Carole Thiebaut

Time-scale and time-frequency analyses of irregularly sampled astronomical time series

We evaluate the quality of spectral restoration in the case of irregular sampled signals in astronomy. We study in details a time-scale method leading to a global wavelet spectrum comparable to the Fourier period, and a time-frequency matching pursuit allowing us to identify the frequencies and to control the error propagation. In both cases, the signals are first resampled with a linear interpolation. Both results are compared with those obtained using Lombs periodogram and using the weighted wavelet-transform developed in astronomy for unevenly sampled variable stars observations. These approaches are applied to simulations and to light variations of four variable stars. This leads to the conclusion that the matching pursuit is more efficient for recovering the spectral contents of a pulsating star, even with a preliminary resampling. In particular, the results are almost independent of the quality of the initial irregular sampling.

Satellite attitude instability effects on stereo images

This paper describes an original method to identify microvibrations of satellite platforms using only a pair of stereo images. Image pairs contain a twofold information: the landscape details and the platforrn microvibration corruptions (roll, pitch and yaw). We propose identification methods in the case of a push-broom acquisition mode with a 6000 pixel CCD linear array (in the case of the French satellite SPOT). This method relies on the image acquisition principle, on the nature of the observed landscape and finally on some concepts of surface geometry. The algorithms providing such identification have been implemented and applied to simulated data, based on a true digital model terrain of Marseille (France). The results of our vibration identification method are presented here. Comparisons with traditional methods are also presented in this paper.

Spectral contents of astronomical unequally spaced time-series: contribution of time-frequency and time-scale analyses

We analyze how time-scale and time-frequency methods can be suited to study unequally spaced astronomical time-series, in terms of recovering precise spectral contents of a pulsating star. In many situations in astronomy, nonuniform sampling proves to be the natural way in which the signal is available for investigation and the spectral information of interest is lost. We propose to study two kinds of methods which enables us to treat this type of signal: first, an approach with global wavelet spectra leading to a wavelet scale that can be compared to the Fourier period. Second, a time-frequency analysis (matching pursuit) providing a progressive constructive process allowing us to identify the fine structure of the driven frequencies and to control the error propagation. We apply, compare and discuss these methods for simulations and for luminosity observations of variable stars. This allows us to conclude that the matching pursuit is the more efficient method, in particular independent of the quality of sampling.

ARAGO: a robotic observatrory for the variable sky

We present the Advanced Robotic Agile Observatory (ARAGO), a project for a large variability survey of the sky, in the range 10-8Hz (year) to 1Hz. Among its scientific objectives are the detection of cosmic gamma-ray bursts, both on alert and serendipitously, orphan afterglows, extrasolar planets, AGNs, quasar microlensing, variable and flare stars, trans-neptunian asteroids, Earth-grazers, orbital debris, etc. A large Education and Public Outreach program will be an important part of the project. The telescope itself will be made of Silicon Carbide, allowing, among other advantages, a very light weight and agile capabilities. ARAGO will be fully autonomous, i.e. there will be no human intervention from the request to the data processing and result dissemination, nor to assist night or day operations. ARAGO will start routine observation by mid-2005.