B. Martin Levine

Adaptive optic correction using microelectromechanical deformable mirrors

A micromachined deformable mirror (?-DM) for optical wavefront correction is described. Design and manufacturing approaches for ?-DMs are detailed. The ?-DM employs a flexible silicon membrane supported by mechanical attachments to an array of electrostatic parallel plate actuators. Devices are fabricated through surface micromachining using polycrystalline silicon thin films. ?-DM membranes measuring 2 mmx2 mmx2 ?m, supported by 100 actuators are described. Figures of merit include stroke of 2 ?m, resolution of 10 nm, and frequency bandwidth dc to 7 kHz in air. The devices are compact, inexpensive to fabricate, exhibit no hysteresis, and use only a small fraction of the power required for conventional DMs. Performance of an adaptive optics system using a ?-DM is characterized in a closed-loop control experiment. Significant reduction in quasistatic wavefront phase error is achieved. Advantages and limitations of ?-DMs are described in relation to conventional adaptive optics systems and to emerging applications of adaptive optics such as high-resolution correction, small-aperture systems, and optical communication.

PICTURE: a sounding rocket experiment for direct imaging of an extrasolar planetary environment

The Planetary Imaging Concept Testbed Using a Rocket Experiment (PICTURE 36.225 UG) was designed to directly image the exozodiacal dust disk of ǫ Eridani (K2V, 3.22 pc) down to an inner radius of 1.5 AU. PICTURE carried four key enabling technologies on board a NASA sounding rocket at 4:25 MDT on October 8th, 2011: a 0.5 m light-weight primary mirror (4.5 kg), a visible nulling coronagraph (VNC) (600-750 nm), a 32x32 element MEMS deformable mirror and a milliarcsecond-class fine pointing system. Unfortunately, due to a telemetry failure, the PICTURE mission did not achieve scientific success. Nonetheless, this flight validated the flight-worthiness of the lightweight primary and the VNC. The fine pointing system, a key requirement for future planet-imaging missions, demonstrated 5.1 mas RMS in-flight pointing stability. We describe the experiment, its subsystems and flight results. We outline the challenges we faced in developing this complex payload and our technical approaches.

Path length control in a nulling coronagraph with a MEMS deformable mirror and a calibration interferometer.

We report progress on a nulling coronagraph intended for direct imaging of extrasolar planets. White light is suppressed in an interferometer, and phase errors are measured by a second interferometer. A 1020-pixel MEMS deformable mirror in the first interferometer adjusts the path length across the pupil. A feedback control system reduces deflections of the deformable mirror to order of 1 nm rms.

Post-Coronagraph Wavefront Sensor for Gemini Planet Imager

The Gemini Planet Imager (GPI)1 will employ an apodized-pupil coronagraph to make direct detections of faint companions of nearby stars to a contrast level of the 10-7 within a few λ/D of the parent star. Such high contrasts from the ground require exquisite wavefront sensing and control both for the AO system as well as for the coronagraph. Un-sensed non-common path phase and amplitude errors after the wavefront sensor dichroic but before the coronagraph lead to speckles which limit the contrast2. The calibration wavefront system for GPI will measure the complex wavefront at the system pupil before the apodizer and provide slow phase corrections to the AO system to mitigate errors that would cause a loss in contrast. Here we describe the low-order and high-order wavefront sensors that compose the calibration wavefront sensor, how they operate, and how their information is combined to form the wavefront estimate before the coronagraph. Detailed simulations that show the expected performance for this wavefront sensor will be described for typical observing scenarios. Finally, we will show preliminary lab results from our calibration testbed that demonstrate the operation of the key hardware.

DAVINCI, A Diluter Aperture Visible Nulling Coronagraphic Instrument

DAVINCI is a dilute aperture nulling coronagraph that has the potential of directly detecting an Earth in the habitable zone around ~100 nearby stars. The novel feature of this mission concept is to replace a filled aperture 5-6 meter telescope with 4 by 1.1 meter telescopes in a phased array, dramatically reducing the cost by potentially by a factor of 5-10.

Design and development of a 329-segment tip-tilt piston mirror array for space-based adaptive optics

We report on the development of a new MEMS deformable mirror (DM) system for the hyper-contrast visible nulling coronagraph architecture designed by the Jet Propulsion Laboratory for NASAs Terrestrial Planet Finding (TPF) mission. The new DM is based largely upon existing lightweight, low power MEMS DM technology at Boston University (BU), tailored to the rigorous optical and mechanical requirements of the nulling coronagraph. It consists of 329-hexagonal segments on a 600μm pitch, each with tip/tilt and piston degrees of freedom. The mirror segments have 1μm of stroke, a tip/tilt range of 600 arc-seconds, and maintain their figure to within 2nm RMS under actuation. The polished polycrystalline silicon mirror segments have a surface roughness of 5nm RMS and an average curvature of 270mm. Designing a mirror segment that maintains its figure during actuation was a very significant challenge faced during DM development. Two design concepts were pursued in parallel to address this challenge. The first design uses a thick, epitaxial grown polysilicon mirror layer to add rigidity to the mirror segment. The second design reduces mirror surface bending by decoupling actuator diaphragm motion from the mirror surface motion. This is done using flexure cuts around the mirror post in the actuator diaphragm. Both DM architectures and their polysilicon microfabrication process are presented. Recent optical and electromechanical characterization results will also be discussed, in addition to plans for further improvement of DM figure to satisfy nulling coronagraph optical requirements.

The visible nulling coronagraph: architecture definition and technology development status

We describe the advantages of a nulling coronagraph instrument behind a single aperture space telescope for detection and spectroscopy of Earth-like extrasolar planets in visible light. Our concept synthesizes a nulling interferometer by shearing the telescope pupil into multiple beams. They are recombined with a pseudo-achromatic pi-phase shift in one arm to produce a deep null on-axis, attenuating the starlight, while simultaneously transmitting the off-axis planet light. Our nulling configuration includes methods to mitigate stellar leakage, such as spatial filtering by a coherent array of single mode fibers, balancing amplitude and phase with a segmented deformable mirror, and post-starlight suppression wavefront sensing and control. With diffraction limited telescope optics and similar quality components in the optical train (λ/20), suppression of the starlight to 10-10 is readily achievable. We describe key features of the architecture and analysis, present the status of key experiments to demonstrate wide bandwidth null depth, and present the status of component technology development.

Optical Design of Dilute Aperture Visible Nulling Coronagraph Imaging (DAViNCI)

This paper presents the optical design of the Dilute Aperture Visible Nulling Coronagraph Imaging (DAViNCI). DAViNCIs dilute aperture approach to the TPF-C extra-solar earth-like detection mission reduces cost and technical risk compared to other filled aperture approaches. DAViNCI has been studied in an ASMCS (Astrophysics Strategic Mission Concept Study) and is included within the ASTRO2010 Decadal review [1]. The DAViNCI team is led by Michael Shao (PI) of JPL.

Athermalized embedding of actuators in cryogenic mirrors

Embedding solid-state ceramic actuators in a bending style deformable mirror presents unique athermalization challenges when operated at cryogenic temperatures. Approaches to athermally embed actuators in a substrate are presented in this study. Each approach is rated according to established design criteria: unmatched displacement, range, compliance ratio, bondline stress, design, and manufacturability. We show the results of our design that allows a large thermal range of operation for the actuators.

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