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The simulations and tentative observations lead to the following remarks:

1. One would like to sample as densely as possible to reach the bottom of the dark speckles, but there should be enough photons per pixel. The optimal sampling is therefore critical and we guess that it should be about 500 pixels per speckle area.

2. A fast photon-counting camera with a low dark noise, high saturation level and many pixels is needed.

3. The observations required a narrow-band filter since the diffraction and speckle pattern are color-dependant. The speckles are themselves dispersed radially. To increase the bandwidth usable in speckle interferometry, Wynne designed a chromatic lens with magnification inversely proportional to wavelength ([Wynne 1979]). D. Kohler built a Wynne corrector and we found it efficiently applicable to the present situation, where the speckle's wavelength dependance is more nearly a linear scaling. The resulting smearing of the planet's peak is acceptable if the spectral band remains less than 100 nm.

4. Different types of apodisation can be achieved, using a classical Lyot coronagraph, the interference coronagraph of Gay & Rabbia (1996), or the phase-mask coronagraph of Roddier & Roddier (1997). Both recent systems favor the detection of planets closer to the central star's Airy peak. Laboratory simulations with these varied devices are considered to compare their respective efficiencies.

Our simulations verify the theoretical expressions given for the signal to noise ratio. The SNR measured from the photon-number variance (Eq.(1)), is consistent with the SNR expected from the model (Eq.(10)). In these preliminary tests, we had to use an interference filter and low saturation level camera which provides a weak signal. We were consequently unable to reach enough sensitivity for detecting extrasolar-planets or even brown-dwarf companions.
The dark-speckle method is also applicable to space telescopes. Even without turbulence, optical defects create static speckles which can be made to fluctuate with a few actuators, arranged in the form of an active optics system, or a fast random scatterer. We proposed a "dark-speckle camera", the Faint Source Coronagraphic Camera for the Hubble Space Telescope ([Gezari et al 1997]). The project is reconsidered for the New Generation Space Telescope.
IR wavelengths are of interest for the detection of extrasolar planets, for two reasons: the planet's contrast is improved and, turbulence is easier to correct at these wavelengths. The forthcoming developement of bidimensional sensors with low read noise should allow red and IR work. We wish to thank D. Kohler and G. Knispel who made simulations possible, as well as D. Mourard and A. Blazit for the CP40 camera assistance. We are also grateful to the ONERA team for providing the adaptive optics system.

next up previous
Next: References Up: Preliminary results of dark-speckle Previous: Simulation and results