R. Foy

LASER GUIDE STAR FOR 3.6- AND 8-M TELESCOPES : PERFORMANCE AND ASTROPHYSICAL IMPLICATIONS

We have constructed an analytical model to simulate the behavior of an adaptive optics system coupled with a sodium laser guide star. The code is applied to a 3.6-m and 8m class telescopes. The results are given in terms of Strehl ratio and full width at half maximum of the point spread function. Two atmospheric models are used, one representing good atmospheric conditions (20 per cent of the time), the other median conditions. Sky coverage is computed for natural guide star and laser guide star systems, with two different methods. The first one is a statistical approach, using stellar densities, to compute the probability to find a nearby reference. The second is a cross-correlation of a science object catalogue and the USNO catalogue. Results are given in terms of percentage of the sky that can be accessed with given performances, and in terms of number of science object that can be observed, with Strehls greater than 0.2 and 0.1 in K and J bands.

Imaging with AMBER/VLTI: The Case of Microjets

The engine that powers pre-main-sequence micro-jets is still unknown and remains a fundamental open question in star formation. The engine can be located solely on the inner disk or in the interaction of the inner disk with the star. In order to ease interpretation problems, imaging the jet engine is the ideal probe to disentangle between the old models and shed evidence for new ones. In this paper, we analyse the feasability of imaging bright southern targets, and show that even at low SNR, accurate image reconstructionis still possible with high contrast. However, the small number of ATs requires a fast reconfigurable array.

Phase retrieval from speckle images

In ground-based astronomy, the inverse problem of phase retrieval from speckle images is a means to calibrate static aberrations for correction by active optics. It can also be used to sense turbulent wavefronts. However, the number of local minima drastically increases with the turbulence strength, mainly because of phase wrapping ambiguities. Multifocal phase diversity has been considered to overcome some ambiguities of the phase retrieval problem. We propose an effective algorithm for phase retrieval from a single focused image. Our algorithm makes use of a global optimization strategy and an automatically tuned smoothness prior to overcome local minima and phase degeneracies. We push the limit of D/r(0)=4 achieved by Irwan and Lane [J. Opt. Soc. Am. A.15, 2302 (1998)] up to D/r(0)=11, which is a major improvement owing to the drastic increase in the problem complexity. We estimate the performances of our approach from consistent simulations for different turbulence strengths and noise levels (down to 1500 photons per image). We also investigate the benefit of temporal correlation.

New generation photon-counting cameras: algol and CPNG

Algol and Comptage de Photons Nouvelle Génération (CPNG) are new generation photon counting cameras developed for high angular resolution in the visible by means of optical aperture synthesis and speckle interferometry and for photon noise limited fast imaging of biological targets. They are intensified CCDs. They have been built to benefit from improvements in photonic commercial components, sensitivity, and personal computer workstations processing power. We present how we achieve optimal performances (sensitivity and spatiotemporal resolution) by the combination of proper optical and electronics design, and real-time elaborated data processing. The number of pixels is 532 x 516 and 1024(2) read at a frame rate of 262 and 100 Hz for CPNG and Algol, respectively. The dark current is very low: 5.5 x 10(-4) e(-) .pixel(-1). s(-1). The saturation flux is approximately 7 photon events /pixel/s. Quantum efficiencies reach up to 36% and 26% in the visible with the GaAsP photocathodes and in the red with the GaAs ones, respectively, thanks to the sensitivity of the photocathodes and to the photon centroiding algorithm; they are likely the highest values reported for intensified CCDs.

Atlas Experiment To Test The Laser Probe Technique For Wavefront Measurements

We discuss that one needs to create an artificial reference source to operate adaptive optics devices at visible wavelengths, since the probability is too low to find a star bright enough in the isoplanatic patch surrounding a given direction in the sky. That artificial source could be the light backscattered from a laser beam emitted through the telescope aperture, e.g. by Sodium atoms in the mesosphere. We describe the ATLAS experiment which is being built to check the validity of the concept of the laser probe technique; we use the 300mJ doubled Nd:YAG laser of the CERGA Lunar Laser Ranging Station and specially developped Shack-Hartmann sensors. The goal is to compare the wavefronts restored from almost simultaneous observations of a real bright star and of a laser spot. The finite distance of the laser source compared to that of the remote star may induce systematic errors in the phase corrections determined from the laser spot and applied to the stellar wavefront. The magnitude of that cone effect depends on the telescope diameter, the isoplanatic patch and the wavelength. We propose to overcome it by measuring the 3D map of the ray deflexions in the turbulent layers in the atmosphere. From SCIDAR measurements, the refractive index structure function shows few well peaked features, so that the layers are assumed discrete. The correction of the cone effect requires to observe a small number of laser spots, typically 4 with an 8m telescope. It is a definite advantage to spread the spots outside the projection of the pupil onto the sky, so that the wavefront can be restored for directions largely beyond the isoplanatic patch. This method to correct the cone effect and to widen the isoplanatic domain could be tested with ATLAS and with real multiple stars.

Speckle interferometry. Data acquisition and control for the SPID instrument

SPID (SPeckle Imaging by Deconvolution) is a new speckle camera currently under construction at CRAL-Observatoire de Lyon (FRANCE). Its high spectral resolution and high image restoration capabilities open new astrophysical programs. The instrument SPID is composed of four main optical modules which are fully automated and computer controlled by a software written in Tcl/Tk/Tix and C. This software provides an intelligent assistance to the user by choosing observational parameters as a function of atmospheric parameters, computed in real time, and the desired restored image quality. Data acquisition is made by a photon-counting detector (CP40). A VME-based computer under OS9 controls the detector and stocks the data. The intelligent system runs under Linux on a PC. A slave PC under DOS commands the motors. These 3 computers communicate through an Ethernet network. SPID can be considered as a precursor for VLTls (Very Large Telescope, four 8-meter telescopes currently built in Chile by European Southern Observatory) very high spatial resolution camera.

Performances of natural and laser guide star adaptive optics for 8-m class telescopes

We have studied the performances of natural and laser guide star adaptive optics systems for the ESO Very Large Telescope. Analytical formulae are used to derive the Strehl and FWHM of the PSF obtained with NGS and LGS, for two atmospheric models (20 % of the time and median). Sky coverage was computed with stellar densities, leading to a 75 % sky coverage at a Strehl of 0.3 in K band at average galactic coordinates with LGS and 8 % with NGS. A cross correlation approach was also used: reference stars were searched in the USNO-A catalogue around science objects found in the Veron-Cetty 96 and SIMBAD catalogues. This approach shows that the number of quasars with strehls greater than 0.2 is increased by a factor of 20 with LGS. In J band LGS is much less efficient, but in good atmospheric conditions, it allows to observe 720 quasars with Strehl greater than 0.2. In median conditions, this number drops to 0, mainly because of the cone effect. This leads to the the need for on-line atmospheric turbulence profile measurement and flexible scheduling of the observation, to optimize the LGS performances in J band.

The Extended Hα Component of T Tau

We report the observation of T Tau by visible speckle interferometry at the CFHT. The observations were carried out at two wavelengths simultaneously. Each image was split into two, allowing improved data processing. The 4-channel photon-counting detector CP40 was used. We have discovered that 30% of the total energy at Ha is emitted by an extended elliptical component, with a FWHM of 70 maresec. In the neighbouring continuum, T Tau is not resolved by the 3.60 m CFHT. We briefly discuss the astrophysical consequences of this new result.