The concept we recently introduced of a coronagraph using a four-quadrant phase mask has been the subject of detailed model calculations and of laboratory validation proving its great potential in planet finding. A nulling factor of 12500 is already demonstrated in the laboratory (Riaud et al., this conference). We first remind the principle of the 4QC : a destructive interference between the two equal fractions of the amplitude with opposite signs produces a very efficient nulling of the star light. We propose to install such a device on several ground-based and space instruments, including present (NAOS/CONICA) of future (Planet Finder) VLT instruments and MIRI, the mid-IR camera of the NGST. The present paper focus on the question of direct imaging of exoplanets using this type of device. Indeed, one advantage of the four-quadrant coronagraph is to permit probing the vicinity of a star down to smaller angular distances than a classical Lyot mask. We examine the sources of uncertainties in different cases of optimized ground-based and space experiments and different situations of planet/star couples, using as far as possible realistic models of planetary evolution. On the VLT, even with an extremely powerful adaptive optics system (Strehl ration of 90%), the speckle noise will be the main limitation : contrast in magnitude as large as Δm = 15 are however possible in the K band. The combination of a 4QC and differential imaging at two wavelength is likely the most promising concept for direct planet Detection from the ground. On the other hand, we show that with a 4QC on MIRI, a classical Jupiter is indeed detectable from space and at 20μm for a star belonging to the closest star forming region at 50 pc.
展开▼
