Stellar Coronagraphy: Study and Test of a Hybrid Interfero-Coronagraph

We present a device for high dynamic range imaging of faint structures in the close angular vicinity of stars. An advantage of this device is the capacity to explore the field of view both uniformly and as close (or near) to the parent star as allowed by the diffraction limit of the telescope. The design is adapted from the Achromatic Interfero-Coronagraph (AIC), which suppresses the light scattered by diffraction from an on-axis unresolved source, but yields two twin images of the stellar environment, displayed symmetrically in the focal plane (Gay & Rabbia 1996; Baudoz et al. 2000a). Our Hybrid Interfero-Coronagraph (HIC) avoids the 180° ambiguity and is therefore well suited for the study of extended objects distributed all around the star. Like the AIC, the on-axis extinction with HIC is based on destructive interference after amplitude division of the incident field from a single telescope. An achromatic π-dephasing and a spatial filtering of the wave front are performed on one arm of the interferometer. The interferometric process occurs between the wave front from the star and a spatially filtered wave front. Because of this spatial filtering, the destructive process no longer remains achromatic. However, the residual chromaticity is compatible with astrophysical applications on ground-based telescopes. Numerical simulations show that HIC performance does not suffer from telescope central obscuration. An appropriate size of the spatial filtering mask and a suitable apodization allow the nulling of an on-axis unresolved star at the level of 10–6, with a spectral resolution R = λ0}/Δλ = 15 and in the absence of wave-front phase or intensity errors. Preliminary laboratory tests with a HIC prototype are reported, and as a first step, an extinction of the source by a factor of 200 was obtained, with a resolution R = 6 in the visible, using a simple setup.