Defocus spot detection of astronomical optical system

Defocusing spot size detection is especially essential for aberration analysis and correction of optical systems. In the case of far defocusing, the celestial forms a pupil image on the detector, and the size of the image is linearly changed with the defocusing distance, and can be used to correct the optical system and analyze the image quality. Based on the focal plane attitude detection of Large Sky Area MultiObject Fiber Spectroscopy Telescope(LAMOST), this paper uses a variety of methods to detect the size of the defocusing spot of LAMOST telescope. For the particularity of the spot, the average value spacing algorithm, the peak value spacing algorithm, the ellipse fitting algorithm, and the multi-peak Gaussian fitting algorithm are used to detect the spot size. This paper will introduce these four methods, in which the average value spacing algorithm is proposed by the author of this paper. The advantages and disadvantages of the four methods are compared. The experimental results show that the average value spacing algorithm can achieve better accuracy of spot size detection in the four algorithms.     

Suppression of atmospheric radio emission fluctuations in radio astronomical observations

The fluctuations of radio emission of the atmosphere (for the present only cloudy) are a substantial limiting factor for observations of cosmic radio sources on centimetre and millimetre wavelengths. For suppressing these fluctuations the dual-beam method (differential receiving from two slightly separated direction) is widely used. At the same time works for perfecting this method are going on as well as in searching other possibilities with the principal aim of doing away limitations on sizes of sources under observation. There are considered here these methods with a competitive evaluation of their possibilities for suppressing fluctuation of the atmosphere radio emissions.     

The selection of filters for reduction of optical contamination in astronomical CCD X-ray images

The European Photon Imaging Camera(EPIC), is the X-ray imaging and medium spectroscopy instrument for theESA X-ray Multi Mirror telescope(XMM) mission. TheCCD detectors to be used in the three focal plane cameras will provide images in the energy band from 0.1 to 10 keV. However, spectral studies may be compromised by low energy, optical photon contamination. In order to reduce this effect, a number of filters will be incorporated onto a rotating mechanism in the camera head. The filters will be chosen to provide a significant reduction in the optical contamination from a source whilst minimising the attenuation of the X-ray flux. Four commercial filters are described here and their effects on calculated typical source fluxes evaluated. In addition, two alternative filter designs are described and their effects on a simulated source spectra are debated. In both cases, particular attention is given to the problem of maintaining high sensitivity at soft X-ray energies (less than 2 keV).     

A radio and optical study of Molonglo radio sources

We present multi-wavelength radio observations with the Very Large Array, and narrow- and broad-band optical observations with the 2.5-m telescope at the Las Campanas Observatory, of a well-defined sample of high-luminosity Fanaroff–Riley class II radio galaxies and quasars, selected from the Molonglo Reference Catalogue 1-Jy sample. These observations were carried out as part of a programme to investigate the effects of orientation and environment on some of the observed properties of these sources. We examine the dependence of the Liu–Pooley relationship, which shows that radio lobes with flatter radio spectra are less depolarized, on size, identification and redshift, and show that it is significantly stronger for smaller sources, with the strength of the relationship being similar for both radio galaxies and quasars. In addition to Doppler effects, there appear to be intrinsic differences between the lobes on opposite sides. We discuss the asymmetry in brightness and location of the hotspots, and present estimates of the ages and velocities from matched-resolution observations in the L and C bands. Narrow- and broad-band optical images of some of these sources were made to study their environments and correlate with the symmetry parameters. An extended emission-line region is seen in a quasar, and in four of the objects possible companion galaxies are seen close to the radio axis.     

Optical Frequency Comb as a general-purpose and wide-band calibration source for astronomical high resolution infrared spectrographs

A spectrally filtered Optical Frequency Comb (OFC) laser is proposed as a versatile calibration source for astronomical spectrometers in the 1?C2 ??m spectral range. Such a source overcomes the limitations of current calibration lamps providing a uniform spectrum of equally spaced lines with similar intensity and extremely high long-term frequency stability. We present preliminary studies and results of a system which filters the OFC from a 100 MHz comb spacing to 16 GHz one, an adequate spacing for spectrometers with resolving power ????/???>30000. The first approach employs two Fabry-Perot cavities in series, made of dielectric coated mirrors, followed by a non-linear optical broadening system. The limitations of such a filtering process are discussed. These can be overcome by the second approach, based on filtering cavities with metallic coated mirrors.     

Refractive optical systems for astronomical instruments using infrared arrays in the 0.8 to 2.5 micron bands

As modern infrared arrays evolve towards larger formats and smaller pixel sizes, their use in instruments requires optics of increasingly higher performance. Refractive systems are attractive for many applications because they can accommodate wide fields of view or fast focal ratios while permitting a compact, unobscured layout. We examine design considerations for lenses which can address the entire near-infrared spectral region of 0.8 to 2.5 m. Achromatization over this broad range is a particular challenge. We discuss a systematic study of candidate materials and combinations of materials having useful chromatic properties, with particular emphasis on alkaline earth halides and metallic oxides. We present an example of a practical design being incorporated into an instrument currently under construction.     

Background–source separation in astronomical images with Bayesian probability theory – I. The method

A probabilistic technique for the joint estimation of background and sources with the aim of detecting faint and extended celestial objects is described. Bayesian probability theory is applied to gain insight into the co-existence of background and sources through a probabilistic two-component mixture model, which provides consistent uncertainties of background and sources. A multiresolution analysis is used for revealing faint and extended objects in the frame of the Bayesian mixture model. All the revealed sources are parametrized automatically providing source position, net counts, morphological parameters and their errors.
We demonstrate the capability of our method by applying it to three simulated data sets characterized by different background and source intensities. The results of employing two different prior knowledge on the source signal distribution are shown. The probabilistic method allows for the detection of bright and faint sources independently of their morphology and the kind of background. The results from our analysis of the three simulated data sets are compared with other source detection methods. Additionally, the technique is applied to ROSAT All-Sky Survey data.     

A generalized measurement equation and van Cittert-Zernike theorem for wide-field radio astronomical interferometry

We derive a generalized van Cittert-Zernike (vC-Z) theorem for radio astronomy that is valid for partially polarized sources over an arbitrarily wide field of view (FoV). The classical vC-Z theorem is the theoretical foundation of radio astronomical interferometry, and its application is the basis of interferometric imaging. Existing generalized vC-Z theorems in radio astronomy assume, however, either paraxiality (narrow FoV) or scalar (unpolarized) sources. Our theorem uses neither of these assumptions, which are seldom fulfiled in practice in radio astronomy, and treats the full electromagnetic field. To handle wide, partially polarized fields, we extend the two-dimensional (2D) electric field (Jones vector) formalism of the standard 'Measurement Equation' (ME) of radio astronomical interferometry to the full three-dimensional (3D) formalism developed in optical coherence theory. The resulting vC-Z theorem enables full-sky imaging in a single telescope pointing, and imaging based not only on standard dual-polarized interferometers (that measure 2D electric fields) but also electric tripoles and electromagnetic vector-sensor interferometers. We show that the standard 2D ME is easily obtained from our formalism in the case of dual-polarized antenna element interferometers. We also exploit an extended 2D ME to determine that dual-polarized interferometers can have polarimetric aberrations at the edges of a wide FoV. Our vC-Z theorem is particularly relevant to proposed, and recently developed, wide FoV interferometers such as Low Frequency Array (LOFAR) and Square Kilometer Array (SKA), for which direction-dependent effects will be important.     

A Minimal Spanning Tree algorithm for source detection in γ-ray images

We developed a source detection algorithm based on the Minimal Spanning Tree (MST), that is a graph-theoretical method useful for finding clusters in a given set of points. This algorithm is applied to γ-ray bi-dimensional images where the points correspond to the arrival direction of photons, and the possible sources are associated with the regions where they clusterize. Some filters to select these clusters and to reduce the spurious detections are introduced. An empirical study of the statistical properties of MST on random fields is carried out in order to derive some criteria to estimate the best filter values. We also introduce two parameters useful to verify the goodness of candidate sources. To show how the MST algorithm works in practice, we present an application to an EGRET observation of the Virgo field, at high Galactic latitude and with a low and rather uniform background, in which several sources are detected.     

A novel method of lossless compression for 2-D astronomical spectra images

A novel method of lossless compression for astronomical spectra images is proposed in this paper. Firstly, Integer Wavelet Transform is adopted to perform decorrelation of the data. Afterwards, Embedded Zero-tree Wavelet encoder is employed to describe the zero-tree structure of wavelet coefficients, and then the resulting stream put through Embedded Zero-tree Wavelet encoder can be transformed to character string including only five characters that is easily compressed by entropy coding. Finally, Arithmetic encoder is chosen as the entropy coder here. Groups of simulation data based on LAMOST and observation data from SDSS are used in the experiment to demonstrate the new method, and the experimental results are much better than those of GZIP and JPEG2000.     

A sample of gigahertz-peaked spectrum radio source candidates

Published in Astrofizika, Vol. 38, No. 4, pp. 697–699, October–December, 1995.     

A test for models of radio source evolution

Some observed astrophysical phenomena such as the blast of a supernova (cf. Zeldovich and Novikov, 1966; Blandfordet al., 1977; Shapiro, 1979) suggest the necessity of study of the motion of shock waves in a relativistic fluid flow in the presence of a magnetic field. This paper deals with the motion of special relativistic shock wave which propagates from the center line outwardly after an explosion with the assumption that the magnetic field which has an axial component only. Similarity solutions which depend on the parameter =r/t are constructed. Two special cases are then studied in detail. In the first case, there is an ultra-relativistic fluid in front of the shock and in the second case, there is a cold fluid in front of the shock.     

Radio source stacking and the infrared/radio correlation at μJy flux densities

We investigate the infrared/radio correlation using the technique of source stacking, in order to probe the average properties of radio sources that are too faint to be detected individually. We compare the two methods used in the literature to stack sources and demonstrate that the creation of stacked images leads to a loss of information. We stack infrared sources in the Spitzer Extragalactic First Look Survey (xFLS) field, and the three northern Spitzer Wide-area Infrared Extragalactic survey (SWIRE) fields, using radio surveys created at 610 MHz and 1.4 GHz, and find a variation in the absolute strength of the correlation between the xFLS and SWIRE regions, but no evidence for significant evolution in the correlation over the 24-μm flux density range 150 μJy to 2 mJy. We carry out the first radio source stacking experiment using 70-μm-selected galaxies, and find no evidence for significant evolution over the 70-μm flux density range 10–100 mJy.     

Fast compression and reconstruction of astronomical images based on compressed sensing

With the fast increase in the resolution of astronomical images, the question of how to process and transfer such large images has become a key issue in astronomy. We propose a new real-time compression and fast reconstruction algorithm for astronomical images based on compressive sensing techniques. We first reconstruct the original signal with fewer measurements, according to its compressibility. Then,based on the characteristics of astronomical images, we apply Daubechies orthogonal wavelets to obtain a sparse representation. A matrix representing a random Fourier ensemble is used to obtain a sparse representation in a lower dimensional space. For reconstructing the image, we propose a novel minimum total variation with block adaptive sensing to balance the accuracy and computation time. Our experimental results show that the proposed algorithm can efficiently reconstruct colorful astronomical images with high resolution and improve the applicability of compressed sensing.     

The infrared and radio continuum of OB-stars

The infrared and radio continuum fluxes of a number of OB-stars are compared to theoretical models. This gives us information about the mass loss rates and the structure of the stellar wind. The more extreme O-star Pup appears to show an effect of clumping in its continuum flux.     

Blind operation of optical astronomical interferometers options and predicted performance

Maximum sensitivity for optical interferometers is achieved only when the optical pathlengths between the different arms can be equalized without using interference fringes on the research object itself. This is called blind operation of the interferometer. In this paper I examine different options to achieve this, focussing on the application to the Very Large Telescope Interferometer (VLTI). It is proposed that blind operation should be done using a so-called coherence autoguider, working on an unresolved star of magnitude V=11 to 13 within the isoplanatic patch for coherencing, which has a diameter of about 1 degree. Estimates of limiting magnitudes for the VLTI are also derived.     

Selection of radio astronomical observation sites and its dependence on human generated RFI

We investigate the influence of population density on radio-frequency interference(RFI) affecting radio astronomy.We use a new method to quantify the threshold of population density in order to determine the most suitable lower limit for site selection of a radio quiet zone(RQZ).We found that there is a certain trend in the population density-RFI graph that increases rapidly at lower values and slows down to almost flat at higher values.We use this trend to identify the thresholds for population density that produce RFI.Using this method we found that,for frequencies up to 2.8 GHz,low,medium and high population densities affecting radio astronomy are below 150 ppl km-2,between 150 ppl km-2and 5125 ppl km-2,and above5125 ppl km-2respectively.We also investigate the effect of population density on the environment of RFI in three astronomical windows,namely the deuterium,hydrogen and hydroxyl lines.We find that a polynomial fitting to the population density produces a similar trend,giving similar thresholds for the effect of population density.We then compare our interference values to the standard threshold levels used by the International Telecommunication Union within these astronomical windows.     

The radio and infrared spectrum of DR 21

The radio and infrared spectrum of DR 21 is established over a wide range of frequencies (from 10² to 10⁸ MHz). Two physical processes, free-free emission from the ionized hydrogen at radio wavelengths and reradiation at infrared wavelengths of the original stellar ultraviolet radiation by dust grains have to be considered in the explanation of the derived spectrum. Physical parameters of the object deduced from its radio emission are also presented.