Presentations
Papers
Abstracts
VINCI links:
Observation logs and statistics
Diameter determinations
Links
Contact info
Presentations (some with anotations, most directly from powerpoint files)
Diameter determinations from VINCI using global calibration solutions
Poster presented at the
workshop entitled:
The power of optical/IR interferometry:
recent scientific results and 2nd generation VLTI instrumentation
held in Garching, Germany, 4-8 April 2005
View Abstract
PDF file of Poster
We were also allowed to submit a 2 page (!) paper for this conference.
Here
is the 3 page "paper" I submitted in postscript format.
MIDI data reduction, analysis and science
workshop 2004:
Presentation on ESO MIDI Data Reduction System (pipeline)
Presentation given 14 October 2004 in Leiden as part of
MIDI data reduction workshop
PDF file produced from presentation
Conceptual Design Review PRIMA Astrometry Operations and Software
29 Sep 2004 through 1 Oct 2004
Presentation given concerning dispersion and
fringe tracking between bands
Includes the "famous" dispersion animations
Powerpoint file used in presentation (no annotations)
Understanding visibility functions
Presentation/tutorial covering the calculation
of visibility functions for various object profiles.
Raw powerpoint file without anotations
Data reduction, calibration, and stellar diameter results using VINCI
J. Meisner
Annotated slides from a presentation made at the September 2003 Workshop
Long Baseline Interferometry in the mid-infrared, Ringberg Castle near Munich.
PDF File
Direct detection of exoplanets using long-baseline
interferometry and visibility phase
Real-time control systems for a
ground-based nulling interferometer:
requirements and configurations
Note: these are published papers for which I was the primary author
or played a major role in preparing.
Coherent integration of complex fringe visibility employing dispersion tracking
Direct detection of the disk around HR 4049
Scientific and technical results from VINCI using coherent
estimation of fringe visibility
Dispersion affecting the VLTI and 10 micron interferometry using MIDI
Direct detection of exoplanets using long-baseline
interferometry and visibility phase
Fringe Tracking and Group Delay Tracking Methods for MIDI
Spatial Filters for Astronomical Interferometry using Discrete Optical
Components
Related paper:
Coherent estimation of complex fringe visibility: a generalized
approach,
Use of a Seeing Monitor to Determine the Velocities of Turbulent
Atmospheric Layers
Concentration
of Starlight from Large Apertures into a Single Spatial Mode for Long-Baseline
Interferometry
Coherent
integration of fringe visibility employing probabilistic determinations
of atmospheric delay
Atmospheric delay tracking in a long-baseline
optical stellar interferometer,
Estimation and tracking of atmospheric delay noise in a long-baseline
optical stellar interferometer and determination of the expected estimation
error, Ph.D. Thesis, Univ. of Minnesota (1995)
Diameter determinations from VINCI using global calibration solutions
ABSTRACT
Visibilities from VINCI raw data are obtained using coherent integration. These raw
visibilities are interpreted using a global solution algorithm which simultaneously solves
for calibration (transfer function) fluctuations and stellar diameters. Using this system,
15500 successful visibilities have been processed on 296 objects, including science objects
and calibrators. Half of these objects were successfully observed at least 15 times and on
at least 5 separate nights, providing a fruitful database for cross-calibration.
The standard approach to calibration based on observations of "calibrator" stars having
assumed diameters, is challenged in the present work. No a priori diameters are input to the
algorithm, which relies on baseline diversity to simultaneously solve both for diameters of
stars having well-behaved characteristics, and a quasi-static transfer function subject to
various hardware fluctuations. The success of this approach depends on a diversified
schedule of observing the same target on different nights in conjunction with a mixture of
other targets, so that all visibilities can be "cross-calibrated."
The results thereby obtained consist mainly of uniform disk diameters, but additionally the
algorithm detects specific departures from that model. In about 20 cases, we detect a "zero-
baseline power" significantly smaller than unity, probably due to circumstellar emission.
Diameter variations of several pulsating stars were also observed. Visibilities beyond the
first null were obtained for several objects, thus directly constraining the magnitude of
limb-darkening in these cases. Among the uniform disk diameters obtained, about one third
had formal errors of better than 1%.
Coherent integration of complex fringe visibility employing dispersion tracking
ABSTRACT
In an instrument with substantial bandwidth, it is also necessary to correct
for fixed and random dispersion. The integrity of phase functions
obtained is dependent on correct modelling of fixed optical phase
functions (obtained from a calibrator observation), dispersion from
air filled delay-lines
(calibratable in principle), and averaging over time to reduce
the effect of random atmospheric water vapor dispersion.
To achieve the best performance, it is necessary to include a dispersion tracker
as well as tracking achromatic OPD, applying each as a phase correction as a
function of time and of optical frequency.
Using MIDI, the N band instrument of the VLTI, which has a wide bandwidth,
it is often possible to uninterruptedly track random dispersion
fluctuations over an observation.
Plots of dispersion fluctuations due to water vapor above
the VLTI are shown, which are used (along with OPD tracking) to coherently
integrate raw frames from that instrument.
The resulting complex visibility includes a unique phase delay signature
reflecting the source structure. A residual ``water-vapor-like'' phase may be
present due to unmonitored humidity in the delay line paths, and to incomplete
averaging of (nominally zero-mean) atmospheric water vapor fluctuations.
Nevertheless, the use of visibility phase results corrupted by
random dispersion is possible.
Direct detection of the disk around HR 4049
ABSTRACT
Direct detection of exoplanets using long-baseline
interferometry and visibility phase
ABSTRACT
We shall limit our discussion to the third method, in which the
effect of a dim companion modulates the visibility function by a
small amount. For a brightness ratio A (generally A < .001),
the magnitude of the visibility amplitude and phase variations
will be of order A, and A radians, respectively. It is expected
that the small phase variations will be more detectable than
the amplitude variations due to symmetry constraints. This
phase can be observed in two ways.
Using a three-telescope interferometer, the closure phase can
be computed by integrating the triple product of the three raw
visibilities. In principle, this estimator needs no calibration, and
will be sensitive when the star-planet separation exceeds .25 *
lambda/B. Spectrally resolved interferometry will allow a
spectrum of the planet's radiation to be directly obtained. It
should be noted that closure phases measured with current
interferometers have error levels an order of magnitude larger
than required for planet detection, however no fundamental
limitation appears to prevent the required improvement.
The alternate method of observing phase is to observe tiny
changes in the phase delay of interference as a function of
wavelength, the so-called "differential phase" method. This
method is particularly sensitive in the case that a planet's
radiation has sharp spectral features within the
interferometer's passband. But even with a smooth spectrum,
a planet can be resolved when the separation multiplied by the
fractional bandwidth is somewhat in excess of .25 * lambda/B.
Such an observation requires careful phase calibration of the
instrument using observation of unresolved or symmetric
calibration sources. Furthermore, random atmospheric
dispersion due to water vapor inhomogeneities, acts as an
additional source of interference, which must be tracked and
removed. The remaining phase signal which cannot be
absorbed by an arbitrary amount of differential water vapor
dispersion, can solve for the planet's brightness ratio and
separation.
We present the mathematical basis for these techniques,
sensitivity estimates, and mention plans for the VLTI to
perform such observations using the MIDI (mid infrared) and
AMBER (near infrared) instruments.
Scientific and technical results from VINCI using coherent
estimation of fringe visibility
ABSTRACT
Such an estimator is thus sensitive to instrumental phase and spectral characteristics, including
the variable component of dispersion introduced
by the excess air paths in the delay lines. Calibration of such instrumental
effects demonstrates the ability to detect
source phase at a fine level as will be required for direct interferometric
detection of extrasolar planets.
We present diameters for five stars obtained by observing the visibility null in their correlated
spectra.
Using coherent integration we have also observed the peculiar correlated spectra
seen in many Mira
variables, possibly
due to changes in the apparent diameter with wavelength. Calibration of the zero-baseline power
from o Ceti
is used with other interferometric observations of this star over a period of 90 days to plot
diameter variations
associated with its pulsation cycle.
Dispersion affecting the VLTI and 10 micron interferometry using MIDI
ABSTRACT
Large amounts of dispersion, if not monitored, can reduce the accuracy of measured visibility
amplitudes. Measurements of the visibility phase as a function of wavelength will be highly sensitive
to dispersion. This will then become a source of noise in results dependent on the phase of
interference, such as imaging of non-symmetric objects, or detection of faint companions. In addition
to dispersion over the 7 - 14 micron region detected by MIDI, observations using phase tracking with
detection in the near IR, could be catastrophically affected by differential phase delays between the
2 micron and 10 micron bands.
Dispersion measurements from VINCI observing in the K band, both due to dry air and to water vapor,
are presented. Combining VINCI results with published data from millimeter wave measurements leads us
to expect atmospheric differential water vapor fluctuations to exceed 1 mole/m^2 rms over typical
baselines. Specific effects from such a level of unmonitored dispersion variations are presented,
which demand corrective action. Various solutions to monitor water vapor dispersion in realtime are
considered.
Fringe Tracking and Group Delay
Tracking Methods for MIDI
ABSTRACT
Group-delay tracking is a technique which is somewhat more robust than
true phase tracking, but supplies a cruder estimate of OPD variations.
Such incoherent techniques are useful for coarse adjustments to an interferometer's
delay lines, and will be part of the real-time software operating in support
of the MIDI instrument. General characteristics of both estimators are
compared as regards sensitivity, detection bandwidth, and behavior in response
to dispersion.
ABSTRACT
While a single-mode waveguide (optical fiber) has excellent properties
in relation to both criteria, it may not be the ideal solution in all cases,
particularly at longer wavelengths where suitable materials may be unavailable.
Instead, concepts for spatial filtering using discrete optical components
are analyzed. Although the proposed configuration is highly wavelength
dependent, one possibility for a wideband implementation is discussed as
well.
ABSTRACT
Use of a Seeing Monitor to Determine the
Velocities of Turbulent Atmospheric Layers
ABSTRACT
Successive frames are later cross-correlated in order to extract the
velocity vectors of winds affecting the turbulent layers responsible for
astronomical seeing. The velocities of strong turbulent layers may be detected
in as little as one second with the present apparatus, allowing the observation
of transient atmospheric turbulent phenomena on such a time scale. Proposed
enhancements would increase the sensitivity of the system to weaker features,
and a system for simultaneously determining the altitude of the atmospheric
layers associated with the wind velocities is suggested.
Due to its ability to identify the individual contributions of distinct
atmospheric layers towards seeing, such an apparatus may have use as a
realtime seeing monitor in conjunction with ground based interferometers
and adaptive optics systems, as well as in statistical studies of seeing,
site selection, and meteorological research.
Atmospheric delay tracking in a long-baseline
optical stellar interferometer,
ABSTRACT
However precise delay-tracking can be achieved at somewhat lower signal
levels by employing an off-line delay-tracking system, in which the raw
data measured by the interferometer is stored for subsequent analysis.
Then the estimate of tau at time t, is based on data collected
both before and after time t.
An optimum delay-tracking algorithm embraces the a priori statistics
of the atmospheric delay process. Rather than simply estimating tau
at a point in time, a superior estimate of tau will be obtained by comparing
all possible functions, tau(t), over a time period. Using Bayes'
theorem, the a posteriori probability density of any tau(t) function
can be determined. An algorithm has been developed which determines
one or more functions which maximize that probability. Even the ambiguous
estimates which result at lower signal levels, can be employed for the
coherent integration of optical correlation.
Coherent integration of fringe visibility
employing probabilistic determinations of atmospheric delay
ABSTRACT
Concentration of Starlight from Large Apertures
into a Single Spatial
ABSTRACT
However the use of an adaptive optics system may enable larger apertures
to concentrate greater amounts of optical power in a single mode, thus
extending the magnitude limit of the interferometer. Aside from systems
using laser guide stars, an adaptive optics system requires a feedback
signal derived from the detection of a portion of the collected starlight
in order to co-phase the sub-apertures. Increasing the portion of light
directed to the feedback system will therefore allow the adaptive optics
system to operate on dimmer objects.
On the other hand, the optical power which is sacrificed for the production
of the feedback signal becomes unavailable for the ultimate use by the
interferometer. However any light which would not successfully be concentrated
into the output mode may be obtained "cost free." This observation leads
one to different design criteria for an adaptive optics system used for
concentrating light into a single mode, as opposed to one designed for
high resolution imaging by a single large aperture.
The sensitivity limits and light concentrating power of any such adaptive
optics system can be found by the analysis of a hypothetical guided wave
optical circuit forming a binary tree structure. Optical power is concentrated
from subapertures feeding the branches of the tree toward the root yielding
starlight concentrated in a single mode which may supply one arm of a long-baseline
interferometer. Concentration of light along the tree structure occurs
at 2-input modules each of which are similar, and are optimized for maximum
light concentration at each stage. The control system for each module operates
independently of the others, and is optimized on the basis of the
power spectrum of phase noise expected for a given level of atmospheric
turbulence. Performance limits are obtained for the resultant optical concentrating
power as a function of incident flux, the Freid parameter, and the atmospheric
coherence time parameter.
Estimation and Tracking of Atmospheric Delay Noise in a Long-Baseline
Optical Stellar Interferometer and Determination of the Expected Estimation Error
ABSTRACT
The conventional
approach to "fringe tracking" involves a control system that serves a rapidly responding path-length
compensator in real-time. At marginal signal levels, the reliability of such a real-time delay-
tracking system suffers. Precise delay-tracking can be achieved at somewhat lower signal levels by
employing an off-line delay-tracking system, in which the raw data measured by the interferometer is
stored for subsequent analysis. Then it is possible to estimate the delay error at time t using raw
data collected both before and after time t, resulting in a superior estimate.
As opposed to point
estimation procedures based upon the estimation of tau at a point in time, the optimum estimation of
tau is based upon the comparison of all possible functions, tau(t), over a time period. Such a path
estimation procedure fully incorporates the a priori statistics of the atmospheric delay process.
Solutions are found using an iterative procedure to maximize the a posteriori probability of the
function tau(t), determined by employing Bayes' theorem. However there will be more than one local
maximum of a posteriori probability. At lower signal-to-noise ratios it becomes increasingly
difficult to differentiate among these multiple solutions, and the resultant estimate contains
ambiguities. However by properly evaluating the array of solutions, sufficient information can be
retained for the purpose of integrating the measurement of correlation.
Very acceptable results are
obtained at signal-to-noise ratios as low as 3.0, corresponding to 9 detected photons per T_0 with
full optical correlation, |V| = 1. T_0 is defined as r _0/V_0) where r _0 is the Fried parameter, and
V _0 is the velocity of the "wind" driving the atmospheric delay process according to the Taylor
model. At this signal level the r.m.s. estimation error of the correctly identified solution is.7
radians.
Leiden Observatory
Postal Address:
Email: meisner@strw.leidenuniv.nl
J. Meisner
Presentation made in Leiden 27 June 2003 based on a poster
presented at XIXth IAP Colloquium
"Extrasolar Planets : Today and Tomorrow" held in Paris, 30 June - 4 July 2003
View Abstract
Slide show presentation (PDF file)
J. Meisner
View
Abstract
Presented at GENIE workshop, 3 - 6 June 2002, Leiden
Annotated version of slides from presentation (pdf)
Other papers which may contain my name can be found using an
ADS Abstract Service query.
J. Meisner, R Tubbs, W. Jaffe
View Abstract
Proceedings SPIE v5491
Interferometry conference, Glasgow, July 2004
Postscript File
Bakker, Eric J.; Meisner, Jeffrey A.; Percheron, Isabelle; Dominik, Carsten
View Abstract
Proceedings SPIE v5491, p35
Interferometry conference, Glasgow, July 2004
J. A. Meisner
View Abstract
Proceedings of the workshop at JENAM 2002,
The Very Large Telescope Interferometer: Challenges for the Future
Astrophysics and Space Science, v286, p119
Postscript File
J. Meisner, R. S. Le Poole
View Abstract
Published in proc. SPIE 4838
Postscript File
J. Meisner
In J.-P. Beaulieu, A. Lecavelier des Etangs, and C.
Terquem, editors, Extrasolar Planets: Today
and Tomorrow, volume 321 of ASP Conference Series,
pages 125–126, 30 June - 4 July
For much more detail on this subject than they let me write in the paper,
see my associated presentation.
Or you can read the two-page "paper"
that they allowed to be published.
Jeffrey Meisner
View Abstract
Published in the proceedings of the 36th Liege International Astrophysics
Colloquim (2001):
"From Optical to Millimetric Interferometry: Scientific and Technological
Challenges"
Postscript File
Jeffrey A. Meisner
View Abstract
Published in the proceedings for the
NOVA/LEIDEN/NEVEC/ESO/ESA Workshop on Space and Ground Based Optical/InfraRed
Interferometry, Leiden, The Netherlands, September 18-22, 2000
Postscript File
Spatial Filtering with Pinholes for MIDI
(unpublished)
Postscript File
Jeffrey Meisner
View Abstract
Published in Proc. SPIE 4006, pp 1068, Astronomical Interferometry,
Proceedings of the conference "Interferometry in Optical Astronomy,"
held March 26 - 29, 2000, Munich, Germany
Postscript
File
Richard Roosen, Jeffrey Meisner
View Abstract
Working on the Fringe: Optical and IR Interferometry from Ground and Space.
Proceedings from ASP Conference Vol. 194. Edited by Stephen Unwin and Robert Stachnik.
p.303
("Working on the Fringe" conference, sponsored
by JPL, May 24-27 1999, Dana Point, CA)
Postscript File
Jeffrey Meisner
ASP Conference Series #174, p193 (1999)
(Contribution to symposium on interferometry and adaptive optics, ASP
meeting, June 28 - July 1 1998, Albuquerque)
Long unpublished Version in HTML format
(Depending on your
browser and installed fonts, you might have some problems with certain
characters in the text! However the display equations are in graphic format
and will come through fine.)
Published version: postscript file
View Abstract
Jeffrey Meisner
Paper in HTML format
(Depending on your
browser and installed fonts, you might have some problems with certain
characters in the text! However the display equations are in graphic format
and will come through fine.)
View Abstract
Proc. SPIE v 3350, p294 (1998)
Jeffrey Meisner
View Abstract
Optical Engineering v35, #7, pp. 1927-1935 (1996)
Scanned paper now downloadable
View Abstract
Online version in pdf format now available, courtesy Richard Mathar.
J. Meisner
VINCI is the test and commissioning instrument of the VLT interferometer (VLTI). It operates
in the K band (2.0 - 2.4 microns) using delay-scanning. Optical fibers serve as spatial
filters, beam combiner, and photometric pick- offs. Although intended as a test and
commissioning instrument, VINCI has produced many scientifically useful results during its 4
years of operation resulting in well over a dozen papers in refereed journals.
J. Meisner
We discuss methods of interferometric data reduction using coherent
integration of fringe visibility. Unlike incoherent estimation
techniques which discard the phase of interference, coherent integration retains
as a complex quantity the contribution from each frame (or scan). In order to
integrate these coherently, one must apply an OPD correction
(or ``phase reference'')
to compensate
for random atmospheric pathlength fluctuations.
Bakker, Eric J.; Meisner, Jeffrey A.; Percheron, Isabelle; Dominik, Carsten
We present direct detections of the spatial extent of the circumbinary disk around HR 4049 and its
companion. Observations were obtained with the ESO Very Large Telescope Interferometer using the VLT
Interferometric Commissioning Instrument (VINCI) at 2 micron and the Mid Infrared Instrument (MIDI)
between 8 and 12 micron. A single uniform disk model fit to the VINCI data gives an angular diameter
of 27 milli-arcseconds. After taking into account the contribution from an unresolved central star we
find that the observed visibilities indicate a second component with a spatial extent of 37 milli-
arcseconds (which is identified as the circumbinary disk). The MIDI interferometric spectra show
features which are due to PAH emission lines (8.6 and 11.3 micron). The visibilities of the emission
lines indicate that the spatial extent in the lines (50 to 60 milli-arcseconds) is larger than in the
continuum (35 to 45 milli-arcseconds). This leads us to propose a three emission components model to
explain the interferometric observations: a central unresolved star, a 37 milli-arcseconds
circumbinary disk and polar lobes emitting in the PAH bands with a size of 50 to 60 milli-arcseconds.
J. Meisner
There are several programs involving long baseline
interferometry in the search for dim exoplanets and brown
dwarfs orbiting around bright stars. These involve either
ground-based observations, or space- based observations
which will be more sensitive. Three techniques will be
employed: indirect detection using precision narrow-angle
astrometry; direct detection using nulling interferometry; and
direct detection using visibility amplitude and/or phase.
J. A. Meisner
Although primarily intended as a test and
alignment instrument in order to commission the
VLTI, VINCI has taken useful scientific data in its first year and a half of operation.
Our results employ coherent integration of fringe visibility in which the actual
amplitudes
of the raw scans are combined linearly after correcting for the position of the fringe
within each scan. In addition to reducing the effect of noise compared to incoherent
integration, the result contains a broader range of information, including
an estimate of the complex visibility spectrum.
J. Meisner, R. S. Le Poole
A wideband interferometer is sensitive to the effects of longitudinal dispersion which affect the
interfering light beams unequally. At shorter wavelengths the major effect of dispersion is from dry
air itself, while at mid infrared wavelengths the effect of water vapor is dominant. MIDI, the future
10 micron instrument of the VLTI, will experience significant effects from the imbalances in the
water
vapor content affecting the paths of the two interfering beams. This imbalance will include terms due
to the unbalanced air paths in the delay line, random atmospheric humidity fluctuations in the lines-
of-sight to the star, and random humidity variations inside of the VLTI delay line tunnels.
Jeffrey Meisner
The tracking of the white light (central) fringe in a broadband optical/IR
interferometer, allows the possibility of long coherent integrations of
fringe visibility. Fringe tracking involves the determination of absolute
OPD offsets (not just narrowband phase) and either real-time correction
of the interferometer's delay lines, or determination of the estimated
OPD time series to be applied to the interferometric data off-line to effect
coherent integration of fringe visibility. Algorithms of this sort will
be included in the data reduction software package being developed for
MIDI, the 10 micron interferometric instrument of the VLTI.
Spatial Filters for Astronomical Interferometry using Discrete Optical
Components
Jeffrey A. Meisner
Spatial filtering of received optical radiation is used to select a
single spatial mode from which interference can be obtained. By constraining
the radiation to a single mode, as is done by the antenna in radio interferometry,
a definite relationship is established between the initial power in the
interfering beams, the strength of the interference signal, and the underlying
correlation (fringe visibility), thus providing a means of calibration
which is insensitive to variations in the atmosphere. In the case of an
infrared interferometer operating at longer wavelengths where the system
noise level is dominated by background (thermal) radiation, the spatial
filter serves a second purpose: that of rejecting (as much as possible)
background radiation responsible for the noise level, while accepting most
of the coherent power in a single mode. These criteria are quantified in
order to evaluate the performance of proposed spatial filter designs.
Coherent estimation of complex fringe visibility: a generalized
approach,
Jeffrey Meisner
The measurement of fringe visibility (correlation) in a long baseline
optical/infrared astronomical interferometer may be performed through a
coherent integration of noisy data if a measurement of the phase producing
the interference is available. That phase reference can be applied in hardware
by rapidly servoing a delay line, or may be applied offline to a data stream
produced while the phase was being monitored. In a spectrally dispersed
detection system, the same data may be used for atmospheric delay tracking
as well as visibility estimation. In that configuration one measures not
only the magnitude, but the phase of the visibility. The squared magnitude
of the measured interference need never be formed, so that all averaging
is performed on the unbiased measurements of complex correlation, optimally
reducing the effect of measurement noise. However accurate results depend
on careful modeling of the system and correction for a number of effects
which are detailed in this paper. The results, supported by simulations,
are applicable to visibility determinations in a wide range of astronomical
interferometers utilizing analog detection or photon counting. In particular
these methods are being developed and proposed for use with the MIDI 10
micron interferometric instrument being built for the VLTI.
Richard Roosen, Jeffrey Meisner
Using the illumination of an arbitrary bright star, a schlieren optical
system at the focus of a .75m telescope has been successful in imaging
atmospheric turbulence. Such an apparatus translates the gradient of atmospheric
phase variations into intensity variations. A CCD camera captures snapshots
of the resulting pattern at a 60Hz frame rate.
Jeffrey Meisner
The performance of a long-baseline optical stellar interferometer is
greatly enhanced if the instantaneous atmospheric delay tau(t), can
be tracked to within a fraction of a wavelength, permitting coherent integration
of the optical correlation (fringe visibility). Real-time fringe-
tracking involves a control system that servos a rapidly responding path-length
compensator in real-time.
Jeff Meisner
Coherent integration of fringe visibility in an optical stellar interferometer
yields much higher signal-to-noise ratios is shorter integration periods
for dim objects. Furthermore coherent integration, if performed simultaneously
over multiple spectral channels, can yield direct determination of the
phase of the visibility function, a quantity that is lost in incoherent
integration. To perform coherent integration it is necessary to estimate
the random atmospheric delay to well within a wavelength. That determination
can be used either to compensate for the delay in hardware, or as a 'phase
reference' for integration of fringe visibility with the correction applied
in software. This paper primarily addresses the latter method, presenting
algorithms for the proper estimation of optical correlation given the interaction
between the statistics of the incoming light and the hardware. Of particular
importance is the effect of error in the estimation of the atmospheric
delay used as a reference phase. The author's previously published method
for estimation of the atmospheric delay achieves well modeled error levels.
In the case of very weak signal, even 'ambiguous' determinations of atmospheric
delay can be used for phase referencing. The delay estimator may employ
the same raw data used for estimation of fringe visibility.
Mode for Long-Baseline Interferometry
Jeffrey Meisner
A long-baseline optical stellar interferometer requires a minimum level
of optical power available from each arm in order to operate in the fringe-tracking
mode which enables coherent integration of fringe visibility. That optical
power must be concentrated in a single spatial mode in order to interfere
coherently. However atmospheric seeing places a limit on the amount of
optical power that will be accepted into a single spatial mode for apertures
much larger than the Fried parameter, thus placing a magnitude limit on
the coherent operation of the interferometer.
Doctoral dissertation, University of Minnesota, 1995
Jeffrey Meisner
A long-baseline optical stellar interferometer is capable of performing superior measurements of
optical correlation (or "fringe visibility") in much shorter observation times if the instantaneous
atmospheric delay tau (t), can be accurately tracked to well within an optical wavelength. This
permits coherent integration of the optical correlation, among other advantages.
Nova-ESO VLTI Expertise
Centre (NEVEC) homepage
VLTI Instrumentation
Optical Long Baseline Interferometry
Newsletter
Massive Page of Interferometry
Links (OLBIN)
Jeffrey Meisner
Postbus 9513
2300 RA Leiden
The Netherlands