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1.
随着技术的发展,下一代引力波探测器的激光功率将得到进一步提高。大光斑半径的应用也将使探测器的热噪声进一步降低,因此,量子噪声将成为在全频段限制引力波探测器灵敏度的首要因素。作为目前最有保障的一种降低量子噪声的技术,频率相关压缩态很可能将被应用于下一代所有引力波探测器中[1]。频率相关压缩态可以通过将频率不相关压缩态与滤波腔相结合而产生。基于滤波腔具有的频率响应特性,这一技术的应用可以使低频波段的辐射压噪声有效降低,同时实现高频波段散粒噪声的降低,从而实现全探测频段灵敏度的提升。基于日本KAGRA引力波探测器的设计,我们预计将9dB压缩度的压缩态与周损失为8×10-5的300m滤波腔相结合,可以使探测器灵敏度在全探测频段提高1倍。此实验于2015年开始,目前滤波腔的安装调试已经基本完成,得到的结果基本与实验前的模拟相符合。与频率不相关压缩态光学实验台的安装过程也已经过半,通过温度和控制回路的调制,二次谐波腔的转化率已经超过50%。  相似文献   

2.
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3.
A new inversion technique for obtaining temperature, pressure, and number density profiles of a planetary atmosphere from an occultation light curve is described. This technique employs an improved boundary condition to begin the numerical inversion and permits the computation of errors in the profiles caused by photon noise in the light curve. We present our assumptions about the atmosphere, optics, and noise and develop the equations for temperature, pressure, and number density and their associated errors. By inverting in equal increments of altitude, Δh, rather than in equal increments of time, Δt, the inversion need not be halted at the first negative point on the light curve as required by previous methods. The importance of the boundary condition is stressed, and a new initial condition is given. Numerical results are presented for the special case of inversion of a noisy isothermal light curve. From these results, simple relations are developed which can be used to predict the noise quality of an occultation. It is found that fractional errors in temperature profiles are comparable to those of pressure and number density profiles. An example of the inversion method is shown. Finally, we discuss the validity of our assumptions. In an appendix we demonstrate that minimum fractional errors in scale height determined from the inversion are comparable to those from an isothermal fit to a noisy isothermal light curve.  相似文献   

4.
We consider the optimum depth of a cluster survey selected using the Sunyaev–Zel'dovich effect. By using simple models for the evolution of the cluster mass function and detailed modelling for a variety of observational techniques, we show that the optimum survey yield is achieved when the average size of the clusters selected is close to the size of the telescope beam. For a total power measurement, we compute the optimum noise threshold per beam as a function of the beam size and then discuss how our results can be used in more general situations. As a by-product we gain some insight into what is the most advantageous instrumental set-up. In the case of beam switching observations one is not severely limited if one manages to set the noise threshold close to the point which corresponds to the optimum yield. Considering a variety of alternative scenarios, we discuss how robust our conclusions are to modifications in the cluster model and cosmological parameters. The precise optimum is particularly sensitive to the amplitude of fluctuations and the profile of the gas in the cluster.  相似文献   

5.
The present generation of weak lensing surveys will be superseded by surveys run from space with much better sky coverage and high level of signal-to-noise ratio, such as the Supernova/Acceleration Probe ( SNAP ). However, removal of any systematics or noise will remain a major cause of concern for any weak lensing survey. One of the best ways of spotting any undetected source of systematic noise is to compare surveys that probe the same part of the sky. In this paper we study various measures that are useful in cross-correlating weak lensing surveys with diverse survey strategies. Using two different statistics – the shear components and the aperture mass – we construct a class of estimators which encode such cross-correlations. These techniques will also be useful in studies where the entire source population from a specific survey can be divided into various redshift bins to study cross-correlations among them. We perform a detailed study of the angular size dependence and redshift dependence of these observables and of their sensitivity to the background cosmology. We find that one-point and two-point statistics provide complementary tools which allow one to constrain cosmological parameters and to obtain a simple estimate of the noise of the survey.  相似文献   

6.
We have developed analysis tools to search for quasi periodic oscillations in light curves from active galactic nuclei, using the following time series techniques: Wavelets, periodogram, Lomb–Scargle periodogram, structure function and multi-harmonic analysis of variance. The analysis tools incorporate different noise models with significant levels for all the techniques that is an improvement over the previous work. By looking for consistently high significance, we make the detection of periodicities more robust. We apply this tool to a previously reported QPO (Espaillat et al. 2008) in the X-ray light curve of 3C 273 with a periodicity of ~3300 s and find that the significance is only 74% in the wavelet and fails to show up above 95% significance in the periodogram and multi-harmonic analysis of variance.  相似文献   

7.
Several techniques have been proposed for measuring speeds of meteoroids observed using radars. A recent technique involves the use of Fresnel transforms to accurately determine the speed of a meteoroid producing the trail. We follow a numerical modeling approach to analyze this technique in detail. Our studies indicate that high sensitivity to background noise levels might be a possible shortcoming of the Fresnel transform method. A matched filtering approach is presented as an alternative to alleviate this sensitivity to the noise problem. Performance of the two techniques is compared using numerical modeling and data from a 30 MHz radar.  相似文献   

8.
The availability of large format, low noise detector arrays has opened the 1 to 20 m region for sub-arcsecond imaging. Using recent results in the investigation of Galactic star forming regions, the Galactic center and external galaxies as examples, we discuss the various techniques that have been employed. These range from image selection and image sharpening, over speckle techniques and adaptive optics to lunar occultation. These examples demonstrate that sub-arcsecond imaging and spatial interferometry in the near- and mid-infrared has a great future potential.  相似文献   

9.
Photon counting strategies with low-light-level CCDs   总被引：1，自引：0，他引：1  
Low light level charge-coupled devices (L3CCDs) have recently been developed, incorporating on-chip gain. They may be operated to give an effective readout noise of much less than one electron by implementing an on-chip gain process allowing the detection of individual photons. However, the gain mechanism is stochastic and so introduces significant extra noise into the system. In this paper we examine how best to process the output signal from an L3CCD so as to minimize the contribution of stochastic noise, while still maintaining photometric accuracy.
We achieve this by optimizing a transfer function that translates the digitized output signal levels from the L3CCD into a value approximating the photon input as closely as possible by applying thresholding techniques. We identify several thresholding strategies and quantify their impact on the photon counting accuracy and the effective signal-to-noise ratio.
We find that it is possible to eliminate the noise introduced by the gain process at the lowest light levels. Reduced improvements are achieved as the light level increases up to about 20 photon pixel−1 and above this there is negligible improvement. Operating L3CCDs at very high speeds will keep the photon flux low, giving the best improvements in signal-to-noise ratio.  相似文献   

10.
In astronomical photometry, the sensitivity of observations is limited by the dark counts of the photomultiplier tube. In the present work, the effect of dark count noise in photon counting systems is investigated by theory and experimental measurements. Dark counts are considered to be originating from two sources, namely: dc fluctuations and random pulses.Experimental measurements were carried out to determine noise effects in different operating regions of noise dominance. The results provide strong evidence that: in normal operating mode, where the effect of random pulses is dominant, dark counts do not follow Poisson statistics. The observed noise shows strong (1/f) power spectrum, where the observed noise power is found to increase with time of observation.The results are important in photon counting systems operating under dark count limited mode. The conclusions drawn can be useful in obtaining more accurate error estimates and in assessing astronomical photometric observations and data reduction techniques.  相似文献   

11.
To determine the apparent diameter of the Sun, it is first necessary to measure the shape of the intensity profile of the solar limb with an imaging optical system (hereafter denoted as a solar-limb profile). The inflection point of the limb profile is usually used as a reference for calculating the diameter. Because this point may be difficult to determine in the presence of noise, it is necessary to define an appropriate filtering process that eliminates noise while preserving the position of the inflection point. In this paper we study two filtering techniques, one based on the compact wavelet transform and the other on the finite Fourier transform definition, that meet these requirements. The application of these two techniques to data gathered by the Solar Disk Sextant experiment shows that the solar radius increased from 1992 to 1996 by about 197 mas. However, a previous analysis of the same data and our present analysis provide a difference in the measured radii of about 92 mas. We show that this difference is entirely traced to the filtering process.  相似文献   

12.
Gravitational lensing is potentially able to observe mass-selected haloes, and to measure the projected cluster mass function. An optimal mass selection requires a quantitative understanding of the noise behaviour in mass maps. This paper is an analysis of the noise properties in mass maps reconstructed from a maximum-likelihood method.
The first part of this work is the derivation of the noise power spectrum and the mass error bars as a straightforward extension of the Kaiser & Squires algorithm for the case of a correlated noise. Very good agreement is found between these calculations and the noise properties measured in the mass reconstructions limited to non-critical clusters of galaxies. It demonstrates that Kaiser & Squires and maximum-likelihood methods have similar noise properties and that the weak lensing approximation is valid for describing these properties .
In a second stage I show that the statistics of peaks in the noise follows accurately the peak statistics of a two-dimensional Gaussian random field (using the BBKS techniques) if the smoothing aperture contains enough galaxies. This analysis provides a full procedure for deriving the significance of any convergence peak as a function of its amplitude and profile.
I demonstrate that a detailed quantitative analysis of the structures in mass maps can be carried out, and that, to a very good approximation, a mass map is the sum of the lensing signal and known two-dimensional Gaussian random noise. A straightforward application is the measurement of the projected mass function in wide-field lensing surveys, down to small mass overdensities that are individually undetectable.  相似文献   

13.
The “TNOs are Cool!: A survey of the trans-Neptunian region” is a Herschel Open Time Key Program that aims to characterize planetary bodies at the outskirts of the Solar System using PACS and SPIRE data, mostly taken as scan-maps. In this paper we summarize our PACS data reduction scheme that uses a modified version of the standard pipeline for basic data reduction, optimized for faint, moving targets. Due to the low flux density of our targets the observations are confusion noise limited or at least often affected by bright nearby background sources at 100 and 160 \(\mu \) m. To overcome these problems we developed techniques to characterize and eliminate the background at the positions of our targets and a background matching technique to compensate for pointing errors. We derive a variety of maps as science data products that are used depending on the source flux and background levels and the scientific purpose. Our techniques are also applicable to a wealth of other Herschel solar system photometric observations, e.g. comets and near-Earth asteroids. The principles of our observing strategies and reduction techniques for moving targets will also be applicable for similar surveys of future infrared space projects.  相似文献   

14.
We use full available array of radial velocity data, including recently published HARPS and Keck observatory sets, to characterize the orbital configuration of the planetary system orbiting GJ876. First, we propose and describe in detail a fast method to fit perturbed orbital configuration, based on the integration of the sensitivity equations inferred by the equations of the original N-body problem. Further, we find that it is unsatisfactory to treat the available radial velocity data for GJ876 in the traditional white noise model, because the actual noise appears autocorrelated (and demonstrates non-white frequency spectrum). The time scale of this correlation is about a few days, and the contribution of the correlated noise is about 2 m/s (i.e., similar to the level of internal errors in the Keck data). We propose a variation of the maximum-likelihood algorithm to estimate the orbital configuration of the system, taking into account the red noise effects. We show, in particular, that the non-zero orbital eccentricity of the innermost planet d, obtained in previous studies, is likely a result of misinterpreted red noise in the data. In addition to offsets in some orbital parameters, the red noise also makes the fit uncertainties systematically underestimated (while they are treated in the traditional white noise model). Also, we show that the orbital eccentricity of the outermost planet is actually ill-determined, although bounded by ~0.2. Finally, we investigate possible orbital non-coplanarity of the system, and limit the mutual inclination between the planets b and c orbits by 5°?C15°, depending on the angular position of the mutual orbital nodes.  相似文献   

15.
We use a model of polarized Galactic emission developed by the Planck collaboration to assess the impact of foregrounds on B -mode detection at low multipoles. Our main interest is in applications of noisy polarization data and in particular in assessing the feasibility of B -mode detection by Planck . This limits the complexity of foreground subtraction techniques that can be applied to the data. We analyse internal linear combination techniques and show that the offset caused by the dominant E -mode polarization pattern leads to a fundamental limit of   r ∼ 0.1  for the tensor–scalar ratio even in the absence of instrumental noise. We devise a simple, robust, template fitting technique using multifrequency polarization maps. We show that template fitting using Planck data alone offers a feasible way of recovering primordial B -modes from dominant foreground contamination, even in the presence of noise on the data and templates. We implement and test a pixel-based scheme for computing the likelihood function of cosmological parameters at low multipoles that incorporates foreground subtraction of noisy data.  相似文献   

16.
Spectral analysis of the residual pulsearrival times of pulsars is a useful tool in understanding the nature of the underlying processes that may be responsible for the timing noise observed from pulsars. Power spectra of pulsar timing residuals may be described by one or a combination of powerlaws. As these spectra are expected to be very steep, it is important to ensure a high dynamic range in the estimation of the spectrum. This is difficult in practice since one is, in general, dealing with timing measurements made at unevenly placed epochs. In this paper, we present a technique based on, ‘CLEAN’ to obtain high dynamic range spectra from unevenly sampled data. We compare the performance of this technique with other techniques including some that were used earlier for estimation of power spectra of pulsar timing residuals.  相似文献   

17.
Given a set of images, whose pixel values can be considered as the components of a vector, it is interesting to estimate the modulus of such a vector in some localized areas corresponding to a compact signal. For instance, the detection/estimation of a polarized signal in compact sources immersed in a background is relevant in some fields like astrophysics. We develop two different techniques, one based on the Neyman–Pearson lemma, the Neyman–Pearson filter (NPF), and another based on pre-filtering before fusion, the filtered fusion (FF), to deal with the problem of detection of the source and estimation of the polarization given two or three images corresponding to the different components of polarization (two for linear polarization, three including circular polarization). For the case of linear polarization, we have performed numerical simulations on two-dimensional patches to test these filters following two different approaches (a blind and a non-blind detection), considering extragalactic point sources immersed in cosmic microwave background (CMB) and non-stationary noise with the conditions of the 70 GHz Planck channel. The FF outperforms the NPF, especially for low fluxes. We can detect with the FF extragalactic sources in a high noise zone with fluxes      Jy for (blind/non-blind) detection and in a low noise zone with fluxes      Jy for (blind/non-blind) detection with low errors in the estimated flux and position.  相似文献   

18.
Scientific CCDs designed in thick high resistivity silicon (Si) are excellent detectors for astronomy, high energy and nuclear physics, and instrumentation. Many applications can benefit from CCDs ultra low noise readout systems. The present work shows how sub electron noise CCD images can be achieved using digital signal processing techniques. These techniques allow 0.4 electrons of noise at readout bandwidths of up to 10?Kpixels per second while keeping the full CCD spatial resolution and signal dynamic range.  相似文献   

19.
介绍一种天线口面温度定标的方法．当用这个方法对目标源进行温度定标时,由于定标源讯号和目标源讯号均由天线口面同路输入,因此波导和微波器件的传输损耗在定标的过程中被自动消除,因而这种温度定标方法能大大地提高观测资料的精度．目前无线口面温度定标方法除了在射电天文和微波天线测量中应用外,还广泛应用于雷达和无线电技术测量及微波遥感控制等．  相似文献   

20.
Due to its sensitivity and speed, the detector still widely used in Cerenkov astrophysics experiments remains the PhotoMultiplier Tube (PMT). For instance, recent pathbreaking experiments in Very High Energy astrophysics (VHE), such as MAGIC and HESS, have used mainstream PMT technology [Aharonian, F. et al Astron. Astrophys. 492(1):L25–L28 (2008)]. Moreover the Cerenkov Telescope Array (CTA) which is now in its design phase, is also planed to be based on PMT’s. However, there are some disadvantages to the PMT technology: the rather poor quantum efficiency, the use of high voltages, the high cost when used in large number in a matrix arrangement and the large weight. Hence, we have investigated the possibility to design future Cerenkov telescopes based on solid state technology, specifically Geiger avalanche photodiodes. In a preliminary development test, we placed HAMAMATSU avalanche photodiodes at the focal plane of a 60 cm diameter telescope at the Pic du Midi in the French Pyrénées, in order to record incident cosmic rays. In this paper, we describe not only the experimental setup but we also put special emphasis to the reduction of the semi-conductor noise. We also show first data that were recorded during two runs in the fall of 2006, and conclude by the presentation of the design of an “integrated, low-cost solid state photodiode arrangement” which might be an alternative to PMT’s for future VHE telescopes.  相似文献   

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