185 research outputs found
Selection of radio sources for Venus balloon-Pathfinder Delta-DOR navigation at 1.7 GHz
In order to increase the success rate of the Delta-DOR (Delta-Differential One-way Range) VLBI navigational support for the French-Soviet Venus Balloon and Halley Pathfinder projects, forty-four extragalactic radio sources were observed in advance of these projects to determine which were suitable for use as reference sources. Of these forty-four radio sources taken from the existing JPL radio source catalogue, thirty-six were determined to be of sufficient strength for use in Delta-DOR VLBI navigation
Performance of single photon-counting X-ray charge coupled devices
Results of intial performance tests on X-ray sensing properties of charge-coupled devices (CCDs) are presented. CCDs have demonstrated excellent spatial resolution and good spectral resolution, superior to that of non-imaging proportional counters
A 2D Electromagnetic PIC Code for Distributed Memory Parallel Computers
The two dimensional electrostatic plasma particle in cell (PIC) code described an [1] has been upgraded to a 2D electromagnetic PIC code running on the Caltech/JPL Mark IIIfp and the Intel iPSC/860 parallel MIMD computers. The code solves the complete time dependent Maxwell’s equations where the plasma responses, i.e., the charge and current density in the plasma, are evaluated by advancing in time the trajectories of ~ 10^6 particles in their self-consistent electromagnetic field. The field equations are solved in Fourier space. Parallelisation is achieved through domain decomposition in real and Fourier space. Results from a simulation showing a two-dimensional Alfèn wave filamentation instability are shown; these are the first simulations of this 2D Alfèn wave decay process
Fiber-Based Interferometry and Imaging
Single-mode optical fibers are playing an increasing role in astronomical interferometry, e.g., in high-accuracy visibility measurements and in nulling interferometry. However, such observing modes typically involve only small numbers of fibers. On the other hand, some recently proposed observing techniques call for arrays of single mode fibers coupled to arrays of sub-apertures within a large telescope pupil. The concepts include pupil-masked visibility measurements (non-redundant masking), pupil-sheared nulling interferometry, and coronagraphic imaging using a fiber-linked phased-array of small optical telescopes. The latter arrangement may also be relevant to optical communications. Here we provide an overview of a number of recent novel applications of single-mode fibers and single-mode fiber arrays
The Vector Vortex Coronagraph: Laboratory Results and First Light at Palomar Observatory
High-contrast coronagraphy will be needed to image and characterize faint
extra-solar planetary systems. Coronagraphy is a rapidly evolving field, and
many enhanced alternatives to the classical Lyot coronagraph have been proposed
in the past ten years. Here, we discuss the operation of the vector vortex
coronagraph, which is one of the most efficient possible coronagraphs. We first
present recent laboratory results, and then first light observations at the
Palomar observatory. Our near-infrared H-band (centered at ~ 1.65 microns) and
K-band (centered at ~ 2.2 microns) vector vortex devices demonstrated excellent
contrast results in the lab, down to ~ 1e-6 at an angular separation of 3 lb/d.
On sky, we detected a brown dwarf companion 3000 times fainter than its host
star (HR 7672) in the Ks band (centered at ~2.15 microns), at an angular
separation of ~ 2.5 lb/d. Current and next-generation high-contrast instruments
can directly benefit from the demonstrated capabilities of such a vector
vortex: simplicity, small inner working angle, high optical throughput (>90%),
and maximal off-axis discovery space
Recent Progress in Vector Vortex Coronagraphy
The optical vortex coronagraph has great potential for enabling high-contrast observations very close to bright stars, and thus for reducing the size of space telescopes needed for exoplanet characterization missions. Here we discuss several recent developments in optical vortex coronagraphy. In particular, we describe multi-stage vortex configurations that allow the use of on-axis telescopes for high-contrast coronagraphy, and also enable the direct measurement of the amplitudes and phases of focal plane speckles. We also briefly describe recent laboratory demonstrations of the optical properties of the dual-stage vortex, and of the broadband performance of single stage vortex masks. Indeed, the demonstrated performance of the vector vortex phase masks already in hand, ≈ 10^(-8), is approximately that needed for an initial coronagraphic mission, such as an exoplanet explorer, aimed at detecting exozodiacal light and jovian exoplanets
New Constraints on Companions and Dust within a Few AU of Vega
We report on high contrast near-infrared (~2.2 μm) observations of Vega obtained with the Palomar Fiber Nuller, a dual sub-aperture rotating coronagraph installed at the Palomar Hale telescope. The data show consistent astrophysical null depth measurements at the 10^(–3) level or below for three different baseline orientations spanning 60 deg in azimuth, with individual 1σ uncertainties ≤7 × 10^(–4). These high cancellation and accuracy levels translate into a dynamic range greater than 1000:1 inside the diffraction limit of the 5 m telescope beam. Such high contrast performance is unprecedented in the near-infrared and provides improved constraints on Vega's immediate ( 20 to 250 mas, or 0.15 to 2 AU) environment. In particular, our measurements rule out any potential companion in the [0.25-1 AU] region contributing more than 1% of the overall near-infrared stellar flux, with limits as low as 0.2% near 0.6 AU. These are the best upper limits established so far by direct detection for a companion to Vega in this inner region. We also conclude that any dust population contributing a significant (≥1%) near-infrared thermal excess can arise only within 0.2 AU of the star, and that it must consist of much smaller grains than in the solar zodiacal cloud. Dust emission from farther than 2 AU is also not ruled out by our observations, but would have to originate in strong scattering, pointing again to very small grains
Improving Interferometric Null Depth Measurements using Statistical Distributions: Theory and First Results with the Palomar Fiber Nuller
A new "self-calibrated" statistical analysis method has been developed for
the reduction of nulling interferometry data. The idea is to use the
statistical distributions of the fluctuating null depth and beam intensities to
retrieve the astrophysical null depth (or equivalently the object's visibility)
in the presence of fast atmospheric fluctuations. The approach yields an
accuracy much better (about an order of magnitude) than is presently possible
with standard data reduction methods, because the astrophysical null depth
accuracy is no longer limited by the magnitude of the instrumental phase and
intensity errors but by uncertainties on their probability distributions. This
approach was tested on the sky with the two-aperture fiber nulling instrument
mounted on the Palomar Hale telescope. Using our new data analysis approach
alone-and no observations of calibrators-we find that error bars on the
astrophysical null depth as low as a few 10-4 can be obtained in the
near-infrared, which means that null depths lower than 10-3 can be reliably
measured. This statistical analysis is not specific to our instrument and may
be applicable to other interferometers
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