Removal of coronagraphy residues with an adaptive hologram,for imaging exo-Earths

Abstract

Exo-planets much fainter than their parent star are likely imageable atvisible wavelengths with a 2 to 8 meter space telescope equipped forcoronagraphy. The dynamic range can be further increased by nulling thespeckled star residue, before detection, with an adaptive hologram. Like theMach-Zehnder interferometer of Codona & Angel (2004), the hologramsubtracts from the stellar residue. a copy of it, made with the light rejectedby the Lyot mask. The final residue can in principle be as low as one photonper speckle, on average. It makes exo-Earths detectable if their Airy peakcontains a few photons. The method can also relax the difficult figuringtolerances for the pre-focal optics. With Lippmann-Bragg holograms, it canbe achromatized.