitive line pair of O IV at 140.4 and 140.7 nm. At 122 nm (H Lyman alpha) we achieved a finesse of 3.8 and a peak transmission of 0.07 percent.

The goal of the Earth Observing System (EOS) SOLar STellar Irradiance Comparison Experiment II (SOLSTICE II) is to measure the solar ultraviolet irradiance (115 nm - 320 nm) to within 5% of its absolute value with a 0.5% per year relative accuracy over the course of a minimum mission lifetime of five years. Most detectors degrade over time while studying the sun. The SOLSTICE instrument design is such that detector and optical system degradation is tracked by routinely observing a series of stable early-type stars. Any changes in the system may then be removed from the solar irradiance. Detector performance and stability lies at the heart of SOLSTICE experimental success. The SOLSTICE detectors are Hamamatsu R2078 PhotoMultiplier Tubes (PMTs). We have developed an integrated PMT package [PMT, PMT housing, (mu) -metal magnetic shield, high voltage divider, and pulse-amplifier discriminator (PAD)] that will achieve our performance objectives. We report here on both the design of the integrated detector package and the laboratory measurements of the operational lifetime performance characteristics of SOLSTICE detectors. These include pulse height distribution, quantum efficiency, photocathode surface uniformity, and magnetic susceptibility.

We have previously described the Vernier Anode, a conductive charge division readout device for use in microchannel plate detectors. The readout pattern comprises nine electrodes each of which varies cyclically, having a sinusoidal form. One of the major benefits offered by the Vernier design is that the spatial resolution can greatly exceed the charge measurement accuracy, unlike devices such as the wedge and strip anode, where the electrode variation is linear. Thus the Vernier anode can exploit the potential position resolution of even the smallest pore microchannel plates at readily achievable microchannel plate gains and electronic signal to noise ratios. We describe a detector utilizing the Vernier anode using the image charge technique. The microchannel plate event charge is collected on a resistive anode composed of Germanium deposited on an insulting substrate. This serves to localize the charge while it is being measured. The Vernier anode is capacitively coupled to the reverse side of the Germanium and the event charge induces signals on the Vernier anode, which are then used to calculate the event centroid position. We present spatial resolution and linearity results from a detector using the Vernier in image charge mode, and discuss the practical and performance advantages offered by this method of operation. The intrinsic spatial resolution of the Vernier anode is shown to be less than 10 microns FWHM and detector resolution is limited by the microchannel plate pore spacing.

The large imaging format, high sensitivity, compact size, and ease of operation of silicon-based sensors have led instrument designers to choose them for most visible-light imagers and spectrometers for space-based applications. This will probably remain the case in the near future. In fact, technologies presently under development will tend to strengthen the position of the silicon-based sensors. CCD-CMOS hybrids currently being developed may combine the advantages of both imagers and new high-gain amplifiers and could permit photon- counting sensitivity even in large-format imagers. Back- illumination potentially enables silicon detectors to be used for photometry and imaging applications for which front- illuminated devices are poorly suited. Successful detection by back illumination requires treatment of the back surface using techniques such as delta doping. Delta-doped CCDs were developed at the Microdevices Laboratory at the Jet Propulsion Laboratory in 1992. Using molecular beam epitaxy, fully- processed thinned CCDs are modified for UV enhancement by growing 2.5 nm of boron-doped silicon on the back surface. Named delta-doped CCDs because of the sharply-spiked dopant profile in the thin epitaxial layer, these devices exhibit stable and uniform 100% internal quantum efficiency without hysteresis in the visible and ultraviolet regions of the spectrum. In this paper we will discuss the performance of delta-doped CCDs in UV and EUV, applicability to electron- bombarded CCD (EBCCD), our in-house thinning capability, and bonding approaches for producing flat focal plane arrays. Recent activities on the extension of delta-doping to other imaging technologies will also be presented.

We have carried out measurements of efficiency as functions of position across the surfaces of replica grating made from the same masters as the UVCS/SOHO flight units. Variations in first order efficiency which significantly affect the interpretation of UVCS data are found along the direction perpendicular to the grooves. Variations are also found along the direction parallel to the grooves, but these do not seriously affect UVCS data interpretation. The measurements and their application to the radiometric calibration of UVCS/SOHO are discussed.