5,832 research outputs found
The CHARA optical array
The Center for High Angular Resolution Astronomy (CHARA) was established in the College of Arts and Sciences at Georgia State University in 1984 with the goals of designing, constructing, and then operating a facility for very high spatial resolution astronomy. The interest in such a facility grew out of the participants' decade of activity in speckle interferometry. Although speckle interferometry continues to provide important astrophysical measurements of a variety of objects, many pressing problems require resolution far beyond that which can be expected from single aperture telescopes. In early 1986, CHARA received a grant from the National Science Foundation which has permitted a detailed exploration of the feasibility of constructing a facility which will provide a hundred-fold increase in angular resolution over what is possible by speckle interferometry at the largest existing telescopes. The design concept for the CHARA Array was developed initially with the contractural collaboration of United Technologies Optical Systems, Inc., in West Palm Beach, Florida, an arrangement that expired in August 1987. In late November 1987, the Georgia Tech Research Institute joined with CHARA to continue and complete the design concept study. Very high-resolution imaging at optical wavelengths is clearly coming of age in astronomy. The CHARA Array and other related projects will be important and necessary milestones along the way toward the development of a major national facility for high-resolution imaging--a true optical counterpart to the Very Large Array. Ground-based arrays and their scientific output will lead to high resolution facilities in space and, ultimately, on the Moon
Exoplanet detection with simultaneous spectral differential imaging: effects of out-of-pupil-plane optical aberrations
Imaging faint companions (exoplanets and brown dwarfs) around nearby stars is
currently limited by speckle noise. To efficiently attenuate this noise, a
technique called simultaneous spectral differential imaging (SSDI) can be used.
This technique consists of acquiring simultaneously images of the field of view
in several adjacent narrow bands and in combining these images to suppress
speckles. Simulations predict that SSDI can achieve, with the acquisition of
three wavelengths, speckle noise attenuation of several thousands. These
simulations are usually performed using the Fraunhofer approximation, i.e.
considering that all aberrations are located in the pupil plane. We have
performed wavefront propagation simulations to evaluate how out-of-pupil-plane
aberrations affect SSDI speckle noise attenuation performance. The Talbot
formalism is used to give a physical insight of the problem; results are
confirmed using a proper wavefront propagation algorithm. We will show that
near-focal-plane aberrations can significantly reduce SSDI speckle noise
attenuation performance at several lambda/D separation. It is also shown that
the Talbot effect correctly predicts the PSF chromaticity. Both differential
atmospheric refraction effects and the use of a coronagraph will be discussed.Comment: 11 pages, 7 figures. To be published in Proc. SPIE Vol. 6269, p.
1147-1157, Ground-based and Airborne Instrumentation for Astronomy; Ian S.
McLean, Masanori Iye; Ed
Vector speckle grid: instantaneous incoherent speckle grid for high-precision astrometry and photometry in high-contrast imaging
Photometric and astrometric monitoring of directly imaged exoplanets will
deliver unique insights into their rotational periods, the distribution of
cloud structures, weather, and orbital parameters. As the host star is occulted
by the coronagraph, a speckle grid (SG) is introduced to serve as astrometric
and photometric reference. Speckle grids are implemented as diffractive
pupil-plane optics that generate artificial speckles at known location and
brightness. Their performance is limited by the underlying speckle halo caused
by evolving uncorrected wavefront errors. The speckle halo will interfere with
the coherent SGs, affecting their photometric and astrometric precision. Our
aim is to show that by imposing opposite amplitude or phase modulation on the
opposite polarization states, a SG can be instantaneously incoherent with the
underlying halo, greatly increasing the precision. We refer to these as vector
speckle grids (VSGs). We derive analytically the mechanism by which the
incoherency arises and explore the performance gain in idealised simulations
under various atmospheric conditions. We show that the VSG is completely
incoherent for unpolarized light and that the fundamental limiting factor is
the cross-talk between the speckles in the grid. In simulation, we find that
for short-exposure images the VSG reaches a 0.3-0.8\% photometric error
and astrometric error, which is a
performance increase of a factor 20 and 5, respectively.
Furthermore, we outline how VSGs could be implemented using liquid-crystal
technology to impose the geometric phase on the circular polarization states.
The VSG is a promising new method for generating a photometric and astrometric
reference SG that has a greatly increased astrometric and photometric
precision.Comment: Accepted for publication in A&
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