525 research outputs found
Station-Keeping Requirements for Constellations of Free-Flying Collectors Used for Astronomical Imaging in Space
The accuracy requirements on station-keeping for constellations of
free-flying collectors coupled as (future) imaging arrays in space for
astrophysics applications are examined. The basic imaging element of these
arrays is the two-element interferometer. Accurate knowledge of two quantities
is required: the \textit{projected baseline length}, which is the distance
between the two interferometer elements projected on the plane tranverse to the
line of sight to the target; and the \textit{optical path difference}, which is
the difference in the distances from that transverse plane to the beam
combiner. ``Rules-of-thumb'' are determined for the typical accuracy required
on these parameters. The requirement on the projected baseline length is a
\textit{knowledge} requirement and depends on the angular size of the targets
of interest; it is generally at a level of half a meter for typical stellar
targets, decreasing to perhaps a few centimeters only for the widest attainable
fields of view. The requirement on the optical path difference is a
\textit{control} requirement and is much tighter, depending on the bandwidth of
the signal; it is at a level of half a wavelength for narrow (few %) signal
bands, decreasing to for the broadest bandwidths expected
to be useful. Translation of these requirements into engineering requirements
on station-keeping accuracy depends on the specific details of the collector
constellation geometry. Several examples are provided to guide future
application of the criteria presented here. Some implications for the design of
such collector constellations and for the methods used to transform the
information acquired into images are discussed.Comment: 13 pages, 6 figures, accepted 6/29/07 for the August 2007 issue of
PAS
Calibration of Low-Frequency, Wide-Field Radio Interferometers Using Delay/Delay-Rate Filtering
We present a filtering technique that can be applied to individual baselines
of wide-bandwidth, wide-field interferometric data to geometrically select
regions on the celestial sphere that contain primary calibration sources. The
technique relies on the Fourier transformation of wide-band frequency spectra
from a given baseline to obtain one-dimensional "delay images", and then the
transformation of a time-series of delay images to obtain two-dimensional
"delay/delay-rate images." Source selection is possible in these images given
appropriate combinations of baseline, bandwidth, integration time and source
location. Strong and persistent radio frequency interference (RFI) limits the
effectiveness of this source selection owing to the removal of data by RFI
excision algorithms. A one-dimensional, complex CLEAN algorithm has been
developed to compensate for RFI-excision effects. This approach allows CLEANed,
source-isolated data to be used to isolate bandpass and primary beam gain
functions. These techniques are applied to data from the Precision Array for
Probing the Epoch of Reionization (PAPER) as a demonstration of their value in
calibrating a new generation of low-frequency radio interferometers with wide
relative bandwidths and large fields-of-view.Comment: 17 pages, 6 figures, 2009AJ....138..219
Comments on the tethered galaxy problem
In a recent paper Davis et al. make the counter intuitive assertion that a
galaxy held `tethered' at a fixed distance from our own could emit blueshifted
light. Moreover, this effect may be derived from the simplest
Friedmann-Robertson-Walker spacetimes and the (0.3,0.7) case which is believed
to be a good late time model of our own universe.
In this paper we recover the previous authors' results in a more transparent
form. We show how their results rely on a choice of cosmological distance scale
and revise the calculations in terms of observable quantities which are
coordinate independent. By this method we see that, although such a tethering
would reduce the redshift of a receding object, it would not do so sufficiently
to cause the proposed blueshift. The effect is also demonstrated to be much
smaller than conjectured below the largest intergalactic scales. We also
discuss some important issues, raised by this scenario, relating to the
interpretation of redshift and distance in relativistic cosmology.Comment: 6 pages, 3 figures, submitted to Am.J.Phy
The Importance of Phase in Nulling Interferometry and a Three Telescope Closure-Phase Nulling Interferometer Concept
We discuss the theory of the Bracewell nulling interferometer and explicitly
demonstrate that the phase of the "white light" null fringe is the same as the
phase of the bright output from an ordinary stellar interferometer. As a
consequence a "closure phase" exists for a nulling interferometer with three or
more telescopes. We calculate the phase offset as a function of baseline length
for an Earth-like planet around the Sun at 10 pc, with a contrast ratio of
at 10 m. The magnitude of the phase due to the planet is radians, assuming the star is at the phase center of the array.
Although this is small, this phase may be observable in a three-telescope
nulling interferometer that measures the closure phase. We propose a simple
non-redundant three-telescope nulling interferometer that can perform this
measurement. This configuration is expected to have improved characteristics
compared to other nulling interferometer concepts, such as a relaxation of
pathlength tolerances, through the use of the "ratio of wavelengths" technique,
a closure phase, and better discrimination between exodiacal dust and planets
Displacement- and Timing-Noise Free Gravitational-Wave Detection
Motivated by a recently-invented scheme of displacement-noise-free
gravitational-wave detection, we demonstrate the existence of
gravitational-wave detection schemes insusceptible to both displacement and
timing (laser) noises, and are thus realizable by shot-noise-limited laser
interferometry. This is possible due to two reasons: first, gravitational waves
and displacement disturbances contribute to light propagation times in
different manners; second, for an N-detector system, the number of signal
channels is of the order O(N^2), while the total number of timing- and
displacement-noise channels is of the order O(N).Comment: 4 pages, 3 figures; mistake correcte
Boundary crossing Random Walks, clinical trials and multinomial sequential estimation
A sufficient condition for the uniqueness of multinomial sequential unbiased
estimators is provided generalizing a classical result for binomial samples.
Unbiased estimators are applied to infer the parameters of multidimensional or
multinomial Random Walks which are observed until they reach a boundary. An
application to clinical trials is presented
Bringing closure to microlensing mass measurement
Interferometers offer multiple methods for studying microlensing events and
determining the properties of the lenses. We investigate the study of
microlensing events with optical interferometers, focusing on narrow-angle
astrometry, visibility, and closure phase. After introducing the basics of
microlensing and interferometry, we derive expressions for the signals in each
of these three channels. For various forecasts of the instrumental performance,
we discuss which method provides the best means of measuring the lens angular
Einstein radius theta_E, a prerequisite for determining the lens mass. If the
upcoming generation of large-aperture, AO-corrected long baseline
interferometers (e.g. VLTI, Keck, OHANA) perform as well as expected, theta_E
may be determined with signal-to-noise greater than 10 for all bright events.
We estimate that roughly a dozen events per year will be sufficiciently bright
and have long enough durations to allow the measurement of the lens mass and
distance from the ground. We also consider the prospects for a VLTI survey of
all bright lensing events using a Fisher matrix analysis, and find that even
without individual masses, interesting constraints may be placed on the bulge
mass function, although large numbers of events would be required.Comment: 23 pages, aastex, submitted to Ap
Direct Detection of the Brown Dwarf GJ 802B with Adaptive Optics Masking Interferometry
We have used the Palomar 200" Adaptive Optics (AO) system to directly detect
the astrometric brown dwarf GJ 802B reported by Pravdo et al. 2005. This
observation is achieved with a novel combination of aperture masking
interferometry and AO. The dynamical masses are 0.1750.021 M and
0.0640.032 M for the primary and secondary respectively. The
inferred absolute H band magnitude of GJ 802B is M=12.8 resulting in a
model-dependent T of 1850 50K and mass range of
0.057--0.074 M.Comment: 4 Pages, 5 figures, emulateapj format, submitted to ApJ
The last gasps of VY CMa: Aperture synthesis and adaptive optics imagery
We present new observations of the red supergiant VY CMa at 1.25 micron, 1.65
micron, 2.26 micron, 3.08 micron and 4.8 micron. Two complementary
observational techniques were utilized: non-redundant aperture masking on the
10-m Keck-I telescope yielding images of the innermost regions at unprecedented
resolution, and adaptive optics imaging on the ESO 3.6-m telescope at La Silla
attaining extremely high (~10^5) peak-to-noise dynamic range over a wide field.
For the first time the inner dust shell has been resolved in the near-infrared
to reveal a one-sided extension of circumstellar emission within 0.1" (~15
R_star) of the star. The line-of-sight optical depths of the circumstellar dust
shell at 1.65 micron, 2.26 micron, and 3.08 micron have been estimated to be
1.86 +/- 0.42, 0.85 +/- 0.20, and 0.44 +/- 0.11. These new results allow the
bolometric luminosity of VY~CMa to be estimated independent of the dust shell
geometry, yielding L_star ~ 2x10^5 L_sun. A variety of dust condensations,
including a large scattering plume and a bow-shaped dust feature, were observed
in the faint, extended nebula up to 4" from the central source. While the
origin of the nebulous plume remains uncertain, a geometrical model is
developed assuming the plume is produced by radially-driven dust grains forming
at a rotating flow insertion point with a rotational period between 1200-4200
years, which is perhaps the stellar rotational period or the orbital period of
an unseen companion.Comment: 25 pages total with 1 table and 5 figures. Accepted by Astrophysical
Journal (to appear in February 1999
A toy model of the five-dimensional universe with the cosmological constant
A value of the cosmological constant in a toy model of the five-dimensional
universe is calculated in such a manner that it remains in agreement with both
astronomical observations and the quantum field theory concerning the
zero-point fluctuations of the vacuum. The (negative) cosmological constant is
equal to the inverse of the Planck length squared, which means that in the toy
model the vanishing of the observed value of the cosmological constant is a
consequence of the existence of an energy cutoff exactly at the level of the
Planck scale. In turn, a model for both a virtual and a real
particle-antiparticle pair is proposed which describes properly some energetic
properties of both the vacuum fluctuations and created particles, as well as it
allows one to calculate the discrete "bare" values of an elementary-particle
mass, electric charge and intrinsic angular momentum (spin) at the energy
cutoff. The relationships between the discussed model and some phenomena such
as the Zitterbewegung and the Unruh-Davies effect are briefly analyzed, too.
The proposed model also allows one to derive the Lorentz transformation and the
Maxwell equations while considering the properties of the vacuum filled with
the sea of virtual particles and their antiparticles. Finally, the existence of
a finite value of the vacuum-energy density resulting from the toy model leads
us to the formulation of dimensionless Einstein field equations which can be
derived from the Lagrangian with a dimensionless (naively renormalized)
coupling constant.Comment: 52 pages, 1 figure; a post-final, rewritten version with a number of
new remarks and conclusion
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