The Kepler spacecraft launched on March 7, 2009, initiating NASA’s first search for Earth-size planets orbiting Sun-like stars. Since launch, Kepler has announced the discovery of 17 exoplanets, including a system of six transiting a Sun-like star, Kepler-11, and the first confirmed rocky planet, Kepler-10b, with a radius of 1.4 that of Earth. Kepler is proving to be a cornucopia of discoveries: it has identified over 1200 candidate planets based on the first 120 days of observations, including 54 that are in or near the habitable zone of their stars, and 68 that are 1.2 Earth radii or smaller. An astounding 408 of these planetary candidates are found in 170 multiple systems, demonstrating the compactness and flatness of planetary systems composed of small planets. Never before has there been a photometer capable of reaching a precision near 20 ppm in 6.5 hours and capable of conducting nearly continuous and uninterrupted observations for months to years. In addition to exoplanets, Kepler is providing a wealth of astrophysics, and is revolutionizing the field of asteroseismology. Designing and building the Kepler photometer and the software systems that process and analyze the resulting data to make the discoveries presented a daunting set of challenges, including how to manage the large data volume. The challenges continue into flight operations, as the photometer is sensitive to its thermal environment, complicating the task of detecting 84 ppm drops in brightness corresponding to Earth-size planets transiting Sun-like stars.

/pdf/uv-x-ray-and-gamma-ray-space-instrumentation-for-astronomy-4owyulify7.pdf

UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XVIII

The effects of photon noise, aliasing, wave front chromaticity, and scintillation on the point-spread function (PSF) contrast achievable with ground-based adaptive optics (AO) are evaluated for different wave front sensing schemes. I show that a wave front sensor (WFS) based on the Zernike phase contrast technique offers the best sensitivity to photon noise at all spatial frequencies, while the Shack-Hartmann WFS is significantly less sensitive. In AO systems performing wave front sensing in the visible and scientific imaging in the near-IR, the PSF contrast limit is set by the scintillation chromaticity induced by Fresnel propagation through the atmosphere. On an 8 m telescope, the PSF contrast is then limited to 10-4 to 10-5 in the central arcsecond. Wave front sensing and scientific imaging should therefore be done at the same wavelength, in which case, on bright sources, PSF contrasts between 10-6 and 10-7 can be achieved within 1'' on an 8 m telescope in optical/near-IR. The impact of atmospheric turbulence parameters (seeing, wind speed, turbulence profile) on the PSF contrast is quantified. I show that a focal plane wave front sensing scheme offers unique advantages, and I discuss how to implement it. Coronagraphic options are also briefly discussed.

https://iopscience.iop.org/article/10.1086/431209/pdf

Limits of adaptive optics for high-contrast imaging

The Keck II Telescope is the first 8-10 m class telescope equipped with a laser guide star adaptive optics (LGS AO) system. Under normal seeing conditions, the LGS AO system produces K-band Strehl ratios between 30% and 40% using bright tip-tilt guide stars, and it works well with tip-tilt guide stars as faint as , with partial correction for stars up to a magnitude fainter. This paper presents the algorithms implemented m p 18 R in the LGS AO system, as well as experimental performance results. A detailed error budget shows excellent agreement between the measured and expected image quality for both bright and faint guide stars.

/pdf/the-w-m-keck-observatory-laser-guide-star-adaptive-optics-451i3x0bph.pdf

The W. M. Keck Observatory Laser Guide Star Adaptive Optics System: Performance Characterization

Several site-testing programs and observatories currently use combined MASS-DIMM instruments for monitoring parameters of optical turbulence. The instrument is described here. After a short recall of the measured quantities and operational principles, the optics and elec- tronics of MASS-DIMM, interfacing to telescopes and detectors, and operation are covered in some detail. Particular attention is given to the correct measurement and control of instru- mental parameters to ensure valid and well-calibrated data, to the data quality and filtering. Examples of MASS-DIMM data are given, followed by the list of present and future applica- tions.

Combined MASS-DIMM instruments for atmospheric turbulence studies

Astronomical seeing is quantified by a single parameter, the turbulence integral, in the framework of the Kolmogorov turbulence model. This parameter can be routinely measured by a Differential Image Motion Monitor (DIMM). A new instrument, the multi-aperture scintillation sensor (MASS), permits one to measure the seeing in the free atmosphere above ∼0.5 km and, together with a DIMM, to estimate the ground-layer seeing. The absolute accuracy of both methods is studied here using analytical theory, numerical simulation and experiments. A modification of the MASS data processing to compensate for partially saturated scintillation is developed. We find that the DIMM can be severely biased by optical aberrations (e.g. defocus) and propagation. Seeing measurements with DIMM and MASS can reach absolute accuracy of ∼ 10 per cent when their biases are carefully controlled. Pushing this limit to 1 per cent appears unrealistic because the seeing itself is just a model-dependent parameter of a non-stationary random process.

http://www.ctio.noao.edu/~atokovin/papers/accurmnras.pdf

Accurate seeing measurements with MASS and DIMM

The vertical distribution of turbulence over Mauna Kea has been measured on four nights in 2002 October, simultaneously using two different instruments based on stellar scintillation—the generalized SCIDAR (scintillation detection and ranging) and MASS (multiaperture scintillation sensor). The turbulence integrals match within 20%, and the low-resolution profiles delivered by MASS correctly reveal the localization of the strongest high-altitude turbulent layers. As deduced from DIMM (differential image motion monitor), MASS, and SCIDAR measurements, optical turbulence in the first 0.7 km above the summit contributed typically half of the total integral, the latter corresponding to a seeing of 0 .5. The ground layer and free atmosphere are not correlated.

https://www.jstor.org/stable/10.1086/428930

A. Ziad

Papers

Optical Turbulence Profiles at Mauna Kea Measured by MASS and SCIDAR