250 research outputs found
Two-Mirror Apodization for High-Contrast Imaging
Direct detection of extrasolar planets will require imaging systems capable
of unprecedented contrast. Apodized pupils provide an attractive way to achieve
such contrast but they are difficult, perhaps impossible, to manufacture to the
required tolerance and they absorb about 90% of the light in order to create
the apodization, which of course lengthens the exposure times needed for planet
detection. A recently proposed alternative is to use two mirrors to accomplish
the apodization. With such a system, no light is lost. In this paper, we
provide a careful mathematical analysis, using one dimensional mirrors, of the
on-axis and off-axis performance of such a two-mirror apodization system. There
appear to be advantages and disadvantages to this approach. In addition to not
losing any light, we show that the nonuniformity of the apodization implies an
extra magnification of off-axis sources and thereby makes it possible to build
a real system with about half the aperture that one would otherwise require or,
equivalently, resolve planets at about half the angular separation as one can
achieve with standard apodization. More specifically, ignoring pointing error
and stellar disk size, a planet at ought to be at the edge of
detectability. However, we show that the non-zero size of a stellar disk pushes
the threshold for high-contrast so that a planet must be at least from its star to be detectable. The off-axis analysis of
two-dimensional mirrors is left for future study.Comment: 21 pages, 7 figures. For author's webpage version see
http://www.orfe.princeton.edu/~rvdb/tex/piaa/ms.pdf This version has improved
figures and addresses comments of a refere
Measurement of HO2 and other trace gases in the stratosphere using a high resolution far-infrared spectrometer at 28 KM
The major events and results to date of the ongoing program of measuring stratospheric composition by the technique of far-infrared Fourier-transform spectroscopy from a balloon-borne platform are reviewed. The highlights of this period were the two balloon flight campaigns which were performed at Palestine, Texas, both of which produced large amounts of scientifically useful data
Speckle nulling with space-based coronagraphs
Following the tracks of Malbet, Yu, & Shao (1995} on dark hole algorithms, we
present analytical methods to measure and correct the speckle noise behind an
ideal coronagraph. We show that, in a low aberration regime, wavefront sensing
can be accomplished with only three images, the next image being fully
corrected (no iterative process needed). The only hardware required is the
coronagraph deformable mirror and an imaging detector in the focal plane, thus
there are no non-common path errors to correct. Our first method, speckle field
nulling, is a fast FFT-based algorithm requiring the deformable mirror
influence functions to have identical shapes. Our second method, speckle energy
minimization is more general and based on matrix inversion. Numerical
simulations show that these methods can improve the contrast by several orders
of magnitude.Comment: 6 pages, 5 figures, to appear in the Proceedings of IAUC 200, Direct
Imaging of Exoplanets: Science & Techniques, October 3-7, 2005, Nice, Franc
High-contrast Imaging from Space: Speckle Nulling in a Low Aberration Regime
High-contrast imaging from space must overcome two major noise sources to
successfully detect a terrestrial planet angularly close to its parent star:
photon noise from diffracted star light, and speckle noise from star light
scattered by instrumentally-generated wavefront perturbation. Coronagraphs
tackle only the photon noise contribution by reducing diffracted star light at
the location of a planet. Speckle noise should be addressed with
adaptative-optics systems. Following the tracks of Malbet, Yu and Shao (1995),
we develop in this paper two analytical methods for wavefront sensing and
control that aims at creating dark holes, i.e. areas of the image plane cleared
out of speckles, assuming an ideal coronagraph and small aberrations. The first
method, speckle field nulling, is a fast FFT-based algorithm that requires the
deformable-mirror influence functions to have identical shapes. The second
method, speckle energy minimization, is more general and provides the optimal
deformable mirror shape via matrix inversion. With a NxN deformable mirror, the
size of matrix to be inverted is either N^2xN^2 in the general case, or only
NxN if influence functions can be written as the tensor product of two
one-dimensional functions. Moreover, speckle energy minimization makes it
possible to trade off some of the dark hole area against an improved contrast.
For both methods, complex wavefront aberrations (amplitude and phase) are
measured using just three images taken with the science camera (no dedicated
wavefront sensing channel is used), therefore there are no non-common path
errors. We assess the theoretical performance of both methods with numerical
simulations, and find that these speckle nulling techniques should be able to
improve the contrast by several orders of magnitude.Comment: 31 pages, 8 figures, 1 table. Accepted for publication in ApJ (should
appear in February 2006
Spectral Evolution of an Earth-Like Planet
We have developed a characterization of the geological evolution of the
Earths atmosphere and surface in order to model the observable spectra of an
Earth-like planet through its geological history. These calculations are
designed to guide the interpretation of an observed spectrum of such a planet
by future instruments that will characterize exoplanets. Our models focus on
spectral features that either imply habitability or are required for
habitability. These features are generated by H2O, CO2, CH4, O2, O3, N2O, and
vegetation-like surface albedos. We chose six geological epochs to
characterize. These epochs exhibit a wide range in abundance for these
molecules, ranging from a CO2 rich early atmosphere, to a CO2/CH4-rich
atmosphere around 2 billion years ago to a present-day atmosphere. We analyzed
the spectra to quantify the strength of each important spectral feature in both
the visible and thermal infrared spectral regions, and the resolutions required
to unambiguously observe the features for each epoch. We find a wide range of
spectral resolutions required for observing the different features. For
example, H2O and O3 can be observed with relatively low resolution, while O2
and N2O require higher resolution. We also find that the inclusion of clouds in
our models significantly affects both the strengths and resolutions required to
observe all spectral features.Comment: 34 pages, 24 fig, pdf, ApJ, TB
Measurement of HO2 and Other Trace Gases in the Stratosphere using a High Resolution Far-Infrared Spectrometer
This report is a continuation of the analysis of data from past flights, exploring issues such as radical partitioning, stratospheric transport, and the ozone budget
Transits of Earth-Like Planets
Transmission spectroscopy of Earth-like exoplanets is a potential tool for
habitability screening. Transiting planets are present-day "Rosetta Stones" for
understanding extrasolar planets because they offer the possibility to
characterize giant planet atmospheres and should provide an access to
biomarkers in the atmospheres of Earth-like exoplanets, once they are detected.
Using the Earth itself as a proxy we show the potential and limits of the
transiting technique to detect biomarkers on an Earth-analog exoplanet in
transit. We quantify the Earths cross section as a function of wavelength, and
show the effect of each atmospheric species, aerosol, and Rayleigh scattering.
Clouds do not significantly affect this picture because the opacity of the
lower atmosphere from aerosol and Rayleigh losses dominates over cloud losses.
We calculate the optimum signal-to-noise ratio for spectral features in the
primary eclipse spectrum of an Earth-like exoplanet around a Sun-like star and
also M stars, for a 6.5-m telescope in space. We find that the signal to noise
values for all important spectral features are on the order of unity or less
per transit - except for the closest stars - making it difficult to detect such
features in one single transit, and implying that co-adding of many transits
will be essential.Comment: 17 pages, 3 figures, 6 tables, to appear in ApJ (accepted) V2:
corrected transit times, corrected values for M4 star radiu
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