Vistas in Astronomy最新文献

It has been claimed that MkIII/EVN experiments do not achieve the expected level of image quality, or dynamic range. An EVN test observation of a strong and compact source, DA193, has allowed us to investigate in detail the possible systematic errors affecting EVN images. Indeed, we have identified two effects which both significantly affect image quality: instrumental polarization and an amplitude error caused by the stepwise delay tracking in XF-correlators. This paper shows how these effects influence total-intensity images and discusses the correction of these non-closing errors. Their removal increased the peak/rms dynamic range from 5000 to 19,000 in our image of DA193.

The Westerbork Northern Sky Survey (WENSS) has been used to select a population of Gigahertz Peaked Spectrum (GPS) sources with flux densities one or two orders of magnitude lower than previous samples. We present preliminary VLBI observations of this new GPS sample.

A short account is presented of the struggle to understand and provide a theory for stellar pulsations. The observational and theoretical developments from the discovery of δ Cephei in 1784 up to Baker and Kippenhahn's key paper of 1962 are described and their significance discussed.

There exists a unique perpetual (solar) calendar with leap week that could become the basis for an eventual world calendar reform. Unlike the Universal Calendar considered for such a reform by the UN in 1956, rejected in a vote, it does not interrupt the continuity of weeks, which was the chief objection against this calendar. Except for the mentioned serious chronological defect, the Universal Calendar would otherwise have had many advantages. The leap week concept had been suggested earlier, but was never elaborated properly. Relevant theoretical questions and a suitable form of the calendar based on this concept are discussed. There is also a glimpse of the possible form of our calendar in a very distant future and of a calendar in cosmic space. The new calendar proposed is an optimum compromise for a solar calendar. While preserving advantages of the Universal Calendar, it fulfills all modern requirements: constancy, uniformity, continuity, simplicity and accuracy. With the leap rule derived (with regard to uniform time) an error of one day would occur in the new calendar in an interval longer than 10000 years. It could obtain a global acceptance, because there would be no discordance in the day of the week with respect to other existing calendars having a weekly cycle (such as, e.g., Jewish, Muslim, etc.).

We describe the use of the wavelet transform for multivariate data analysis problems. In prediction, a multiscale transform of time-varying data can allow forecasts of each scale, followed by a combining of the individual forecasts. The use of a wavelet transform with noise modeling for point pattern clustering can lead to the result, which initially appears counterintuitive, of clustering in constant computational time, O (1).

The concentration of information is a general problem in image processing. We illustrate this in a particularly simple case, the ISO Serendipity Survey, where a detector of 2 × 2 pixels slews through the sky and the task is to detect point sources. There is a theoretically optimal solution for information concentration: the Eigenvalue Transform. The practical shortcomings of the ET are solved by the Karhunen-Loèv Transform concept; and the application of the KLT to our particular problem yields the first-order two-dimensional Haar Transform. The above mentioned transform concentrates the information of the point sources and the glitches in different coefficients (gradient and curvature respectively). A one-dimensional (along the slew) Adaptive Filtering of the gradient is used to detect the point sources.

Over the last few years quite a lot of research has been devoted to restoring astronomical images, but not much work has been reported on the use of multichannel techniques to restore such images. In this work, we apply the Bayesian paradigm to restore multichannel astronomical images. We compare classical methods which are formulated as Bayesian deconvolution techniques and propose a new method based on a multichannel prior distribution that preserves the flux within each channel. The methods are compared on real multichannel astronomical images.

A survey of southern X-ray binary stars from the van Paradijs catalogue has been made using the Australia Telescope Compact Array. Detection sensitivities of under 1 mJy were achieved for each of the 91 fields of declinations < −30° observed at 6 and 3 cm wavelength. Eight objects were detected within the inner imaged field of 128² arc-sec² at flux densities of ≥ 1 mJy. Preliminary work on improving the sensitivity of the survey by mapping the whole primary field of the ATCA antennas is reported.

Improvements in the technique of high precision differential astrometry in VLBI based on the use of triangles of radio sources are reported. Such geometry provides new constraints by taking advantage of “closure”. Results on the relative separations (ranging from 4.6 to 6.8°) of the sources in the triangle 1803+784/1928+738/2007+777 from ionosphere-free delays are shown and “sky closure” verified. A comparison with previous results on the pair 1928+738/2007+777 is also presented.

We present very deep VLA radio maps of the field around the Cyg X-1 radio counterpart. These images were obtained in an attempt to search for radio jets emanating from this black hole candidate.