Optical Planet Discoverer: how to turn a 1.5-m class space telescope into a powerful exo-planetary systems imager

Optical Planet Discoverer (OPD) is a 1.5m class space telescope concept working as a visible nulling-interferometer imager. It is designed to detect Jupiter-like planets orbiting main sequence stars 10pc away in a few minutes of integration and carry out a low resolution (~20) spectroscopy of their atmosphere. OPD would fit in the budget envelope of a discovery class mission. It would serve as an efficient precursor to a Visible Terrestrial Planet Finder (VTPF), a scaled-up 4m class version based on the same optical scheme and allowing direct detection of 10pc Earthlike planets in a few hours. We detail here OPD's optical principle layout, which is primarily driven by an integrated stellar light attenuation of 1e-6 in the final focal plane. The optical concept is based on a double-shearing nulling interferometer followed by an array of single-mode waveguides. The waveguides array ensures high residual starlight suppression - as already demonstrated at the 1e-6 level by preliminary JPL visible LASER nulling experiments - together with diffraction limited imaging of the circumstellar environment over a 2 arcsec field. During the observations, the telescope is spun around the line of sight to allow for proper detection of fixed planetary signatures against residual off-axis speckle patterns at the 1e-9 level. Use of the single-mode waveguide array to filter out scattered starlight eliminates the requirements for pristine λ/4000 rms wavefronts anywhere in the optical train. With OPD, stringent phase requirements apply only to scales larger than 5 cm - the equivalent size of the pupil regions to be recombined and nulled in a given fiber, so that phase specifications can be met using low order active optics.