Timothy P. Sasseen

Far-Ultraviolet Performance of the Berkeley Spectrograph duringthe ORFEUS-SPAS II Mission*

The Berkeley spectrograph aboard the ORFEUS telescope made its second flight on the 14 day ORFEUS-SPAS II mission of the Space Shuttle Columbia in 1996 November/December. Approximately half of the available observing time was dedicated to the Berkeley spectrograph, which was used by both principal and guest investigators. The spectrographs full bandpass is 390-1218 A; here we discuss its in-flight performance at far-ultraviolet wavelengths, at which most of the observations were performed. The instruments effective area peaks at 8.9±0.5 cm2 near 1020 A, and the mean spectral resolution is 95 km s-1 FWHM for point sources. Over most of the spectral range, the typical nighttime background event rate in each spectral resolution element was ~0.003 s-1. Simultaneous background observations of an adjacent blank field were provided through a secondary, off-axis aperture. The Berkeley spectrographs unique combination of sensitivity and resolution provided valuable observations of approximately 105 distinct astronomical targets, which ranged in distance from the Earths own Moon to some of the brightest active galactic nuclei.

Radiative Transfer Analysis of Far-Ultraviolet Background Observations Obtained with the Far Ultraviolet Space Telescope

In 1992, the Far Ultraviolet Space Telescope provided measurements of the ultraviolet (140-180 nm) diffuse sky background at high, medium, and low Galactic latitudes. A significant fraction of the detected radiation was found to be of Galactic origin, resulting from scattering by dust in the diffuse interstellar medium. To simulate the radiative transfer in the Galaxy, we employed a Monte Carlo model that utilized a realistic, nonisotropic radiation field based on the measured fluxes (at 156 nm) and positions of 58,000 TD-1 stars, and a cloud structure for the interstellar medium. The comparison of the model predictions with the observations led to a separation of the Galactic scattered radiation from an approximately constant background, attributed to airglow and extragalactic radiation, and to a well-constrained determination of the dust scattering properties. The derived dust albedo a = 0.45 ± 0.05 is substantially lower than albedos derived for dust in dense reflection nebulae and star-forming regions, while the phase function asymmetry g = 0.68 ± 0.10 is indicative of a strongly forward-directed phase function. We show the highly nonisotropic phase function to be responsible, in conjunction with the nonisotropic UV radiation field, for the wide range of observed correlations between the diffusely scattered Galactic radiation and the column densities of neutral atomic hydrogen. The low dust albedo is attributed to a size distribution of grains in the diffuse medium with average sizes smaller than those in dense reflection nebulae.

Delay-line detectors for the UVCS and SUMER instruments on the SOHO Satellite

Microchannel plate based detectors with cross delay line image readout have been rapidly implemented for the SUMER and UVCS instruments aboard the Solar Orbiting Heliospheric Observatory (SOHO) mission to be launched in July 1995. In October 1993 a fast track program to build and characterize detectors and detector control electronics was initiated. We present the detector system design for the SOHO UVCS and SUMER detector programs, and results from the detector test program. Two deliverable detectors have been built at this point, a demonstration model for UVCS, and the flight Ly (alpha) detector for UVCS, both of which are to be delivered in the next few weeks. Test results have also been obtained with one other demonstration detector system. The detector format is 26mm x 9mm, with 1024 x 360 digitized pixels,using a low resistance Z stack of microchannel plates (MCPs) and a multilayer cross delay line anode (XDL). This configuration provides gains of approximately equals 2 X 107 with good pulse height distributions (<50% FWHM) under uniform flood illumination, and background levels typical for this configuration (approximately equals 0.6 event cm-2 sec-1). Local counting rates up to approximately equals 400 event/pixel/sec have been achieved with no degradation of the MCP gain. The detector and event encoding electronics achieves approximately equals 25 micrometers FWHM with good linearity (+/- approximately equals 1 pixel) and is stable to high global counting rates (>4 X 105 events sec-1). Flat field images are dominated by MCP fixed pattern noise and are stable, but the MCP multifiber modulation usually expected is uncharacteristically absent. The detector and electronics have also successfully passed both thermal vacuum and vibration tests.

Microchannel plates for the UVCS and SUMER instruments on the SOHO satellite

The microchannel plates for the detectors in the SUMER and UVCS instruments aboard the Solar Orbiting Heliospheric Observatory (SOHO) mission to be launched in late 1995 are described. A low resistance Z stack of microchannel plates (MCPs) is employed in a detector format of 27 mm multiplied by 10 mm using a multilayer cross delay line anode (XDL) with 1024 by 360 digitized pixels. The MCP stacks provide gains of greater than 2 multiplied by 107 with good pulse height distributions (as low as 25% FWHM) under uniform flood illumination. Background rates of approximately equals 0.6 event cm-2 sec-1 are obtained for this configuration. Local counting rates up to approximately equals 800 events/pixel/sec have been achieved with little drop of the MCP gain. MCP preconditioning results are discussed, showing that some MCP stacks fail to have gain decreases when subjected to a high flux UV scrub. Also, although the bare MCP quantum efficiencies are close to those expected (approximately equals 10%), we found that the long wavelength response of KBr photocathodes could be substantially enhanced by the MCP scrubbing process. Flat field images are characterized by a low level of MCP fixed pattern noise and are stable. Preliminary calibration results for the instruments are shown.

A Minisurvey of Interstellar Titanium from the Southern Hemisphere

We describe the results of a minisurvey of interstellar Ti II and Ca II absorption toward 42 early-type stars observed from the southern hemisphere at a spectral resolution of 4.5 km s-1. Results are also presented for the Na I ultraviolet line (3302 A) detected toward nine of these targets. We examine the dependence of the integrated column densities of N(Ti II), N(Ca II), and N(Na I) on distance, reddening, neutral hydrogen column density, and their Galactic elemental abundance. Our findings support the proposition that Ti II and Ca II absorption originates in the same regions of the pervasive, warm, and neutral intercloud gas of the interstellar medium. We have observed a correlation of decreasing Ti and Ca abundance with increasing line-of-sight gas density. The Ti II/Ca II abundance ratio has been found to be essentially constant under all the interstellar density conditions we have sampled. Thus, we conclude that the general absorption properties of titanium (and calcium) are similar throughout the entire disk of our Galaxy.

A study of the impact of the Space Shuttle environment on faint far-UV geophysical and astronomical phenomena

FAUST is a far ultraviolet (1400–1800 A) photon-counting imaging telescope featuring a wide field of view (7.6°) and a high sensitivity to extended emission features. During its flight as part of the ATLAS-1 payload aboard the STS-45 mission in March 1992, nineteen deep-space nighttime viewing opportunities were utilized by FAUST. Here we report the observed fluxes and their time and space variations, and identify the signatures of post-sunset airglow phenomena and Orbiter vernier attitude-control thruster firing events. We find that the Space Shuttle nighttime environment at 296 km altitude is often sufficiently dark to permit geophysical and astronomical UV observations down to levels on the order of 1000 photons/cm2 sr A s, i.e., 0.01 Rayleighs/A. We also find evidence for occasional geophysical fluxes of some tens or hundreds of Rayleighs in the upward-looking direction.

FAUST Observations of Ultraviolet Sources toward the Virgo Cluster

We analyze three UV images covering a ~100 square degree field toward the Virgo cluster, obtained by the FAUST space experiment. We detect 191 sources to a signal-to-noise ratio of 4.4 and identify 94% of them. Most sources have optical counterparts in existing catalogs, and about half are identified as galaxies. Some sources with no listed counterpart were observed at the Wise Observatory. We present the results of low-resolution visible spectrophotometry and discuss the foreground 101 stellar sources and the 76 detected galaxies, both in the cluster and in the foreground or background. We derive conclusions on star formation properties of galaxies and on the total UV flux from discrete and diffuse sources in the cluster. We test for the presence of intracluster dust, determine the clustering properties of UV emitting galaxies, and derive the UV luminosity function of Virgo galaxies.

Delay line microchannel plate detectors for the far-ultraviolet spectroscopic explorer satellite

Delay line detectors have been chosen for the Far Ultraviolet Spectroscopic Explorer1 mission to be launched in 2000. The demands of the FUSE detectors include large format (220mm x 10mm format), high spatial resolution (15im x 35im FWHM) and linearity, high image stability, low power consumption and weight, and counting rates in excess of 3 x i0 events sec1. The FUSE program builds on the previous work, which includes two delay line detectors (95mm x 27mm double delay line format) that have already been successfully employed in the ORFEUSASTROSPAS2 ultraviolet spectrometer launched by shuttle in September 1993. We present the plans for the FUSE detector program, and results from double delay line (DDL) detectors that are under investigation to meet the requirements of the FUSE program. Our current detector achieves 15im x 25p,m FWHM (<4000 x 500 resolution elements) over the 65 x 15mm format used for the FUSE demonstration detector (90% of the flight detector segment format length), with good linearity (±1 resolution element) and high stability. State of the art analog to digital converter (ADC), gated integrator, and digital signal processor (DSP) technology have been employed to develop novel event position encoding electronics with high count rate capability (<5 x104 events sec). Microchannel plates with lOj.tm pores and an 80: 1 pore length to diameter ratio, with a 70mm x 20mm format have been used in a Z stack configuration to provide the photon amplification (gain 2 x 10). These show good pulse height distributions (<35% FWHM) even with uniform flood illumination, and background levels typical for this configuration (<1 event cm 2 sec 1). Flat field images are dominated by the microchannel plate fixed pattern noise due to the multifiber boundaries, and are stable. High efficiency photocathodes, such as KBr have been extensively studied, and provide quantum detection efficiencies of 40-50% in the 900 - 1200A range for FUSE.

Observations of terrestrial far UV emissions by the FAUST telescope

The FAUST telescope was developed to provide precise photometry of extended astronomical objects in the 1400–1800 A wavelength interval, and has a field of view of 7.6° and an angular resolution of 2 arc minutes. It was carried into orbit by the space shuttle Atlantis in March 1992 as part of the ATLAS-1 mission. During this flight, on several occasions it was pointed at the Earth to provide an estimate of the terrestrial nightglow, auroral and shuttleglow emissions. Our preliminary findings on these non-astronomical measurements are that auroral precipitation activity dominates in the high magnetic latitude, and that outside the high magnetic latitude region we consistently see diffuse emission of .005 to .02 Rayleighs/A, slowly varying over the nightside. We explore interpretations of this emission as terrestrial nightglow and shuttleglow, and find a possible explanation as NO chemiluminescent nightglow.

Results from the Far Ultraviolet Space Telescope (FAUST)

Abstract We present early results from the Far Ultraviolet Space Telescope (FAUST), which flew in March 1992 with the ATLAS space shuttle mission. The telescope provides wide-field images in the far ultraviolet (1400–1800A). Studies underway using the data obtained on this mission include establishing the brightness and distribution of far ultraviolet stars in the halo of our Galaxy, establishing the far ultraviolet properties of nearby galaxies and nearby clusters of galaxies, analyzing the diffuse galactic light, and searching for the origin of the extragalactic ultraviolet light. We discuss the instrument performance, and early results from these observations.