Michael Shao

Long- baseline optical interferometer for astrometry

A proposed astrometric interferometer capable of relative stellar position measurements with 10-4 arcsec accuracy is described. The instrument is a long-baseline Michelson interferometer modified to track the fringe motion caused by atmospheric turbulence. Simultaneous fringe amplitude and phase measurements at two wavelengths are used to correct atmospheric distortion when the field of view is much larger than the isoplanatic patch. Relative positions of stars brighter than ~ 10 mag in an ~ 1° field of view can be measured with an accuracy of ~10-4 arcsec after several hours of observation. Such an instrument should have a number of interesting astrophysical and geophysical applications, such as a search for planets around nearby stars, the gravitational deflection of light around the sun, and changes in the earth’s axis of rotation.

First fringe measurements with a phase-tracking stellar interferometer

A prototype two-telescope stellar interferometer with a 1.5-m base line has been used to track the white-light fringes, 0.4-0.9 microm, from Polaris. Continuous fringe phase and amplitude measurements were made with ~220-photon/4-msec integration time and 1.27-cm(2) collecting area under 2-arc sec seeing conditions. The same control algorithm should be able to track fringes from an 8.7-mg star using the light from two 13-cm (5-in.) telescopes and a 10-msec integration time under 1-arc sec seeing conditions. When tracking, the servo maintained equal path lengths to 0.1-microm rms in the two arms of the interferometer, thus cancelling the path-length variations caused by earth rotation and atmospheric turbulence. In the future, two-color phase measurements will make optical aperture synthesis and optical very long-base-line astrometry possible.

The New Millennium Formation Flying Optical Interferometer

Spaceborne opt i ca l inte~erometry has been identified as a critical technology for many of NASA’s 21* centu~ science visions. Included in this m“sion are interferometers that can probe the orighs of stars and can ultimately study Earthlike planets around nearby stars. To accomplish this feat, separation of an interferometer’s collecting apertures by large baselines are required from hundreds of meters up to thousands of kilometers. Thus the large separations require multiple spacecraft jlying in a formation. Furthermore, optical pathlengths over these distances must be controlled to the nanometer level. This level of control demands precision spacecraft controls, active optics, metrology, and starlight detection technologies, To date, some of these technologies have been demonstrated only in ground applications with baselines of order of a hundred meters; space operation will require a significant capability enhancement. This paper describes the New Millennium formation flying optical interferometer concept and associated technologz”es. The mission is designed to provide a technology demonstration for multiple spacecraft precision formation flying and very long baseline optical interferometry. The interferometer would be distributed over three spacecraft: two spacecraft would serve as collectors, directing ——— ——— ————-— ———---— ——— ——— ——--— —— —--— Copyright@ 1997 by the American Institute of Aeronautics and Astronautics, Inc. The U.S. Governmertt has a royalty-free license to exercise all rights under the copyright claimed herein for governmental purposes. All other rights are reserved by the copyright owner. starlight toward a third spacecraft which would combine the light and perform the in terferometric detection. The interferometer baselines would be variable, allowing baselines of 100 m to 1 km in an equilateral formation, providing angular resolutions from 1 to 0.1 milliarcsec.

Precision astrometry mission for exoplanet detection around binary stars

We propose an innovative low-cost mission capable of detecting potentially habitable planets around a sample of solartype stars near the sun. The finding of rocky planets in temperate orbits among our immediate stellar neighbors will be a signature discovery. Our mission will deliver relative measurements of stellar position and motion at sub-micro arcsecond precision. These data, in turn, will reveal the presence of orbiting exoplanets. For the case of our primary targets Alpha Centauri A and B, objects below one Earth mass will be accessible when the end-of-mission astrometric precision requirement of 0.4 micro arcsecond is achieved. TOLIMAN will directly reveal the presence of sub-earth mass planets and constrain it orbit and mass This paper describes the optical and mechanical architecture of the mission, and first order instrument design. We also explain the instrument stability requirements imposed by the diffractive pupil post-processing calibration limitations. Our design baseline is a stable two-mirror telescope that images the field directly on CCD camera minimizing the number of reflections and optical components.

The TOLIMAN space telescope

The TOLIMAN space telescope is a low-cost, agile mission concept dedicated to astrometric detection of exoplanets in the near-solar environment, and particularly targeting the Alpha Cen system. Although successful discovery technologies are now populating exoplanetary catalogs into the thousands, contemporary astronomy is still poorly equipped to answer the basic question of whether there are any rocky planets orbiting any particular star system. Toliman will make a first study of stars within 10 PC of the sun by deploying an innovative optical and signal encoding architecture that leverages the most promising technology to deliver data on this critical stellar sample: high precision astrometric monitoring. Here we present results from the Foundational Mission Study, jointly funded by the Breakthrough Prize Foundation and the University of Sydney which has translated innovative underlying design principles into error budgets and potential spacecraft systems designs.

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