Astronomical Systems Research Articles

Acceleration of adaptive optics simulations using programmable logic

Numerical Simulation is an essential part of the design and optimisation of astronomical adaptive optics systems. Simulations of adaptive optics are computationally expensive and the problem scales rapidly with telescope aperture size, as the required spatial order of the correcting system increases. Practical realistic simulations of AO systems for extremely large telescopes are beyond the capabilities of all but the largest of modern parallel supercomputers. Here we describe a more cost effective approach through the use of hardware acceleration using field programmable gate arrays. By transferring key parts of the simulation into programmable logic, large increases in computational bandwidth can be expected. We show that the calculation of wavefront sensor image centroids can be accelerated by a factor of four by transferring the algorithm into hardware. Implementing more demanding parts of the adaptive optics simulation in hardware will lead to much greater performance improvements, of up to 1000 times.

Measuring the centroid gain of a Shack–Hartmann quad-cell wavefront sensor by using slope discrepancy

Shack-Hartmann wavefront sensors (SH WFS) are used by many adaptive optics (AO) systems to measure the wavefront. In this WFS, the centroid of the spots is proportional to the wavefront slope. If the detectors consist of 2 x 2 quad cells, as is the case in most astronomical AO systems, then the centroid measurement is proportional to the centroid gain. This quantity varies with the strength of the atmospheric turbulence and the angular extent of the beacon. The benefits of knowing the centroid gain and current techniques to measure it are discussed. A new method is presented, which takes advantage of the fact that, in a SH-WFS-based AO system, there are usually more measurements than actuators. Centroids in the null space of the wavefront reconstructor, called slope discrepancy measurements, contain information about the centroid gain. Tests using the W. M. Keck Observatory AO system demonstrate the accuracy of the algorithm.

Translating Ancient Chinese Astronomical Systems with Excel: How Not to Stew the Strawberries?

Translating Ancient Chinese Astronomical Systems with Excel: How Not to Stew the Strawberries?

The Master Mobile Astronomical System. Optical Observations of Gamma-Ray Bursts

The operating principles of the telescope-robot system MASTER (Mobile Astronomical System of Telescopes-Robots, http://observ.pereplet.ru), designed to search for fast transient phenomena in the optical range, are described. The robot-telescope includes the following: a Richter-Slefogt telescope with D=355 mm, F/D=2.4, a Richter-Slefogt telescope with D=200 mm, F/D=2.4, a Flugge telescope with D=280 mm, F/D=2.5, a TV camera with a field of 20x40 degrees, and three CCD cameras. A German mount with a slew rate of 8 deg/s is used. MASTER obtains images down to 19m in a field of 6 square degrees in a 1.5 minute exposure. We present some observations of the optical afterglow of cosmic gamma-ray bursts. MASTER was the first system in Europe to record optical emission from GRB030329.

한국우주전파관측망(KVN)의 영상모의실험

한국우주전파관측망(Korean VLBI Network: KVN)은 우리나라 최초의 초장기선 전파간섭계로 새로운 천문우주 연구 분야 및 측지, 지구물리 등의 연구를 위하여 3기의 21m 최첨단 밀리미터파 전파 안테나를 이용하여 우주를 바라보는 새로운 영역을 개척할 예정이다. 전파간섭계의 성능은 안테나 배열 및 관측 대상 천체의 적위, 그리고 천체와 크기 및 형태에 따른 의존성이 매우 크다. 본 연구는 현재 설정되어있는 KVN관측소 및 시스템 자료를 근거로 단일 점광원, 다중 점광원, 원형광원 2개, Cygnus A의 5개 모델 영상에 대하여 가상관측을 수행하였다. 적위에 따른 KVN의 UV 궤적 분포로부터 가장 이상적이라고 여겨지는 적위 60도인 천체에 대하여 22GHz의 관측 파장으로 12시간 동안 가상관측을 수행하여 얻어진 UV데이터를 가지고 간섭계 표준 영상처리를 하였다. 그 결과 22GHz에서 예상되는 KVN의 빔 크기에 의해 분해가 되지 않는 모델들의 RMS 대비 최대밝기 강도(Jy/Beam)가 대략 점광원의 징우 l0000:1, 지름 6mas(milli arcsecond) 원형 광원의 경우 5000:1 정도로 매우 높은 반면, 빔 크기보다 큰 모델에서는 115:1 및 34:1정도로 현저하게 감소하였으며 영상의 복원 정도도 이와 같은 결과를 보였다. 이것은 KVN이 상대적으로 적은 기선의 개수로 인하여 UV평면을 충분히 채우지 못하며, 동시에 짧은 기선이 부족하여 넓은 분포를 가지는 천체들에 대한 영상화능력이 떨어지기 때문이다. 그러나 각 모델과 CLEAN 영상과의 픽셀 좌표비교에서는 12mas 원형광원을 제외하고는 정확하게 일치함을 보였다. 그러므로 KVN의 주요한 관측 대상은 콤팩트한 천체들이 적당할 것이며, 이러한 천체들에 대하여 KVN은 위치측정에 우수한 성능을 보였다. The Korean VLBI Network (KVN) will open a new field of research in astronomy, geodesy and earth science using the newest three Elm radio telescopes. This will expand our ability to look at the Universe in the millimeter regime. Imaging capability of radio interferometry is highly dependent upon the antenna configuration, source size, declination and the shape of target. In this paper, imaging simulations are carried out with the KVN system configuration. Five test images were used which were a point source, multi-point sources, a uniform sphere with two different sizes compared to the synthesis beam of the KVN and a Very Large Array (VLA) image of Cygnus A. The declination for the full time simulation was set as +60 degrees and the observation time range was -6 to +6 hours around transit. Simulations have been done at 22GHz, one of the KVN observation frequency. All these simulations and data reductions have been run with the Astronomical Image Processing System (AIPS) software package. As the KVN array has a resolution of about 6 mas (milli arcsecond) at 220Hz, in case of model source being approximately the beam size or smaller, the ratio of peak intensity over RMS shows about 10000:1 and 5000:1. The other case in which model source is larger than the beam size, this ratio shows very low range of about 115:1 and 34:1. This is due to the lack of short baselines and the small number of antenna. We compare the coordinates of the model images with those of the cleaned images. The result shows mostly perfect correspondence except in the case of the 12mas uniform sphere. Therefore, the main astronomical targets for the KVN will be the compact sources and the KVN will have an excellent performance in the astrometry for these sources.

Levi ben Gershom's Criticism of Ptolemy's Astronomy

Levi ben Gershom or Gersonides (1288-1344), one of the most original thinkers of the Middle Ages, devoted the first section of the fifth book of his Milhamot ha-Shem to an exposition of his astronomical system, which was based on his own observations and departed in many respects from prevailing views. Heir to a long tradition of criticism of Ptolemy, which he associated notably with al-Bitrūjī Book on the Configuration [of the World], but which reached him also through Maimonides' Guide and Averroes' Epitome of the Almagest, Gersonides tried to reconcile the physical and the mathematical descriptions of the heavenly motions and provide a true picture of the configuration of the world. The long chapters 43 and 44 of his Astronomy are devoted, respectively to the demonstration that neither Ptolemy's nor al-Bitrūjī's astronomical theories can be true. Critical editions of the Hebrew and Latin versions of chapter 43 are presented here, along with an annotated English translation.

Dark Matter: Astronomical Aspects

AbstractThe chief evidence for appreciable dark matter in the universe comes from the monotonic increase in mass to light ratios measured for various astronomical systems as one looks on larger and larger length scales. Though the evidence comes from photons, most of the dark matter is non-photonic, and, for that matter, non-baryonic. There remain several questions about the nature and behavior of dark matter to which conventional astronomical observations are (probably) relevant. The most germain to this JD is whether astronomers have seen decay or annihilation products from dark matter particles, to which the current answer seems to be no. We look at a few of the others.

An X-ray view of radio sources

We review recent examples where the synergy between radio and X-ray observations has led to substantial progress in understanding astronomical systems. The sub-arcsecond imaging capabilities of the Chandra X-ray observatory provides a 100-fold improvement for com- paring X-ray and radio structures. We specifically discuss examples which provide insight into the outflow of material and energy from pul- sars and supernovae, the centers of clusters of galaxies, and the nuclei of quasars.

A Robust Algorithm for the Pointing Refinement and Registration of Astronomical Images

ABSTRACTWe present a generic algorithm for performing astronomical image registration and pointing refinement. The method is based on the matching of positions and fluxes of available point sources in image overlap regions. This information is used to compute a set of image offset corrections by globally minimizing a weighted sum of all matched point‐source positional differences in a prespecified reference image frame. A fast linear sparse matrix solver is used for the minimization. From these corrections, the pointings and orientations of images can be refined in either a relative sense in which pointings become fixed (registered) relative to a single input image, or in an absolute sense (in the International Celestial Reference System [ICRS]) if absolute point‐source information is known. The latter provides absolute pointing refinement to an accuracy that depends on the robustness of point‐source extractions, match statistics, and accuracy of the astrometric catalog used. The software is currently used in the Spitzer image‐processing pipelines, although it is adaptable to any astronomical imaging system that uses the FITS image format and world coordinate system (WCS) pointing standard. We test the algorithm using Monte Carlo simulations and compare them to image data acquired with the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope. We find that dispersions in matched source separations after refinement are entirely consistent with centroiding errors in source extractions, implying that systematic uncertainties due to inaccurately calibrated distortions are negligible. For these data, we predict refinements to better than ∼70 and ∼280 mas (2 σ radial) for the IRAC 3.6 and 8 μm bands, respectively. These bands bracket two extremes in available source matches, and for the data under study, correspond to an average of about 55 and 8 matches per image in these two bands, respectively.

Wave-front reconstruction from shear phase maps by use of the discrete Fourier transform

Fast wave-front reconstruction methods are becoming increasingly important, for example, in large astronomical adaptive optics systems and high spatial resolution shear interferometry, where pseudoinverse matrix methods scale poorly with problem size. Wave-front reconstruction from difference measurements can be achieved by use of fast implementations of the discrete Fourier transform (DFT), obtaining performance comparable with that of the pseudoinverse in terms of the noise propagation coefficient. Existing methods that are based on the use of the DFT give exact results (in the absence of noise) only for the particular case in which the shear is a divisor of the number of samples to be reconstructed. We present two alternate solutions for the more general case when the shear is any integer. In the first solution the dimensions of the problem are enlarged, and in the second the problem is subdivided into a set of smaller problems with shear amplitude equal to one. We also show that the retrieved solutions have minimum norm and calculate the noise propagation coefficient for both methods. The proposed algorithms are implemented and timed against pseudoinverse multiplication. The results show a speed increase by a factor of 50 over the pseudoinverse multiplication for a grid with N = 3 x 10(3) samples.

Missing Mass as Evidence for Modified Newtonian Dynamics at Low Accelerations

Milgrom has proposed that the appearance of discrepancies between the Newtonian dynamical mass and the directly observable mass in astronomical systems could be due to a breakdown of Newtonian dynamics in the limit of low accelerations rather than the presence of unseen matter. Milgrom's hypothesis, modified Newtonian dynamics or MOND, has been remarkably successful in explaining systematic properties of spiral and elliptical galaxies and predicting in detail the observed rotation curves of spiral galaxies with only one additional parameter — a critical acceleration which is on the order of the cosmologically interesting value of cH0. Here we review the empirical successes of this idea and discuss its possible extension to cosmology and structure formation.

Effect of scintillations in curvature sensing

The effect of scintillations on the curvature sensing which is used in astronomic adaptive systems is investigated. It is shown that the magnitude of the effect depends on two atmospheric turbulence parameters: on the Fried parameter which characterizes the seeing conditions, and on the turbulence inner scale which is responsible for small-sized turbulence inhomogeneities. Depending on the turbulence conditions, the magnitude of the effect ranges from 3% to 20%, while for the typical ones it is of the order of 10%-15%.

Biotechnology in agriculture and food systems: an example of science and society at a crossroad

Biotechnology applications in agriculture and food systems have placed science and society at a crossroad in North America, Europe and large parts of Asia. Revolutionary advancesin the biological sciences, such as cloning and other genetic manipulations of plants, animals and microorganisms, open up avenues of discovery of unprecedented potential. Such revolutionary moments, however, have historically attracted resistance and opposition. Most recently, use of genetically modified organisms (GMO's) in food has spawned debate among scientists and attentive members of the public. This is not the first time that rapid advances in science and the results of scientific experimentation have generated controversy. One of the earliest crossroads between science and society occurred in the 16th and 17th centuries between scientists and the prevailing belief system. Well documented are the arguments by the established church againstthe works of Galileo Galilei (1564-1642), who postulated an astronomical system that was, not geocentric, but heliocentric. In those early times, the crossroad was clearly set between the proponents of scientific theory and established societal beliefs. So, with a long historical lens, the present day controversy over GMO's and biotechnology may be seen as an extension of a time worn historical process of scientific controversy. With a shorter historical lens, it may be seen as the aftermath of previous societal concerns over the use of nuclear power, over the irradiation of foods, and as the consequence of a growing mistrust of scientists, government officials, and corporate agriculture in the Western world. The concerns expressed are wide-ranging andobjections are often strongly held. Objections to the applications of biotechnology range from1)gene pollution of important world food crops and livestock, 2)concern over ownership ofgenes, 3)allergic reactions to engineered proteins, 4)industrialization of the food system,5)corporate control in agriculture, to 6)the demise of local food systems. This paper will address the crossroad issues now prevalent in North America and relate these to biotechnology opportunities and concerns that may arise in the Arabian Gulf.

Horizons and new possibilities for astronomical system's mega spectroscopy

Results from helioseismology and periodicity spectrum of the Sun and stars can only be adequately understood and effectively interpreted in the context of unified theoretical representations. The Wave Universe concept allows to understand the Solar and any astronomical systems as in terms of the principle Wave dynamic System (WDS). Investigations using the theoretically calculated Fundamental Wave spectrum (periods) of those objects, its Megaspectroscopy (including helioseismology and asteroseismology for the Sun and stars) lead to the reconstruction of the dynamical and physical properties, and their internal and external structure.

Methods for correcting tilt anisoplanatism in laser-guide-star-based multiconjugate adaptive optics.

Multiconjugate adaptive optics (MCAO) is a technique for correcting turbulence-induced phase distortions in three dimensions instead of two, thereby greatly expanding the corrected field of view of an adaptive optics system. This is accomplished with use of multiple deformable mirrors conjugate to distinct ranges in the atmosphere, with actuator commands computed from wave-front sensor (WFS) measurements from multiple guide stars. Laser guide stars (LGSs) must be used (at least for the forseeable future) to achieve a useful degree of sky coverage in an astronomical MCAO system. Much as a single LGS cannot be used to measure overall wave-front tilt, a constellation of multiple LGSs at a common range cannot detect tilt anisoplanatism. This error alone will significantly degrade the performance of a MCAO system based on a single tilt-only natural guide star (NGS) and multiple tilt-removed LGSs at a common altitude. We present a heuristic, low-order model for the principal source of tilt anisoplanatism that suggests four possible approaches to eliminating this defect in LGS MCAO: (i) tip/tilt measurements from multiple NGS, (ii) a solution to the LGS tilt uncertainty problem, (iii) additional higher-order WFS measurements from a single NGS, or (iv) higher-order WFS measurements from both sodium and Rayleigh LGSs at different ranges. Sample numerical results for one particular MCAO system configuration indicate that approach (ii), if feasible, would provide the highest degree of tilt anisoplanatism compensation. Approaches (i) and (iv) also provide very useful levels of performance and do not require unrealistically low levels of WFS measurement noise. For a representative set of parameters for an 8-m telescope, the additional laser power required for approach (iv) is on the order of 2 W per Rayleigh LGS.

Astronomical Data Analysis Software and Systems X11Conference was held in Boston, Massachusetts, in 2000 November. Proceedings will be edited by F. R. Harnden, F. A. Primini, and H. E. Payne and published in the ASP Conference Series.

Astronomical Data Analysis Software and Systems X11Conference was held in Boston, Massachusetts, in 2000 November. Proceedings will be edited by F. R. Harnden, F. A. Primini, and H. E. Payne and published in the ASP Conference Series.

Pyramid Wavefront Sensor behavior in partial correction Adaptive Optic systems

In this letter we evaluate the limiting magnitude for a Pyramid Sensor operating in a closed loop astronomical Adaptive Optics system. A first heuristic analysis has shown that, when a point-like reference source is used a pyramid sensor exhibits a significant gain in terms of limiting magnitude over the widely used Shack-Hartmann sensor. This when diffraction limited conditions are reached. However, in current astronomical Adaptive Optics, diffraction limited regime at the sensing wavelength is difficult to achieve. Our simulations quantify the pyramid sensor limiting magnitude considering an Adaptive Optics system working in a partial correction regime. The simulations show that the considered gain is retained even in partial correction. An average gain of two magnitude is found in the considered case. This feature of a pyramid sensor can be very important in reducing the fundamental limit of today's Astronomical Adaptive Optics systems using natural reference sources, i.e. the limited sky-coverage.

Sodium Laser Guide Star Experiment with a Sum‐Frequency Laser for Adaptive Optics

ABSTRACTA sodium laser guide star (LGS) experiment for astronomical adaptive optics (AO) using a diode‐laser pumped sum‐frequency laser is described. Results of the LGS experiment from 1997 October to 1998 February are presented and discussed. An LGS as bright as 9.5 mag and as small as a natural star has been formed with about 1.1 W of laser power incident at the mesosphere. The experimental data indicate that the optical pumping effects have enhanced the LGS brightness by about 40%–50%. The laser has been shown to be efficient and robust and can already be used as a facility instrument on the astronomical AO system. The predicted wave‐front sensor measurement errors for typical AO systems have been calculated from the observed LGS flux and spot profile. Wave‐front errors of 83 nm with the current Chicago Adaptive Optics System (ChAOS) at the winter (maximum) sodium abundance level and 96 nm with the improved ChAOS and laser system at the summer (minimum) sodium abundance level are predicted.

Influence of scintillations on the performance of adaptive astronomical systems with Hartmann-like wavefront sensors

The influence of scintillations on image centroid measurements and on the phase reconstruction from Hartmann-like wavefront sensors is investigated quantitatively by means of computer simulations. It is shown that under the conditions of astronomical observations, the magnitude of the effect is between 10% (for excellent seeing) and 18% (for poor seeing). However, because the magnitude of the effect increases with the increasing of the turbulence strength, one can expect that under the strong-turbulence conditions the influence of scintillations on the image centroid can be quite strong. So, starting from agiven turbulence strength, it can be impossible to make a successful phase reconstruction from image centroid measurements. It is also shown that the scintillations affect in a different and complicated way the reconstruction quality of different aberrations. Nevertheless one can notice some general tendencies: The scintillations affect more strongly the reconstruction quality of the tip-tilt and high-order aberrations than the reconstruction quality of intermediate aberrations.

Magnetohydrodynamic production of relativistic jets.

A number of astronomical systems have been discovered that generate collimated flows of plasma with velocities close to the speed of light. In all cases, the central object is probably a neutron star or black hole and is either accreting material from other stars or is in the initial violent stages of formation. Supercomputer simulations of the production of relativistic jets have been based on a magnetohydrodynamic model, in which differential rotation in the system creates a magnetic coil that simultaneously expels and pinches some of the infalling material. The model may explain the basic features of observed jets, including their speed and amount of collimation, and some of the details in the behavior and statistics of different jet-producing sources.