Rhonda M. Morgan

Nulling interferometric beam combiner utilizing dielectric plates: experimental results in the visible broadband

This paper describes a laboratory experiment of achromatic nulling in the visible to SIM requirements. The experiment employs phase shifting interferometry techniques on a spectrally dispersed fringe to measure the phase as a function of wavelength. This phase is then used by a control system that adjusts the tilt of the plates and the air path difference until a satisfactory level of null is achieved. A fast servo adjusts the air path difference to stabilize the phase to the nanometer level. The paper includes a discussion of the design issues, the experimental measure of phase versus wavelength, a description of the control system, and a preliminary measure of the null.

Visible light Terrestrial Planet Finder: planet detection and spectroscopy by nulling interferometry with a single aperture telescope

Planet detection around a bright star is dependent on the resolution of the imaging system and the degree of light suppression of the star relative to the planet. We present a concept and a scaled precursor for a visible light Terrestrial Planet Finding (VTPF) mission. Its major feature is an imaging system for planet detection using a nulling interferometer behind a single aperture telescope. This configuration is capable of detecting earth-like planets with a 4m aperture using both imaging and spectroscopic imaging modes. We will describe the principles of the system, and show results of studies demonstrating its feasibility.

Final laboratory results of visible nulling with dielectric plates

Nulling stellar interferometry may enable the discovery of earth-like planets around other stars. In nulling mode, the zero order fringe is destructive and on axis, thus cancelling light from a bright source and allowing detection of dimer off-axis features. To create a deep on-axis null, the phase must be shifted half a wave achromatically over a broad band. The phase shift is created by adding optical path thickness with dielectric plates. Plates of different materials can balance dispersion. The nulling solutions found for TPF (infrared) and SIM (visible) are promising. This paper presents the final results of a dissertation that developed a nulling beam combiner testbed. The deepest null achieved over the spectral region of 600 to 800 nm was 7x10-3. The test bed revealed the extreme challenges of this technique and provided very valuable lessons to enable further implementations. The testbed first achromatized the null by actively controlling the optical thicknesses of the plates. The phase as a function of wavelength was measured by PSI on a spectrally dispersed fringe. The phase was fit to a model to determine the optical thicknesses. The eigenfunctions of the model were nearly collinear and consequently the dynamic range required of the phase data was very high and not supported by the hardware. The testbed then searched for the null fringe and locked on the null using a 300 Hz servo loop and on a grey fringe. The OPD was stabilized to 6 nm peak-to-valley.

Adaptive Cross-Correlation Algorithm and Experiment of Extended Scene Shack-Hartmann Wavefront Sensing

We have developed a new, adaptive cross-correlation (ACC) algorithm to estimate with high accuracy the shift as large as several pixels in two extended-scene images captured by a Shack-Hartmann wavefront sensor (SH-WFS). It determines the positions of all extended-scene image cells relative to a reference cell using an FFT-based iterative image-shifting algorithm. It works with both point-source spot images as well as extended scene images. We have also set up a testbed for extended-scene SH-WFS, and tested the ACC algorithm with the measured data of both point-source and extended-scene images. In this paper we describe our algorithm and present our experimental results.

Actuator fault detection via electrical impedance testing

Lead magnesium niobate (PMN) actuators are electrostrictive actuators with high dynamic range used in deformable mirrors. Actuator fault detection in deformable mirrors typically occurs through optical testing. We developed a nonoptical method for detecting actuator faults via low electric field resonance testing. The low electric field resonance method is standard practice for characterizing piezoelectric materials. The piezoelectric/electrostrictive coefficient couples the electrical and mechanical impedance of the actuator; a change in the mechanical boundaries (force) on the actuator results in a shift of the impedance resonances. We demonstrate experimentally that a PMN actuator can fracture but retain functionality under compression and that the fracture can be detected by measuring the impedance resonances at various bias voltages (various values of tension and compression). A concurrent optical test using a displacement interferometer was used to corroborate the results. We propose the impedance resonance approach as a non-optical fault detection test for in-situ actuators.

Testbed for extended-scene Shack-Hartmann and phase retrieval wavefront sensing

We have implemented a testbed to demonstrate wavefront sensing and control on an extended scene using Shack-Hartmann and MGS phase retrieval simultaneously. This dual approach allows for both high sensitivity and high dynamic range wavefront sensing. Aberrations are introduced by a silicon-membrane deformable mirror. The detailed characterization of this mirror and its sensitivity matrix are presented. The various Shack-Hartmann algorithms, including a maximum likelihood approach are discussed and compared to phase retrieval results using a point source. The next phase of the testbed will include results with extended scenes.

The StarLight space interferometer: optical design and performance modeling

The StarLight mission aimed to place the first formation flying optical interferometer into space in year 2006. Utilizing two spacecraft to form a long baseline Michelson interferometer, it would measure white light fringes on a number of partially resolved stars of magnitudes >5 in the wavelength range 600 to 1000 nm. The interferometer baseline is variable between 30 and 125 m, and also has a fixed 1.3 m mode. The spacecraft are flown in a parabolic geometry which requires an optical delay line to build up more than 14 m of delay on one arm of the interferometer. To obtain high fringe visibility, starlight wavefront, pointing and intensity must be preserved through 22 reflections from mirrors and beamsplitters. The alignment of a total of 27 optics is maintained through careful thermal design and the use of two actuated mirrors on each arm. This paper describes the optical layout, including the beam combiner design which allows star tracking, optical system alignment and fringe formation on a single CCD. The effects of diffraction of the starlight transferred from a distant spacecraft and from optical surface imperfections are modeled. Other contributors to the visibility budget and the resulting variation of fringe visibility across the focal plane are discussed.

Technology maturity for the habitable-zone exoplanet imaging observatory (HabEx) concept

HabEx Architecture A is a 4m unobscured telescope mission concept optimized for direct imaging and spectroscopy of potentially habitable exoplanets, and also enables a wide range of general astrophysics science. The exoplanet detection and characterization drives the enabling core technologies. A hybrid starlight suppression approach of a starshade and coronagraph diversifies technology maturation risk. In this paper we assess these exoplanet-driven technologies, including elements of coronagraphs, starshades, mirrors, jitter mitigation, wavefront control, and detectors. By utilizing high technology readiness solutions where feasible, and identifying required technology development that can begin early, HabEx will be well positioned for assessment by the community in 2020 Astrophysics Decadal Survey.

Two-laser heterodyne metrology for a separated spacecraft interferometer

The proposed New Millennium Interferometer consists of three spacecraft separated by up to several kilometers. A heterodyne laser metrology system is proposed to measure the relative distances between the spacecraft. Because diffraction losses for a round-trip measurement are prohibitively large, a two-laser metrology system has been suggested in which each spacecraft has both a laser and a receiver. The system has been successfully demonstrated with a one meter baseline and verified by a conventional single- laser system in a laboratory experiment. The precision was limited by thermal effects in the room environment for time scales greater than one minute. The single-laser system obtained a precision of 3 nm for integration times up to 0.5 seconds. The two-laser system obtained a precision of 20 and was limited by self-interference and electronics noise. The resolution of the two-laser metrology system was (lambda) 30.

HabEx yield modeling with for systems engineering (Conference Presentation)

We present yield modeling results for the HabEx concept study using EXOSIMS. EXOSIMS (Exoplanet Open-Source Imaging Mission Simulator) provides a parametric estimate of science yield of mission concepts using contrast curves from physics-based diffraction model codes and Monte Carlo simulations of design reference missions using realistic observing constraints. Two baseline architecture configurations and two extended configurations are compared. We compare a configuration with a coronagraph to a configuration with a starshade for both detection and spectral characterization. The input parameters, including astrophysical assumptions, are detailed. We show sensitivity to key design parameters around design space local to the point designs. The yield results provide an analysis of the relative performance of telescope and instrument design that enable system engineering decisions.