C. Aime

Signal-to-noise ratio in differential speckle interferometry

Expressions are given for the signal-to-noise ratio of the amplitude and for the uncertainty of the phase of the cross spectrum of two speckle images in the presence of photon shot noise. Applications to astronomical observations are discussed. A maximum-likelihood estimate is derived for the speckle displacement in a differential speckle-interferometric experiment, and its variance is computed. A relation is given between the size of the smallest detectable feature and the stellar magnitude, showing that this technique is quite promising.

Phase mask coronagraphy using a Mach-Zehnder interferometer

Aims. We report results obtained with a four-quadrant and an eight-octant phase-mask coronagraphs (4QC and 8OC) produced using amplitude masks inside a Mach-Zehnder interferometer (MZI). We describe the laboratory implementation of these coronagraphs operated in laser light and provide a detailed comparison between theory and experiment. Methods. The +1 and –1 (π phase) amplitude transmissions required to produce a 4QC or an 8OC were obtained using complementary binary-masks (transmission of 1 or 0 for quadrants of similar parities) in the two arms of a MZI, taking advantage of the achromatic π phase shift between the two paths. In one output of the MZI, the reconstructed image of the focal plane is similar to those obtained for sectorised phase-masks coronagraphs, and the image of the aperture corresponds to the characteristic patterns of the 4QC/8OC coronagraphs. Results. Observations are compared with the theory published by other authors and developed further here. The expressions for the light diffracted outside the aperture image are simplified using the Zernike radial polynomials Z 1 instead of the rapidly diverging hypergeometric functions otherwise used. Experimental results are found to be in very good agreement with the theory. With the present laboratory experiment, about 99% of the light is rejected outside the Lyot stop and the contrast obtained beyond 5λ/D is higher than 10 5 for both the 4QC and 8OC. As expected, the 8OC is less affected by a pointing error than the 4QC, but more sensitive to an aperture central obstruction. Incidently, the loss of transmission for a planet crossing two phase masks is given theoretically as a function of the Lyot stop size, in agreement with observations.

Multiple-stage apodized pupil Lyot coronagraph for high-contrast imaging

We describe the multiple-stage Lyot coronagraph first proposed by Aime, Soummer and Ferrari (A&A 2002, 389,334344) for the detection of exoplanets. This coronagraph uses several stages. The first stage is a Prolate Apodized Lyot Coronagraph (PALC). It produces a residual wavefront on the aperture that is proportional to the entrance prolate spheroidal apodized wavefront. This permits the use of a second stage of coronagraphy, needing only a Lyot mask, identical to the first one, without any further aperture apodization. The resulting extinction factor is the square of the initial PALC. Using several stages permits to obtain considerable rejection factors for small-size coronagraphic masks and good overall throughput.

Imaging with hypertelescopes: a simple modal approach

Aims. We give a simple analysis of imaging with hypertelescopes, a technique proposed by Labeyrie to produce snapshot images using arrays of telescopes. The approach is modal: we describe the transformations induced by the densification onto a sinusoidal decomposition of the focal image instead of the usual point spread function approach. Methods. We first express the image formed at the focus of a diluted array of apertures as the product

Theoretical performance of solar coronagraphs using sharp-edged or apodized circular external occulters

R_0(alpha) X_{rm F}(alpha)

Strategies for the deconvolution of hypertelescope images

of the diffraction pattern of the elementary aperturesxa0

Statistical approach to bias effects in the techniques of speckle interferometry and speckle masking

R_0(alpha)

New observations of alpha Orionis with a rotation shearing interferometer

by the object-dependent interference termxa0

ARC: an Achromatic Rotation-shearing Coronagraph

X_{rm F}(alpha)

Polychromatic transfer functions in stellar speckle interferometry

between all apertures. The interference term, which can be written in the form of a Fourier Series for an extremely diluted array, produces replications of the object, which makes observing the image difficult. We express the focal image after the densification using the approach of Tallon and Tallon-Bosc. Results. The result is very simple for an extremely diluted array. We show that the focal image in a periscopic densification of the array can be written as