Laurent Koechlin

Limit in field-resolution ratio for interferometric arrays

This paper presents a theoretical limit to the field/resolution ratio for the imaging mode of aperture synthesis interferometers. This limit is a function of both the number of apertures in the interferometric array and the dynamic range of the visibility and phase data. It does not depend on the optical setup of the instrument. This work is based on the theory of information.

Field-resolution ratio progresses for interferometric arrays

This paper presents progress made regarding the field to resolution ratio for aperture synthesis interferometers. In order to overcome a limit established for the field to resolution ratio of interferometric arrays, we propose an interferometer configuration which allows a better coverage of the spatial frequency plane. This setup requires large sub-apertures, which can be built more easily with a diffractive Fresnel plates than with large mirrors. We compare a dense array of 9 Fresnel sub-apertures, which gives a snapshot field-resolution ratio of 400, versus a sparse array of 150 small apertures, which yields a field-resolution ratio of 150.

GI2T/REGAIN control system and data reduction package

We present the general architecture of the GI2T/REGAIN control system. Based on a Graphical User Interface and different client-server communications, the system has to control both telescopes, the delay line, the beam-combiner, the data acquisition system and the real-time processing used as fringe tracker. We also describe in details the implementation of a real-time fringe tracker based on 4 monochromatic images and which used the fractional excess algorithm. Numerical simulations are shown. The control system is also dedicated to the acquisition of all the relevant data for the visibility calibration. We will also describe in details the data reduction package that provides the corrected visibilities. This architecture is very general and robust and has been developed having in mind that GI2T/REGAIN should be used by a wide community of astronomers.

Fringe tracking using a priori information on the optical path difference drift

Our goal is to improve fringe tracking in ground-based Michelson interferometry in order to reach fainter limiting magnitudes, and lower fringe visibility thresholds. The classical technique is the Fourier analysis of dispersed fringes (peak detection). It can be regarded as a maximum likelihood estimator. Although such an estimator is optimal by complying with Cramer-Rao bound rule, it does not use a priori information about the optical path difference (OPD) to be measured. We introduce a new signal analysis procedure based on the OPD drifts measured at the GI2T interferometer: if the signal is autocorrelated, then it would be possible to use a linear estimator giving a likelihood function from previous OPD values, reducing the noise in fringe Fourier analysis. Keywords: fringe tracking, photon counting, Fourier analysis, autoregressive modeling

DELTA photon-counting camera: concept and current status

We plan to build a photon counting camera able to yield photo-event coordinates (x,y, and t) at maximum rates superior to a million per second with a high temporal resolution (2.6 μs) and a 512 X 592 field. Ground-based interferometric techniques (single or multi-aperture) require detectors providing short frames, to deal with atmospheric turbulence. In order to reach the high signal-to-noise ratios required for imaging capabilities with aperture synthesis, such photon counting detectors are required.

Image recovery of moving objects from quantum limited data

We propose a method for faint object imaging in the case of displacementin the field of the detector during data acquisition. This method is adaptedto photon noise and unknown but linear displacements. We show that it canprovide some improvement also in the case of an arbitrarily non-linearmotion. The algorithms proposed involve a space-time three-dimensionalFourier transform and can take advantage of the sparse matrix nature ofphoton counting images.