Spectrometer Channels Research Articles

A PROGRAMMABLE 36-MHZ DIGITAL FILTER BANK FOR RADIO SCIENCE

ABSTRACT We have designed and constructed an agile 8-channel digital filter bank on a 9U VME board that operates at a maximum clock rate of 36 MHz. A set of these boards have been employed to create 100-200 MHz, 50-100 channel spectrometers. Our applications involve detection of weak signals in a background of noise--pulsar radio astronomy and the search for narrow-band radio emissions from extraterrestrial civilizations. The agility factors include total bandwidth, spacing of filter channels, selection of filter response and choice of output data format (voltage or detected power). The input is a complex analog voltage centered on zero frequency which is passed to the board via coaxial ports on the front panel. The computational kernels on this board are Harris HSP 43168 Finite Impulse Response (FIR) filter devices. We can also operate the board in a 16-channel output mode. The 2-4 bit format digital output is presented to a custom backplane. Along with our development we have written a suite of simulation software tasks both to determine the sensitivity loss and non-linear gain response that result from quantization and to optimize filter responses.

Twenty Years of Serendip, The Berkeley Seti Effort: Past Results and Future Plans

AbstractThe Berkeley SETI effort has been ongoing for more than twenty years. During this period we have carried out extensive SETI searches, but of at least equal importance, we have been on the forefront of instrumentation development, the development of interference rejection and detection algorithms, and educational efforts. Our instrumentation has evolved from a 100 channel spectrometer (SERENDIP I) to our latest machine (SERENDIP IV) with 168 million channels. In addition to supporting our SETI work, our hardware has been used by NASA for RFI surveys and to check the Mars Survey relay transmitter. Our designs have influenced several of the major SETI hardware systems, and a SERENDIP III system is currently being used in the Ohio State SETI group. Our hardware has also been incorporated in pulsar instrumentation at NAIC (Arecibo), NRAO (Greenbank) and Bonn, Germany. We have just completed a 4 year search at the world’s largest radio telescope at Arecibo. We covered 93% of the observable sky at least once, and 44% of the sky at least 5 times, with a sensitivity of ~ 3 × l0−25Wm−2. We collected and analyzed information from over 1014spectral bins and logged and analyzed over 2 × 108signals of potential special interest. We have developed extensive analysis programs to identify and remove interference and to search for signal characteristics which are expected from an extraterrestrial signal. Our algorithms were successful in removing all but 0.01% of spurious RFI in the Arecibo data set. We have involved over 30 undergraduate and graduate students in our effort, and 5 Master's theses have been awarded for work on the SERENDIP project. Our next effort will use our SERENDIP IV hardware at Arecibo Observatory to carry out a 21 cm survey of the Arecibo sky.

Optimal tuning of the discriminator of the spectrometer channel in a mossbauer experiment

We consider here the choice of optimal tuning of the spectrometer recording channel discriminator in a Mossbauer experiment in transmission configuration. It is established that the width of the discriminator “window” should be one and a half times the widths of the recorded resonance γ-radiation peak at its half-height. The position of the center of the discriminator window should then correspond to the position of the maximum of the resonance γ-radiation peak.

Quantitation of phosphorus metabolites in newborn human brain using internal water as reference standard.

A new method for noninvasive, in vivo quantitation of cerebral phosphorus (31P) metabolites is described. The technique employs point-resolved spectroscopy (PRESS) to obtain both 31P-metabolite and proton (1H) water spectra: brain water is used as an internal concentration reference. Spin-spin relaxation times (T2s) of cerebral 31P metabolites are much longer than the minimum echo time (TE) usable on a spectrometer equipped with actively shielded gradient coils. With short-TE (approximately 10 ms) 31P PRESS, T2 relaxation is minimal and phase modulation of the nucleotide triphosphate (NTP) multiplets can be accounted for 1H water spectra were acquired using several TEs so that extra- and intracellular water signals could be separated from that due to cerebrospinal fluid. Prior calibration of the 31P and 1H spectrometer channels and an assumed brain-water concentration enabled estimations of metabolite concentrations. Using this method, mean 31P metabolite concentrations in the brains of eight normal infants of gestational plus postnatal age 34 to 39 wk were: phosphomonoester (PME) 5.6 (SD 0.9); inorganic phosphate 1.4 (0.4); mobile phosphodiester 2.3 (0.6); phosphocreatine 2.9 (0.3); nucleotide triphosphate 2.8 (0.6); and total mobile phosphate 21.4 (2.8) mmol/kg wet.

Extreme-Ultraviolet Spectroscopy and Photometry of EQ Pegasi

We present the first spectroscopic extreme-ultraviolet observations of the star EQ Peg (dM4e+dM6e) obtained during a guest observer pointing with the Extreme Ultraviolet Explorer (EUVE) satellite. EQ Peg was detected in all three spectrometer channels (70-760 Å) and was successfully monitored at the same time by the EUVE Deep Survey photometer (67-178 Å). We have identified the main features observed in the spectra and conclude that a number of lines are produced by the highest ionization stages of iron. The detections in the 280-760 Å range are mainly caused by second-order throughput, except for the He II λ304 line. Evidence of activity modulation during the observation is present in both the photometric and spectroscopic data.

64 channel fiber optic spectrometer for a dual wavelength interferometric pressure sensor array

As part of a fiber optic, interferometric pressure sensing array system for hypersonic wind tunnel studies, a 64 channel spectrometer has been developed to separate the return signals by both position and wavelength. The spectrometer is designed to optically separate the quadrature signals from the interferometer. A ribbon of single-mode fibers carrying the dual wavelengths (811.7 and 813.3 nm) from the sensing surface array forms the entrance slit to the spectrometer. This spectrometer uses a unique optical layout consisting of a 32 element microlens array, narrow passband interference filter and concave mirror to separate and focus the optical signals onto the photosensitive surface of a 1×64 photodiode array. The microlens array collimates the emerging beams from the single-mode fibers to minimize cross talk between channels. The detector array and its control electronics are an integral part of the spectrometer and are mounted on the same baseplate as the other optical components. Fast scanning and digitizing electronics produce an effective data rate of 160 kHz for 64 channels. Details of the construction and results of the calibration are presented.

Coherent variations of monthly mean total ozone and lower stratospheric temperature

Space‐time patterns of correlation between total ozone and lower stratospheric temperature are documented, based on 14 years (1979–1992) of global monthly mean observations. Data are obtained from the total ozone mapping spectrometer (TOMS) and microwave sounding unit (MSU) channel 4, the latter being a weighted mean temperature of the 150‐ to 50‐mbar layer. These data are analyzed (separately) for linear trend, solar cycle, quasi‐biennial oscillation (QBO), and El Niño‐Southern Oscillation (ENSO) variations via linear regression: significant signals are identified for each term, and the corresponding structures in ozone and temperature are found to be highly coherent. The temperature trends derived here show significant cooling of the lower stratosphere over Northern Hemisphere (NH) midlatitudes in winter‐spring and over Antartica in Southern Hemisphere (SH) spring; the overall space‐time patterns are similar to those determined for ozone trends. Interestingly, temperatures do not decrease over SH midlatitudes during midwinter, in spite of large ozone losses. These data furthermore show globally coherent ozone and temperature perturbations associated with both QBO and ENSO variations; a new result here shows large total ozone anomalies in middle‐to‐high latitudes of both hemispheres associated with ENSO events. Residuals from the ozone and temperature time series (defined as the deseasonalized total minus the regression fits) show strong positive correlation in middle‐to‐high latitudes but weak correlations in the tropics. Time periods following the volcanic eruptions of El Chichon and Pinatubo are clearly identified from the coupled signatures of decreased ozone and increased temperature, opposite to the positive ozone‐temperature correlations observed at other times. The ratios of ozone to temperature anomalies derived here show quantitative signatures indicating that either radiative (trend, solar, and QBO) or dynamical (ENSO and residuals) processes are responsible for the strong ozone‐temperature correlations.

Absolute calibration and monitoring of a spectrometric channel using a photomultiplier

A method for measuring the photomultiplier gain using a realistic photomultiplier response function is described. A precision of about 1% for the deconvoluted gain parameter can be achieved.

Calibration facility and preflight characterization of the photometer in the Infrared Space Observatory

A calibration facility simulating the optical and cryogenic environment of the Infrared Space Observatory (ISO) satellite has been built for characterizing the ISO photometer (ISOPHOT). This facility uses a commercially calibrated 900-K blackbody radiation source and optics at room temperature to provide an f /15 beam to the instrument, which is contained in a LHe cryostat. The low-level infrared flux levels of the ISO are obtained by using a light-sealed instrument chamber and cold attenuation filters. The infrared flux can be calculated using the known blackbody emission and the cold calibrated transmission spectra of the filters. The calibration facility further provides a scanning mechanism, a light modulator, and filters for polarization measurements and wavelength calibrations of the ISOPHOT spectrometer channels. Electrical support equipment for the instrument operation and software for data archiving and analysis have been customized for this project. The test program comprised a standardized acceptance test for the entire instrument and special tests addressing individual instrument properties. The data obtained contain the photometric sensitivities of ISOPHOT, optimized instrument settings, reference data for the integrated system tests, and inputs for the ongoing mission planning.

Background correction procedure for multi‐channel x‐ray fluorescence spectrometers

AbstractA background correction procedure is described for multi‐channel x‐ray fluorescence spectrometers/quantometers with spectrometric channels of a fixed type. The background intensities are measured for a special background sample. Background intensities of the unknown sample are calculated from these measurements with the help of s and f corrections, taking account of qualitative and quantitative differences in the spectral composition of the background radiation for both an unknown specimen and the background sample. Determinations of the major elements in rocks on a multi‐channel x‐ray fluorescence quantometer demonstrated the capabilities of this procedure.

Radio-frequency bandpass filter for NMR applications

A high-power, lumped-elements, interdigital bandpass filter is described which has been used to improve the isolation between decouple and observe channels of NMR spectrometers, and to ‘‘clean’’ the transmitter signals. With no calculations, and with the help of a sweeper or a spectrum analyzer, filters of this type can be easily assembled and tuned over the 90–400 MHz range, typical of the proton decoupling frequencies.

Spectral interferences on arsenic and mercury prominent emission lines in vacuum inductively coupled plasma-atomic emission spectrometry and use of methanol vapor as optical filter for spectral order sorting

Wavelength profiles for eleven prevalent concomitant elements on arsenic and mercury emission lines at 189.04 and 184.95 nm in the third order, were obtained by simultaneously scanning the arsenic and mercury channels of a vacuum spectrometer with an argon inductively coupled plasma (ICP) as an excitation source. The displacement of the polychromator primary slit was used for scanning. Vacuum ultraviolet (VUV) lines as well as ultraviolet (UV) lines above 200 nm were observed because the interference filter located before the channels' photomultipliers had some residual transmission to radiation above 200 nm. The VUV emission lines were differentiated from the UV lines by purging the optical path between the plasma and the spectrometer, with methanol vapor. The vapor absorbed the VUV lines and allowed the UV lines above 200 nm to pass through, acting as a gas optical filter. The arsenic channel experienced spectral interferences from Ti(200 mgl −1), Mn(200 mgl −1), Cr(200 mgl −1), and Fe(1000 mgl −1) due to VUV lines and from V(200 mgl −1), and Cr due to UV lines, while Mg(1000 mgl −1), Al(1000 mgl −1), Si(11000 mgl −1), Ca(1000 mgl −1) and Fe exhibited background enhancement. Ni(200 mgl −1) and Cu(200 mgl −1) did not interfere. The mercury channel experienced spectral interferences from Si and Ca due to VUV lines and from Mn, Cr and Fe due to UV lines, whereas Mg and Al exhibited background enhancement, V, Ti, Ni and Cu did not interfere.

The spectrum of comet halley obtained by VEGA-2

During the pass of VEGA-2 near Comet Halley spectra in the 2750 ÷ 7000 Å region are registered by the three channel spectrometer on board. Spectra from different parts of the comet coma are obtained by scanning. The spectra are deconvoluted by means of regularizational algorithm. Bands of the following cometary atmospheric species are identified: OH, NH, CN, Ca, CH, C 3, NH 2, H 2O +, CO +, CO 2 +.

A multi-channel spectrometer for airglow observations

An account is given of the design, construction, and performance of a six channel spectrometer using tilting interference filters. Spectra of resolution of approximately 2 nm and extending over a range of about 15 nm are obtained once every 2 min at each of six wavelength regions in the visible and near infrared, and are recorded digitally for subsequent analysis. Examples are given of data obtained between 1981 and 1984 showing how the variation of intensities of atomic and molecular emissions and of the temperature deduced from the OH bands can be displayed.

Upon the application of cumulant analysis to the interpretation of quasielastic light scattering spectra

Monte Carlo simulations were used to study how noise limits the interpretation of quasielastic light scattering spectra with cumulant analysis. The effects of signal-to-noise ratios, channel spacing, line shape, and the truncation order of the cumulant series were all examined. The underlying spectra included narrow and broad unimodal and bimodal forms, as expected for ternary polymer:polymer:solvent mixtures. The upper bound on the usable number of cumulants is determined by the signal-to-noise ratio, the spectral nonexponentiality, and the number and spacing of the spectrometer channels. Upper limits on the acceptable degree of spectral nonexponentiality—below which the cumulant expansion is useful—can be stated in terms of the measured second cumulant. Use of logarithmically rather than linearly spaced correlator channels greatly increases the region over which cumulant fits are useful.

Spectrometer for the study of three‐photon annihilation of three‐photon annihilation of positrons

AbstractA three channel spectrometer for studying three‐photon annihilation is reported. It is based on a fast‐fast coincidence circuit and includes NaJ(Tl) scintillation detectors, constant fraction differential discriminators, a fast coincidence circuit with variable resolving time (6 ns—60 ns), a linear gate and a multichannel analyzer.The possibility of using positron source 64Cu in the spectrometer, its advantages and disadvantages are discussed. In this connection we describe the special units coupled to the spectrometer as well as a method which helps to avoid the influence of the short decay time of 64Cu of the accuracy of the experimental results.

A search for narrow band signals with SERENDIP II: A progress report

Commensal programs for the Search for Extraterrestrial Intelligence (SETI), carried out concurrently with conventional radio astronomical observing programs, can be an attractive and cost-effective means of exploring the large multidimensional search space intrinsic to this effort. Our automated commensal system, SERENDIP II, is a high resolution 131,072 channel spectrometer. It searches for 0.49 Hz signals in sequential 64,700 Hz bands of the IF signal from a radio telescope being used for an astronomical observation. Upon detection of a narrow band signal with power above a preset threshold, the frequency, power, time, and telescope direction are recorded for later study. The system has been tested at the Hat Creek Radio Astronomy Observatory 85 ft telescope and the NASA-JPL Deep Space Station (DSS 14) 64 m telescope. It is currently collecting data at the National Radio Astronomy Observatory 300 ft telescope.

Extreme Ultraviolet Explorer spectrometer

The design and calculated performance is described for a spectrometer included on the Extreme Ultraviolet Explorer (EUVE) astronomical satellite. The instrument is novel in design, consisting of three plane reflection gratings mounted in the converging beam behind a grazing incidence telescope. This configuration is based on new varied line-space (VLS) gratings which have recently been proposed. A sample EUVE grating has been mechanically ruled and experimentally characterized. It recovered over 80% of the theoretical efficiency of perfectly formed grooves, reaching 38% absolute at a wavelength of 114 Å. The grating was used to construct a laboratory spectrographic version of the EUVE spectrometer and recorded the spectrum of helium from 228 to 320 Å. The spectral resolution was λ/Δλ ∼ 2000 and the image heights were ∼10 sec of arc. Individual spots were ∼25 × 50 μm, which is a significant improvement over existing grazing incidence spectrographs. A line profile measurement at 1 Å away from first order 304 Å showed <1.5%/ Å of grating stray light and a rapid decline toward the wings. In visible light, no grating stray or ghost light could be seen. Three flight spectrometer channels in combination span the 70–760-Å band with an effective collecting area of 0.3–1 cm2 over the 80–600-Å region. The spectrometer has an inherent resolution of λ/Δλ ∼ 300, but if combined with a worst-case satellite performance will yield a spectral resolution of λ/Δλ = 110–240 and a spatial resolution of 1–2 min of arc. For a 40,000-sec observation, the average 3σ sensitivity to continuum flux is ∼2 × 10−27 erg/cm2/sec/Hz. This is a factor of 100 dimmer than a bright known EUV source and is comparable to the sensitivity of the all-sky survey which will be carried out on the EUVE. At a 5σ detection threshold, the spectrometer sensitivity to individual spectral lines is 1–4 × 10−3 photons/cm2/sec, which is a factor of 50 better than available with the EUVE wide bandpass telescopes. Simulated observations of two known classes of extrasolar EUV sources reveal rich spectra. During a six-month spectroscopic phase, target selection will be conducted by guest investigators chosen by NASA.

Background limitations in x-ray fluorescence analysis

For the x-ray fluorescence analysis of solutions formed in the technological processes of the nuclear fuel cycle, a method of preliminary selection was developed for the content of heavy elements [i, 2] according to the radiation energy of the sample, by means of a cylindrical pyrographite Bragg reflector, located between the sample and an Si(Li) detector. This method allows the background created by scattered perturbing radiation to be reduced. Limits of detection %0.15 ppm were achieved for uranium and neighboring elements in solution with quasimonochromatic excitation of a transmission x-ray tube with a 25-W power; the loading of the spectrometric channel for this amounted to less than 300 sec -z.

C-13H3OH in OMC-1

Transition line data for C-13H3OH in OMC-1 were gathered with a superconducting tunnel junction receiver and a 512 channel spectrometer on a 10.4 m telescope at the Owens Valley Radio Observatory. The methanol was scanned at 236 GHz and an observational efficiency of 85 percent. The survey was carried out to complement the data base on the line frequencies of internal rotors such as methanol and thereby the resolution of the C-12/C-13 ratio toward the galactic center. The data indicated that previous emission lines attributed to CO(+) and CH3CHO are actually methanol emissions, and the associated C-12/C-13 ratio is about 30.