Overview of spatial and timing resolution of event counting detectors with Microchannel Plates
Not Available
Tremsin, A. S.↗
Engineering topics
Vallerga, J. V.
Publications and source records attributed to Vallerga, J. V..
Epsilon Canis Majoris and the ionization of the local cloud
The Lyman continuum radiation from the brightest extreme ultraviolet (EUV) source, the B2 II star epsilon Canis Majoris (Adara), is so intense that it dominates the local stellar EUV radiation field at wavelengths longer than 450 A and therefore sets a lower limit to the ionization of hydrogen in the Local Cloud. Using the EUV (70-730 A) spectrum of epsilon CMa taken with the Extreme Ultraviolet Explorer Satellite (EUVE) and simple models that extrapolate this spectrum to the Lyman edge at 912 A, we have determined the local interstellar hydrogen photionizatin parameter Gamma solely from epsilon CMa to be 1.1 x 10(exp -15)/s. This fiugre is a factor of 7 greater than previous estimates of Gamma calculated for all nearby stars combined (Bruhweiler & Cheng 1988). Using measured values of the density and temperature of neutral interstellar hydrogen gas in the Local Cloud, we derive a particle density of ionized hydrogen n(H(+)) and electrons n(sub e) of 0.015-0.019/cu cm assuming ionization equilibrium and a helium ionization fraction of less than 20%. These values correspond to a hydrogen ionizatin fraction, chi(sub H) from 19% to 15%, respectively. The range of these derived quantities is due to the uncertainties in the local values of the neutral hydrogen and helium interstellar densities derived from both (1) solar backscatter measurements of Ly alpha lines of hydrogen and helium (1216 and 584 A), and (2) the average neutral densities along the line of sight to nearby stars. The local proton density produced by epsilon CMa is enough to allow the ionization mechanism of Ripken & Fahr (1983) to work at the heliopause and explain the discrepancy between the neutral hydrogen density derived from solar backscatter measurements and line-of-sight averages to nearby stars. A large value of electron density in the Local Cloud of n(sub e) is approximately 0.3-0.7/cu cm (T = 7000 K) has recently been reported by Lallement et al. (1994) using observations of Mg II and Mg I toward Sirius A. We show that if such a high value exists, it cannot result from the EUV stellar radiation field and, therefore, must be due to a strong diffuse source of EUV radiation.
Vallerga, J. V.↗
First spectral observations of the diffuse background with the Extreme Ultraviolet Explorer
We present the first results from the analysis of the spectroscopic observations of diffuse extreme ultraviolet (EUV) emission taken with the Extreme Ultraviolet Explorer (EUVE) spectrometers in the wavelength range 160-740 A. Although not designed or optimized for diffuse observation, the EUVE spectrometers are the most sensitive diffuse EUV spectrometer in orbit. The spectral resolution for diffuse emission of the medium and long-wavelength spectrometers are 17 and 34 A FWHM, respectively. During the period from 1992 July 25 to 1992 August 19, the spectrometers surveyed a 2 x 20 deg field scanned from (l, b) = (24 deg, -28 deg) to (44 deg, -74 deg) with a total effective exposure time of 575,232 s. The only emission lines detected were those of He I and He II (584, 537, 304 A) with intensities consistent with local geocoronal and/or interplanetary scattering of solar radiation (584 A = 1.30 rayleighs; 537 A = 0.040 R; and 304 A = 0.029 R). Models of the soft X-ray background, which results from a 10(exp 6) K plasma (Local Bubble) surrounding the neutral gas near the Sun (Local Cloud), predict that most of the flux from the hot plasma appears as emission lines in the EUV. We have compared these spectral predictions with our observations to place limits on the emission measure versus temperature of the proposed hot plasma. Using the same plasma model, we derived emissions measures for our data and the C and B soft X-ray bands of the Wisconsin rocket survey. We find that our limits for the plasma emission measure are a factor of 5-10 below the C- and B-band emission measures over the temperature range from 10(exp 5.7) to 10(exp 6.4) K. We explore possible scenarios that could reconcile our results with the X-ray surveys and conclude that depletion or a nonequilibrium plasma state rather than absorption are the more likely explanations of the discrepancy. We also show that our spectrum is inconsistent with the spectrum from the approximately 10(exp 5) K gas at the conductive interface between the hot Local Bubble and the cooler Local Cloud given by Slavin (1989). In addition, we place new limits on the helium ionization parameter in the Sun's vicinity caused by the 10(exp 6) K plasma in the Local Bubble.
Jelinksy, P.↗
EUVE spectroscopy of epsilon Canis Majoris (B2 II) from 70 to 730 A
We present spectra of the brightest stellar source of extreme ultraviolet (EUV) radiation longward of 400 A, the B2 II star, epsilon CMa. These data were taken with the three spectrometers aboard the NASA Extreme Ultraviolet Explorer satellite (EUVE) during the first cycle of pointed observations. We report on our initial studies of the continuum and line spectrum of the stellar photosphere in the 320 to 730 A region, and on the wind emission lines observed in the 170-375 A region. This is the first EUV spectrum of an early-type star, and thus makes epsilon CMa the most comprehensively observed B star from the X-ray to infrared regimes. The radiation in both the H Lyman continuum and He I continuum (shortward of 504 A) are found to be significantly greater than predicted by both Local Thermodynamic Equilibrium (LTE) and non-LTE model atmospheres. Since epsilon CMa also exhibits a mid-infrared excess, this points to the outer layers being warmer than the models indicate. The anomalously large Lyman continuum flux, combined with the very low column density measured in the direction toward this star implies that it is the dominant source of hydrogen ionization of the local interstellar medium in the immediate vicinity of the sun. All of the lines predicted to be strong from model atmospheres are present and several wind absorption features are also identified. We have detected emission lines from highly ionized iron that are consistent with the ROSAT Position Sensitive Proportional Counter (PSPC) observations if a multi-temperature emission model is used, and the assumption is made that there is significant absorption beyond that of the neutral phase of the ISM. The spectrum shows strong O III 374 A line emission produced by the Bowen flourescence mechanism, which has not previously been observed in the spectra of hot stars.
Cassinelli, J. P.↗
The local distribution of Na I interstellar gas
We present high-resolution absorption measurements (lambda/Delta lambda approximately 75,000) of the interstellar Na I D lines at 5890 A toward 80 southern hemisphere early-type stars located in the local interstellar medium (LISM). Combining these results with other sodium measurements taken from the literature, we produce galactic maps of the distribution of neutral sodium column density for a total of 293 stars generally lying within approximately 250 pc of the Sun. These maps reveal the approximate shape of the mid-plane contours of the rarefied region of interstellar space termed the Local Bubble. Its shape is seen as highly asymmetric, with a radius ranging from 30 to 300 pc, and with an average radius of 60 pc. Similar plots of the Galactic mid-plane distribution of sources emitting extreme ultraviolet radiation show that they also trace out similar contours of the Local Bubble derived from Na I absorption measurements. We conclude that the Local Bubble absorption interface can be represented by a hydrogen column density, Nu(sub ETA) = 2 x 10(exp 19) cm(exp -2), which explains both the local distribution of Na I absorption and the observed galactic distribution of extreme ultraviolet sources. The derived mid-plane contours of the Bubble generally reproduce the large-scale features carved out in the interstellar medium by several nearby galactic shell structures.
Welsh, B. Y.↗
Serendipitous EUV sources detected during the first year of the Extreme Ultraviolet Explorer right angle program
We report the detection of 114 extreme ultraviolet (EUV; 58 - 740 A) sources, of which 99 are new serendipitous sources, based on observations made with the imaging telescopes on board the Extreme Ultraviolet Explorer (EUVE) during the Right Angle Program (RAP). These data were obtained using the survey scanners and the Deep Survey instrument during the first year of the spectroscopic guest observer phase of the mission, from January 1993 to January 1994. The data set consists of 162 discrete pointings whose exposure times are typically two orders of magnitude longer than the average exposure times during the EUVE all-sky survey. Based on these results, we can expect that EUVE will serendipitously detect approximately 100 new EUV sources per year, or about one new EUV source per 10 sq deg, during the guest observer phase of the EUVE mission. New EUVE sources of note include one B star and three extragalactic objects. The B star (HR 2875, EUVE J0729 - 38.7) is detected in both the Lexan/B (approximately 100 A) and Al/Ti/C (approximately 200 A) bandpasses, and the detection is shown not to be a result of UV leaks. We suggest that we are detecting EUV and/or soft x rays from a companion to the B star. Three sources, EUVE J2132+10.1, EUVE J2343-14.9, and EUVE J2359-30.6 are identified as the active galactic nuclei MKN 1513, MS2340.9-1511, and 1H2354-315, respectively.
Mcdonald, K.↗
High-resolution Ca II observations of the local interstellar medium
High-resolution absorption measurements of the interstellar Ca II K line observed toward 46 early-type stars in the local ISM (LISM) are presented. Ca II was detected in 36 of the 46 stars with 82 individual cloud components identified. Ca II was detected to most of the stars closer than 50 pc, except in the region of the Galactic quadrant l = 180-270 degrees which also contains the empty line of sight to B CMa at 220 pc. The mean local standard of rest velocity of the 82 Ca II components implies that the LISM clouds are associated with the motion of the solar neighborhood and not the sun. If the present data are combined with other nearby Ca II component velocities taken from the literature, then a cloud centered approximately at l = 90 deg, b = -40 deg moving coherently with the local interstellar wind vector is supported at a significance level of 99 percent. The Ca II data have been combined with Na I data for the same stars to produce a N(Na I)/N(Ca II) ratio for each identified absorption feature. This ratio plotted against the local standard of rest velocities of the clouds shows that the Routly-Spitzer effect exists down to +/- 10 km/s, which supports grain desorption/destruction models that are efficient at returning calcium to the gas phase at these low velocities.
Vallerga, J. V.↗
Experiences with thin film filter development for the Extreme Ultraviolet Explorer
The design, development, and optimization of the thin film filters used on the Extreme Ultraviolet Explorer (EUVE) Satellite to define the EUV wavelength bandpasses of the individual instruments was a complicated task. The bandpasses had to be optimized for the astrophysical goals of the EUVE mission and constrained by the strong geocoronal EUV background emission. Materials with optical constants that met these requirements had to be found and tested. In many cases these materials were not compatible or were not strong enough to survive the intense vibrations of a rocket launch. Other effects, such as photoelectron 'halo' produced in the filters, were not discovered until flight qualification. The final set of flight filters included: lexan/boron, aluminum/carbon, titanium/antimony/aluminum, and tin/silicon monoxide. This paper discusses the lessons learned in the development of these filters, including the optimization process, material interactions and problems, calibration techniques, vibration susceptibility, thermal tests, and photoelectron emission. We feel the experiences gained over the last 10 years creating the filter sets for EUVE will be invaluable for future missions that use thin film filters.
Vallerga, J. V.↗
Thin film filter lifetesting results in the extreme ultraviolet
We present the results of the thin film filter lifetesting program conducted as part of the NASA Extreme Ultraviolet Explorer (EUVE) satellite mission. This lifetesting program is designed to monitor changes in the transmission and mechanical properties of the EUVE filters over the lifetime of the mission (fabrication, assembly, launch and operation). Witness test filters were fabricated from thin film foils identical to those used in the flight filters. The witness filters have been examined and calibrated periodically over the past seven years. The filters have been examined for evidence of pinholing, mechanical degradation, and oxidation. Absolute transmissions of the flight and witness filters have been measured in the extreme ultraviolet (EUV) over six orders of magnitude at numerous wavelengths using the Berkeley EUV Calibration Facility.
Vedder, P. W.↗
Further high-resolution Na I observations of the local interstellar medium
High-resolution absorption measurements of the interstellar Na I D lines at 5890 A observed toward 27 early-type stars in the local interstellar medium (LISM) are presented. These results are combined with other high-resolution sodium measurements to map the space distribution of neutral sodium column density for some 118 stars out to less than 200 pc. These measurements indicate an upper limit to the neutral sodium column density of log N(Na I) less than 10.0/sq cm can be inferred out to a distance of 50 pc in most directions in the LISM. Also, the rarefield region of the Local Bubble may extend beyond 60 pc in at least 35 percent of the directions sampled thus far. Evidence is shown for a ubiquitous, comoving vector for neutral NaI gas clouds in the LISM which is in a different direction to LISM vectors previously reported for more ionized local gas clouds. A comparison of the measured sodium columns with those of interstellar Ca II for a sample of 12 stars within 95 pc results in a ratio of Na I/Ca II less than 0.5 for most stars. This value implies that there could be warm, neutral gas with T of about 12,500 K beyond 50 pc in the LISM.
Welsh, B. Y.↗
Expected scientific performance of the three spectrometers on the extreme ultraviolet explorer
The expected in-orbit performance of the three spectrometers included on the Extreme Ultraviolet Explorer astronomical satellite is presented. Recent calibrations of the gratings, mirrors and detectors using monochromatic and continuum EUV light sources allow the calculation of the spectral resolution and throughput of the instrument. An effective area range of 0.2 to 2.8 sq cm is achieved over the wavelength range 70-600 A with a peak spectral resolution (FWHM) of 360 assuming a spacecraft pointing knowledge of 10 arc seconds (FWHM). For a 40,000 sec observation, the average 3 sigma sensitivity to a monochromatic line source is 0.003 photons/sq cm s. Simulated observations of known classes of EUV sources, such as hot white dwarfs, and cataclysmic variables are also presented.
Vallerga, J. V.↗
Imaging characteristics of the Extreme Ultraviolet Explorer microchannel plate detectors
The Extreme Ultraviolet Explorer (EUVE) satellite will conduct an all-sky survey over the wavelength range from 70 A to 760 A using four grazing-incidence telescopes and seven microchannel-plate (MCP) detectors. The imaging photon-counting MCP detectors have active areas of 19.6 cm2. Photon arrival position is determined using a wedge-and-strip anode and associated pulse-encoding electronics. The imaging characteristics of the EUVE flight detectors are presented including image distortion, flat-field response, and spatial differential nonlinearity. Also included is a detailed discussion of image distortions due to the detector mechanical assembly, the wedge-and-strip anode, and the electronics. Model predictions of these distortions are compared to preflight calibration images which show distortions less than 1.3 percent rms of the detector diameter of 50 mm before correction. The plans for correcting these residual detector image distortions to less than 0.1 percent rms are also presented.
Vallerga, J. V.↗
Flat field response of the microchannel plate detectors used on the Extreme Ultraviolet Explorer
The results of the extreme ultraviolet (EUV) flat field calibrations of two of the flight detectors to be flown on the Extreme Ultraviolet Explorer Satellite (EUVE) are presented. Images of about 40 million detected events binned 512 by 512 are sufficient to show microchannel plate fixed pattern noise such as hexagonal microchannel multifiber bundle interfaces, 'dead' spots, edge distortion, and differential nonlinearity. Differences due to photocathode material and dependencies on EUV wavelength are also described. Over large spatial scales, the detector response is flat to better than 10 percent of the mean response, but, at spatial scales less than 1 mm, the variations from the mean can be as large as 20 percent.
Vallerga, J. V.↗
The filters for the Extreme Ultraviolet Explorer - Calibration and lifetesting results
The results of the transmission calibrations of the seven sets of flight integration filters for the Extreme Ultraviolet Explorer (EUVE) satellite are presented. The Berkely EUV Calibration Facility was used to determine absolute transmissions over six orders of magnitue at 28 wavelengths. A lifetesting program designed to monitor changes in the transmission and mechanical properties of the EUVE filters over the lifetime of the mission is described.
Vedder, P. W.↗
Extreme ultraviolet quantum efficiency of opaque alkali halide photocathodes on microchannel plates
Comprehensive measurements are presented for the quantum detection efficiency (QDE) of the microchannel plate materials CsI, KBr, KCl, and MgF2, over the 44-1800 A wavelength range. QDEs in excess of 40 percent are achieved by several materials in specific wavelength regions of the EUV. Structure is noted in the wavelength dependence of the QDE that is directly related to the valence-band/conduction-band gap energy and the onset of atomic-like resonant transitions. A simple photocathode model allows interpretation of these features, together with the QDE efficiency variation, as a function of illumination angle.
Siegmund, O. H. W.↗
Ultraviolet quantum detection efficiency of potassium bromide as an opaque photocathode applied to microchannel plates
The quantum detection efficiency (QDE) of potassium bromide as a photocathode applied directly to the surface of a microchannel plate over the 250-1600 A wavelength range has been measured. The contributions of the photocathode material in the channels and on the interchannel web to the QDE have been determined. Two broad peaks in the QDE centered at about 450 and about 1050 A are apparent, the former with about 50 percent peak QDE and the latter with about 40 percent peak QDE. The photoelectric threshold is observed at about 1600 A, and there is a narrow QDE minimum at about 750 A which correlates with 2X the band gap energy for KBr. The angular variation of the QDE from 0 to 40 deg to the channnel axis has also been examined. The stability of Kbr with time is shown to be good with no significant degradation of QDE at wavelengths below 1216 A over a 15-day period in air.
Siegmund, Oswald H. W.↗
A high-speed, dual-CCD imaging photometer
The SNAPSHOT is a high-speed, dual-CCD imaging photometer, with special capability for occultation observations. It is capable of recording occultation light curves in the presence of bright, nonuniform background illumination, such as occultations of stars by Saturn's rings or by comets. Such events are difficult or impossible to observe with conventional photometers. The fundamental idea that makes high time-resolution observations possible is to reduce the data rate by digitizing only those pixels in subframes containing the objects of interest.
Dunham, E. W.↗
The explosive transient camera (ETC) - An instrument for identification of gamma ray burst optical counterparts
The ETC is a wide-field (3-steradian) electronic camera array which can detect coincident optical flashes with durations of 0.1 to 100 sec. Each array element is a 20 x 30 deg FOV, cooled CCD detector. An optical transient (possibly associated with a gamma ray burst) as faint as B = +11 (1 sec duration) can be detected with an S/N of not less than 20, and its position determined to an accuracy of + or - 10 arcsec. Candidate events 1000 times fainter than the archived event (plate taken in 1928) reported for the November 19, 1978 gamma ray burst by Schaefer (1981) should be detectable in real time. A prototype camera was tested in April 1982 to establish sky background levels and spurious event rejection schemes; a 1/2-ster test version of the ETC was planned for operation in early 1984; expansion to the full 3-ster complement of 16 detectors at each of two sites was planned for the 1984-1985 period.
Ricker, G. R.↗