Stop-less Lyot coronagraph for exoplanet characterization

The characterization of directly-imaged exoplanets at very small angular separations requires high-contrast spec-troscopic capabilities. For this purpose, the new generation of instruments dedicated to direct imaging of massive exoplanets at large orbital radii, such as VLT/SPHERE and Gemini/GPI, includes integral field spectroscopy (IFS) and/or long slit spectroscopy (LSS) coupled with coronography. LSS is particularly challenging since observations will be stronly limited by quasi-static speckles and diffraction residuals that need to be removed with a posteriori data analysis methods. It is therefore necessary to limit as much as possible the influence of diffraction in the data. In this work we compare the use of the classical Lyot coronagraph (CLC) and the stop- less Lyot coronagraph (SLLC) with LSS for the characterization of exoplanets. SLLC uses a grey apodization to suppress the diffraction above 4.53 λ/D and does not require the use of any Lyot stop, offering a convenient implementation. We show that this apodized long slit spectroscopy (ALSS) improves notably the performance at small angular separations (0.3"-0.4"), allowing the spectral analysis of colder planets.