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Less than 3% of the known exoplanets were directly imaged for two mainreasons. They are angularly very close to their parent star, which is severalmagnitudes brighter. Direct imaging of exoplanets thus requires a dedicatedinstrumentation with large telescopes and accurate wavefront control devicesfor high-angular resolution and coronagraphs for attenuating the stellar light.Coronagraphs are usually chromatic and they cannot perform high-contrastimaging over a wide spectral bandwidth. That chromaticity will be critical forfuture instruments. Enlarging the coronagraph spectral range is a challenge forfuture exoplanet imaging instruments on both space-based and ground-basedtelescopes. We propose the multi-stage four-quadrant phase mask that associatesseveral monochromatic four-quadrant phase mask coronagraphs in series.Monochromatic device performance has already been demonstrated and themanufacturing procedures are well-under control since their development forprevious instruments on VLT and JWST. The multi-stage implementation simplicityis thus appealing. We present the instrument principle and we describe thelaboratory performance for large spectral bandwidths and for both pupil shapesfor space- (off-axis telescope) and ground-based (E-ELT) telescopes. Themulti-stage four-quadrant phase mask reduces the stellar flux over a widespectral range (30%) and it is a very good candidate to be associated with aspectrometer for future exoplanet imaging instruments in ground- andspace-based observatories.

Multi-stage four-quadrant phase mask: achromatic coronagraph for space-based and ground-based telescopes