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.