Extra-solar Planet Imaging with a Space Telescope and a Nulling Interferometric Coronagraph

This paper describes a space mission for the direct detection and spectroscopy of Jupiter-like and Earthlike extrasolar planets in visible light using a modest aperture (1-4m) space telescope with a nulling interferometer based coronagraphic instrument. This concept is capable of satisfying the scientific objectives of the Terrestrial Planet Finder mission at a fraction of the complexity and at less cost than previous concepts. We discuss the key features of our mission design, and we present latest results of the technology developments needed for achieving a ten billion to one star light suppression ratio required. INTRODUCTION With a flux ratio in the optical of ~10-10 between a planet and its star, the hardest problem in imaging extra solar planets is that of contrast suppression, and achieving a very low background against which to detect a planet requires control of both scattered and diffracted light. The Hubble Space Telescope (D=2.4m) can detect a V = 30 object, so a 27 magnitude object takes much less than 1 hr of integration. In terms of resolution the orbit of a Jupiterlike planet at 10 parsec subtends an angle approximately 0.5 arc seconds, which requires a diffraction limited telescope of only 30cm or greater (at 0.75μm wavelength), and an earth-like planet at 0.1as can be resolved with a 1.5m diameter aperture. A nulling interferometer, however, can be used to suppress both diffraction and scattering, and an imaging instrument can be located behind a modest sized single aperture to resolve an extrasolar planet (Shao, 1990). In principle, a nulling interferometer effectively cancels the starlight and has 100% transmission for planet light when the optical path from the planet is λ/2 different from the star. For a modest sized aperture, about D=1m, a Jupiter-like planet could be resolved by synthesizing an interferometer with a 30 cm baseline, and at D=4m, an earth-like planet can be resolved with a 1.5m baseline. This paper describes a instrument for direct planet detection that we call the nulling coronagraph. The schematic system is shown in Figure 1. It synthesizes a four element nulling interferometer from the telescope pupil to suppress the diffraction from a central star. After nulling, an array of coherent single mode optical fibers is used to negate the effects of residual stellar leakage (scattering) due to imperfections in the telescope optics and optical train. A simple imaging system after this array forms the final extrasolar planet image, or a spectrometer can measure spectra for signs of life. This concept combines all the advantages of a nulling interferometer with the simplicity of a modest size, diffraction limited single aperture telescope. Advances in nulling technology enable this approach (Wallace, Shao, Levine and Lane, 2003). A further key element of the nulling approach is the use of single mode fiber spatial filter in conjunction with the nulling interferometer (Liu, Levine, and Shao, 2003) . The progress toward demonstration of these subsystems is all presented below. This combination makes very deep nulling possible without the requirement to achieve and maintain extreme (λ/4000) wavefront quality over a (large) full aperture of the space telescope. IMAGING PROPERTIES OF THE VISIBLE NULLING CORONAGRAPH A nulling interferometer interferes the light from two apertures, destructively. This is shown in the figure below as a two telescope interferometer, (Shao 2002). Light that is “on axis” is destructively interfered, but planet light “off axis” passes through the nuller and is detected. Behind the interferometer we can place a camera to image the field of view. The use of a camera for a visible nulling coronagraph is in contrast to an IR nulling interferometer where a single pixel detector is used. It’s important to understand what the nuller does to the image. The nuller effectively projects a transmission grating on the sky. The camera images the sky but the transmission of the camera/nuller depends on the angular position of the object. In this way the nulling coronagraph is similar to a Lyot Coronagraph where the transmission of the coronagraph is less when the light is blocked by the coronagraphic stop. Single Aperture telescope Pointing/Tracking control: Diffraction Control: Achromatic Nulling Scattered Light Control: Fiber-optic Spatial Filter Array Imaging System / Low Resolution Spectrometer Figure 1: Schematic of an imaging extrasolar planets with a shearing interferometer based instrument behind a single aperture telescope. Space 2003 23 25 September 2003, Long Beach, California AIAA 2003-6303 Copyright © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.