FIR filter effects and nucleation phases

The symmetric impulse response of linear phase Finite Impulse Response (FIR) filters most commonly used in modern seismic recording systems produces precursory signals to impulsive arrivals. These acausal filter-generated artefacts may result in misinterpretations of various onset properties. Prior to any onset interpretation, these effects have to be removed from the seismic record. This can be achieved without loss of bandwidth by post-filtration of the digital seismograms if the filter coefficients and the decimation ratios are known. We have analysed numerous signals from different instruments and sampling rates for precursory phases and found that—in contrast to commonly held beliefs—FIR-filter-related precursory signals are not always easy to recognize visually from their waveform signature. Furthermore, they can exhibit surprisingly similar properties to those reported for nucleation phases, although the majority of nucleation phases reported in the past have been obtained on instruments with a causal response. We demonstrate examples of filter-related precursory signals for events scanning nine orders of moment, from 10¹⁰ N m to 10¹⁹ N m. Surprisingly, the lower bound of the artefact durations as a function of seismic moment scales close to the cube root of the seismic moment. We interpret this as being caused by the fact that above a certain seismic moment, the attenuated source signal acts as a causal lowpass filter of a smaller bandwidth than the FIR filter. Assuming an ω^-2 source model, constant stress drop and an empirical relationship between the maximum artefact duration and the cut-off frequency of the FIR filter, the artefact durations are expected to scale proportional to the 1/2.5 power of the seismic moment, in comparison to 1/3 as proposed for nucleation phases.     

Arome,a balloon-borne instrument for astronomical near-infrared spectrophotometry

We present here the French balloon-borne experiment named AROME. This astronomical instrument is specifically designed for the detection of near infrared emission bands (=3–12m) from extended sources of low surface brightness with an angular resolution of 0.5°. The rms sensitivity achieved is of the order of 10^-8 Wm^-2sr^-1, in terms of the power radiated in the 3.3m band. We recall that this program led to the first detection of the 3.3m feature in the spectrum of the diffuse galactic emission. The scientific results were presented and interpreted in Paper I, Giard et al., 1989. Future work will include both the observation of fainter objects at 3.3m and the detection of other emission bands at 6.2 and 11.3m.     

Estuarine Physical Processes Research: Some Recent Studies and Progress

The literature on estuarine physical studies is vast, diverse and contains many valuable case studies in addition to pure, process-based research. This essay is an attempt to summarize both some of the more recent studies that have been undertaken during the last several years, as well as some of the trends in research direction and progress that they represent. The topics covered include field and theoretical studies on hydrodynamics, turbulence, salt and fine sediment transport and morphology. The development and ease-of-application of numerical and analytical models and technical software has been essential for much of the progress, allowing the interpretation of large amounts of data and assisting with the understanding of complex processes. The development of instrumentation has similarly been essential for much of the progress with field studies. From a process viewpoint, much more attention is now being given to interpreting intratidal behaviour, including the effects of tidal straining and suspended fine sediment on water column stratification, stability and turbulence generation and dissipation. Remote sensing from satellites and aircraft, together with fast sampling towed instruments and high frequency radar now provide unique, frequently high resolution views of spatial variability, including currents, frontal and plume phenomena, and tidal and wave-generated turbidity. Observations of fine sediment characteristics (floc size, aggregation mechanisms, organic coatings and settling velocity) are providing better parameterizations for sediment transport models. These models have enhanced our understanding both of the estuarine turbidity maximum and its relationship to fronts and intratidal hydrodynamic and sedimentological variability, as well as that of simple morphological features such as intertidal mudflats. Although few, interdisciplinary studies to examine the relationships between biology and estuarine morphology show that bivalve activity and the surface diatom biofilm on an intertidal mudflat can be important in controlling the erosion of the surface mud layer.     

Rugate filters for OH-suppressed imaging at near-infrared wavelengths

Ground-based observations at near-infrared wavelengths are severely affected by atmospheric OH bands. Many authors have recognized the potential gains in sensitivity from suppressing these features. Dispersive instruments show some promise but are both expensive and complicated to build. OH suppression filters using single or periodic notches have the advantage of simplicity but significant gains have not yet been realized.   Rugate filters (with graded index inhomogeneous coatings) offer key advantages for astronomical imaging. It is possible to produce a transmission profile comprising a series of irregular and sharply defined bandpasses. We demonstrate through numerical simulation of rugate filters that it should be possible to achieve 95 per cent suppression of the OH features in the J photometric band, while retaining roughly half of the spectral coverage. This would lead to extraordinary gains in sensitivity even for observations of continuum sources. In addition, these filters allow longer exposures before the detector saturates on the sky background. I - and z -band filters can also be envisaged.   In 1-arcsec seeing, a J -band rugate filter used in conjunction with a 4-m telescope would detect a J  = 23 continuum source at 5.0σ in a single 10-min exposure. In comparison, a conventional J filter requires multiple exposures for a 10-min integration time and achieves only a 2.5σ detection. For emission-line sources, the rugate filter has an even bigger advantage over conventional filters, with a fourfold increase in signal-to-noise ratio possible in certain instances.   Astrophysical studies that could benefit from rugate filters are searches for very low-mass stars and galaxy evolution out to z  = 3.     

映射函数对天文大气折射的改进

本文利用大气折射积分母函数方法,分别给出在射电波段和光学波段上天文大气折射改正的映射函数,并完整地考虑了天文学和空间技术所需要的物理和地球物理因素引入的改正．本文还利用探空气球的资料分析了新天文大气折射改正公式的实际精度;计算结果证明：它在2°高度角时达到5”左右,而在5°高度角时约为1”．我们认为：限制计算精度的主要因素是真实地球大气分布与理论大气模型的区别．     

ULTRACAM: an ultrafast, triple-beam CCD camera for high-speed astrophysics

ULTRACAM is a portable, high-speed imaging photometer designed to study faint astronomical objects at high temporal resolutions. ULTRACAM employs two dichroic beamsplitters and three frame-transfer CCD cameras to provide three-colour optical imaging at frame rates of up to 500 Hz. The instrument has been mounted on both the 4.2-m William Herschel Telescope on La Palma and the 8.2-m Very Large Telescope in Chile, and has been used to study white dwarfs, brown dwarfs, pulsars, black hole/neutron star X-ray binaries, gamma-ray bursts, cataclysmic variables, eclipsing binary stars, extrasolar planets, flare stars, ultracompact binaries, active galactic nuclei, asteroseismology and occultations by Solar System objects (Titan, Pluto and Kuiper Belt objects). In this paper we describe the scientific motivation behind ULTRACAM, present an outline of its design and report on its measured performance.     

Exact optics – V. Null tests for the mirrors of Schwarzschild's spectrograph camera

Null tests are described for the strongly curved and highly aspheric mirrors of a set of 'exact optics' spectrograph cameras based on the design of Schwarzschild. For f /1.0 and f /1.333 cameras having a focal length of 100 mm the tests of each mirror are well within the Rayleigh limit and would permit construction on a larger scale. The ray-theoretical image diameters at the edge of the field of view of these two cameras are under 10 μm unless they are scaled up to an aperture greater than 450 mm.     

Searches for ultracompact dwarf galaxies in galaxy groups

We present the results of a search for ultracompact dwarf galaxies (UCDs) in six different galaxy groups: Dorado, NGC 1400, 0681, 4038, 4697 and 5084. We searched in the apparent magnitude range  17.5 ≤ b _J≤ 20.5  (except NGC 5084:  19.2 ≤ b _J≤ 21.0  ). We found one definite plus two possible UCD candidates in the Dorado group and two possible UCD candidates in the NGC 1400 group. No UCDs were found in the other groups. We compared these results with predicted luminosities of UCDs in the groups according to the hypothesis that UCDs are globular clusters formed in galaxies. The theoretical predictions broadly agree with the observational results, but deeper surveys are needed to fully test the predictions.