Stuart B. Shaklan

Coupling starlight into single-mode fiber optics.

We have calculated the efficiency with which starlight can be coupled into a single-mode fiber optic that is placed in the focal plane of a telescope. The calculations are performed for a wide range of seeing conditions, with and without rapid image stabilization, and for a wide range of wavelengths. The dependence of coupling efficiency on the f-ratio of the incident beam is explored. Also, we calculate the coupling efficiency as a function of displacement for a perfect Airy pattern. We have also used a computer program which simulates atmospheric wavefronts to determine the variance of instantaneous coupling efficiency as a function of seeing. In perfect conditions, the maximum efficiency at the LP(11) mode cutoff is 78% due to the mismatch of the Airy pattern and the nearly Gaussian mode of the fiber. Maximum total coupled power is attained at d/r(0) = 4 with rapid image stabilization.

Broadband wavefront correction algorithm for high-contrast imaging systems

Great strides have been made in recent years toward the goal of high-contrast imaging with a sensitivity adequate to detect earth-like planets around nearby stars. It appears that the hardware − optics, coronagraph masks, deformable mirrors, illumination systems, thermal control systems − are up to the task of obtaining the required 10-10 contrast. But in broadband light (e.g., 10% bandpass) the wavefront control algorithms have been a limiting factor. In this paper we describe a general correction methodology that works in broadband light with one or multiple deformable mirrors by conjugating the electric field in a predefined region in the image where terrestrial planets would be found. We describe the linearized approach and demonstrate its effectiveness through laboratory experiments. This paper presents results from the Jet Propulsion Laboratory High Contrast Imaging Testbed (HCIT) for both narrow-band light (2%) and broadband light (10%) correction.

Closed Loop, DM Diversity-based, Wavefront Correction Algorithm for High Contrast Imaging Systems

High contrast imaging from space relies on coronagraphs to limit diffraction and a wavefront control systems to compensate for imperfections in both the telescope optics and the coronagraph. The extreme contrast required (up to 10(-10) for terrestrial planets) puts severe requirements on the wavefront control system, as the achievable contrast is limited by the quality of the wavefront. This paper presents a general closed loop correction algorithm for high contrast imaging coronagraphs by minimizing the energy in a predefined region in the image where terrestrial planets could be found. The estimation part of the algorithm reconstructs the complex field in the image plane using phase diversity caused by the deformable mirror. This method has been shown to achieve faster and better correction than classical speckle nulling.

Reflectivity and optical surface height requirements in a broadband coronagraph. 1.Contrast floor due to controllable spatial frequencies

We derive the broadband contrast floor in a coronagraphic telescope having nonideal optical surfaces. We consider only fundamental spatial frequencies within the control bandwidth of the coronagraphs deformable mirror. Cross terms arising from the beating of spatial frequencies beyond the deformable mirror control bandwidth will be considered in a second paper. Two wavefront control systems are analyzed:a zero-path difference Michelson interferometer with two deformable mirrors at a pupil image, and a sequential pair of deformable mirrors with one placed at a pupil image. We derive requirements on optical surface figure and reflectivity uniformity for both cases.

The Near-Earth Asteroid Tracking (NEAT) Program: An Automated System for Telescope Control, Wide-Field Imaging, and Object Detection

The Near-Earth Asteroid Tracking (NEAT) system operates autonomously at the Maui Space Surveillance Site on the summit of the extinct Haleakala Volcano Crater, Hawaii. The program began in 1995 December and continues with an observing run every month. Its astrometric observations result in discoveries of near-Earth objects (NEOs), both asteroids (NEAs) and comets, and other unusual minor planets. Each six-night run NEAT covers about 10% of the accessible sky, detects thousands of asteroids, and detects two to five NEAs. NEAT has also contributed more than 1500 preliminary designations of minor planets and 26,000 detections of main-belt asteroids. This paper presents a description of the NEAT system and discusses its capabilities, including sky coverage, limiting magnitude, and detection efficiency. NEAT is an effective discoverer of NEAs larger than 1 km and is a major contributor to NASAs goal of identifying all NEAs of this size. An expansion of NEAT into a network of three similar systems would be capable of discovering 90% of the 1 km and larger NEAs within the next 10–40 yr, while serving the additional role of satellite detection and tracking for the US Air Force. Daily updates of NEAT results during operational periods can be found at JPLs Web site (http://huey.jpl.nasa.gov/~spravdo/neat.html). The images and information about the detected objects, including times of observation, positions, and magnitudes are made available via NASAs SkyMorph program.

MASSES OF ASTROMETRICALLY DISCOVERED AND IMAGED BINARIES: G78-28AB AND GJ 231.1BC

The Stellar Planet Survey (STEPS) is an ongoing astrometric search for giant planets and brown dwarfs around a sample of ~30 M dwarfs. We have discovered several low-mass companions by measuring the motion of our target stars relative to their reference frames. The highest mass discovery thus far is G78-28B, a companion to the M dwarf G78-28A. The orbital period is 4.18 ± 0.03 yr, the system mass is 0.565 ± 0.055 M_☉, and the semimajor axis is 2.19 ± 0.10 AU. Imaging observations with the Keck laser guide star adaptive optics (LGSAO) and the Palomar AO instruments resolved the system and also yielded JHK-band delta magnitudes. We use the orbital solution, light ratios, and mass-luminosity relationships to derive component masses of M_A = 0.370 ± 0.034 M_☉ and M_B = 0.195 ± 0.021 M_☉. G78-28B is of type M4 V based on its colors and mass. We also discovered GJ 231.1C, a companion to GJ 231.1B, with STEPS and imaged the companion with LGSAO and Palomar AO, but the orbital period is longer than our observing baseline; thus, the system parameters are less constrained. In GJ 231.1BC the masses are M_B = 0.25 ± 0.06 M_☉ and M_C = 0.12 ± 0.02 M_☉. The inferred spectral type of GJ 231.1C is M5 V. We demonstrate the results of the current state of mass estimation techniques with our data.

Single-mode fiber optics in a long-baseline interferometer

We have investigated the potential for using single-mode fiber optics to link two or more telescopes in a large optical to near-IR astronomical interferometer. On an optical bench, we observed the effects of dispersion, temperature, and birefringence on wide-bandwidth interference fringes using up to 30 m of single-mode fiber in each arm of a Twyman-Green interferometer.

Low-Order Aberration Sensitivity of Eighth-Order Coronagraph Masks

In a recent paper, Kuchner, Crepp, and Ge describe new image-plane coronagraph mask designs that reject to eighth order the leakage of starlight caused by image motion at the mask, resulting in a substantial relaxation of image centroiding requirements compared to previous fourth-order and second-order masks. They also suggest that the new masks are effective at rejecting leakage caused by low-order aberrations (e.g., focus, coma, and astigmatism). In this paper, we derive the sensitivity of eighth-order masks to aberrations of any order and provide simulations of coronagraph behavior in the presence of optical aberrations. We find that the masks leak light as the fourth power of focus, astigmatism, coma, and trefoil. This has tremendous performance advantages for the Terrestrial Planet Finder Coronagraph.

ACCESS – A Concept Study for the Direct Imaging and Spectroscopy of Exoplanetary Systems

ACCESS is one of four medium-class mission concepts selected for study in 2008-9 by NASAs Astrophysics Strategic Mission Concepts Study program. ACCESS evaluates a space observatory designed for extreme high-contrast imaging and spectroscopy of exoplanetary systems. An actively-corrected coronagraph is used to suppress the glare of diffracted and scattered starlight to contrast levels required for exoplanet imaging. The ACCESS study considered the relative merits and readiness of four major coronagraph types, and modeled their performance with a NASA medium-class space telescope. The ACCESS study asks: What is the most capable medium-class coronagraphic mission that is possible with telescope, instrument, and spacecraft technologies available today? Using demonstrated high-TRL technologies, the ACCESS science program surveys the nearest 120+ AFGK stars for exoplanet systems, and surveys the majority of those for exozodiacal dust to the level of 1 zodi at 3 AU. Coronagraph technology developments in the coming year are expected to further enhance the science reach of the ACCESS mission concept.

Astrometric discovery of GJ 802b : In the brown dwarf oasis?

The Stellar Planet Survey is an ongoing astrometric search for giant planets and brown dwarfs around a sample of ~30 M dwarfs. We have discovered several low-mass companions by measuring the motion of our target stars relative to their reference frames. The lowest mass discovery thus far is GJ 802b, a companion to the M5 dwarf GJ 802A. The orbital period is 3.14 ± 0.03 yr, the system mass is 0.214 ± 0.045 M☉, and the semimajor axis is 1.28 ± 0.10 AU or 81 ± 6 mas. Imaging observations indicate that GJ 802b is likely to be a brown dwarf with the astrometrically determined mass 0.058 ± 0.021 M☉ (1 σ limits). The remaining uncertainty in the orbit is the eccentricity that is now loosely constrained. We discuss how the system age limits the mass and the prospects of further narrowing the mass range when e is more precisely determined.