| IV. Review Paper: COMPENSATION SCHEMES FOR TROPOSPHERIC PHASE INSTABILITY David Woody |
Correcting for the atmospheric phase fluctuations at millimeter
and sub-millimeter wavelengths is one of the major challenges
that must be overcome to realize the full potential of current and
future interferometer arrays. This review will describe the
characteristics of "seeing" as it pertains to radio
interferometric observations and discuss several of the methods
currently in use or being developed to improve the maps
obtained using millimeter wave interferometers.
The primary cause of phase variation at
these wavelengths is the variation in the column density
of water vapor over the array. Several methods and observing
strategies to improve image quality will be discussed,
with emphasis on the use of radiometers to measure the
column density of water vapor along the line of sight of each telescope
in the array. These radiometers provide a real time measurement
of the water vapor induced path delay through the atmosphere
that can be applied to the interferometric phase output from
the array. Preliminary results show that the image
quality can be improved significantly using these techniques.
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| IV.1 COMPARISON OF TROPOSPHERIC PHASE FLUCTUATIONS WITH A 22GHz WATER VAPOR LINE MEASUREMENTS ALONG A SINGLE LINE OF SIGHT Yoshiharu Asaki, Masato Ishiguro, & Hideyuki Kobayashi |
Tropospheric phase fluctuation due to the water vapor content is one of difficult problems which degrade imaging performances of radio interferometry. One of the potential solutions is differential radiometry observations to measure the differential water vapor content along the lines of sight. We developed a 22-GHz line radiometer to be mounted on a ground data-link antenna which
supplies the timing reference signal for a space VLBI satellite, HALCA. The satellite was launched by the Institute of Space and Astronautical Science (ISAS) in 1997 to conduct international space VLBI experiments. The phase transfer system for HALCA supplies a highly stabilized frequency standard from the data-link antenna. As the astronomical data is transmitted back from the satellite to the link antenna, the round-trip phase can provide unique experimental measurements to investigate tropospheric delay along a single line of sight. This phase transfer system combined with the radiometer will allow us to compare directly the atmospheric phase fluctuation with the water vapor content along the single line of sight measured by
the radiometer |
| IV.2 ATMOSPHERIC PHASE CORRECTION FOR INTERFEROMETRY AND VLBI Michael Bremer |
In interferometric observations with the Plateau de Bure
Interferometer, operated by the Institute de Radio Astronomie Millimetrique (IRAM), we apply atmospheric phase corrections derived from calibrated power measurements of the line-of-sight atmospheric emission in the 210-245 GHz window. This method is only reliable for atmospheric water vapour but not for clouds.
This method is available since 1995 after the development of
astronomical receivers with a relative stability of 2E-04. Currently, only the improved amplitudes of the interferometric visibilities are used, as total power assisted phase tracking between calibrator and target
is not yet reliable.
Setting the modeled phases to zero average on a source observation can improve the results, and may even come close to an absolute (i.e. phase tracking during source changes) technique. The potential of this method is illustrated with an example of a map without and with phase correction.
An absolute phase correction scheme with cloud correction based on radiometric measurements at the 22 GHz water vapour line is currently under development at IRAM. A prototype receiver for this purpose has been built and tested. Some specially adapted stability criteria were
employed to qualify the instrument.
At the IRAM 30m telescope, the 200 GHz sky emission has been used for phase monitoring during recent intercontinental VLBI experiments. The status of this project will be reported.
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| IV.3 PHASE CORRECTION OF 12GHz INTERFEROMETRIC MEASUREMENTS USING A PAIR OF 183GHz RADIOMETERS AT CHAJNANTOR Guillermo Delgado |
Spatial variations of the inhomogeneous distribution of water vapour in the
atmosphere introduce phase variations to a plane wave front at millimetre
and sub-millimetre wavelengths, degrading the imaging quality possible to
achieve with an interferometer telescope.
Among other proposed methods of correcting these phase variations, a
promising technique is the monitoring of water vapour variations by
brightness temperature measurements using suitable radiometers.
Two radiometers operating near the water vapour line at 183 GHz have been
installed at Llano de Chajnantor in Northern Chile at an altitude of 5.000
m. They are situated at the ends of a 300-m baseline of a 12 GHz
interferometer.
Care has been taken to have the radiometers observing the same path of
atmosphere as the interferometers, with both beams matched as close as
possible.
Here we present results of phase correction, discussing the error sources
and possible limitations of the technique as applied at Chajnantor.
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| IV.4 PHASE CALIBRATION WITH VLBI Abderrahmane Mezaoui |
In VLBI , telescopes are separated by such large
distances that atmospheric fluctuations are different.
Moreover, the angular resolution is so high (often
lower than the m.a.s) that generally ther is not a
radio source which can be used as a calibrator in the
neighbourhood . The solution to this difficulty is to
calculate completely random phases, then by using
closure phases one eliminates the shifts introduced by
atmospheric fluctuations. Nowadays, in VLBI technique
the phase error due to the terrestrial atmosphere
predominate on the others error sources. We shall
explain how with the aid of an initial model, we can
recover partially the phase information and so to map
extragalactic sources the OVV quasar 3C454.3. The use
of the radio source confinement principle or the
brightness positivity makes the process convergence faster.
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| IV.5 ANOMALOUS REFRACTION MEASUREMENTS AT FCRAO AND TIP-TILT COMPENSATION ON THE LARGE MILLIMETER TELESCOPE Luca Olmi |
Radio seeing shows up on filled-aperture telescopes as
an anomalous refraction (AR), i.e. an apparent displacement of a radio source from its true position. The magnitude of this effect, as a fraction of the beam width, is
bigger on larger telescopes, and thus its impact on the pointing is likely to become critically important in the next generation of electrically large filled-aperture radio telescopes (D/wavelength > 10E+4) and in particular on the Large Millimeter Telescope (LMT). AR effects are expected
to reduce the total effective observing time at the highest frequencies and will affect on-the-fly mapping, which will become the most common observing mode on large telescopes.
Here we present the results of systematic AR measurements carried out at FCRAO. We will discuss the structure functions and powera spectra of AR as well as their correlations with the observing parameters.
Using a model study of AR effects, by producing simulations of two-dimensional phase screens, we then discuss the basic design and simulate the operation of a tip-tilt compensation method at millimeter
wavelengths for the LMT that would use a scanning 183~GHz radiometer. Finally, we will also present the current
evaluation of the LMT site. |
| IV.6 PHASE FLUCTUATION AT THE ALMA SITE AND THE HEIGHT OF THE TURBULENT LAYER Yasmin Robson & al. |
Site-testing for the Atacama Large Millimetre Array (ALMA)
is well under way, on the Chajnantor plateau at an altitude
of 5000m, in the Atacama Desert of Northern Chile. Even on
such a high, dry site, the inhomogeneous distribution of
water vapour in the atmosphere causes significant phase
fluctuations at millimetre wavelengths; a knowledge of the
fluctuation timescales and the height of the turbulent water
vapour is an essential requirement when designing phase
correction schemes for the array. Although radiosonde data can reveal the presence of any distinct inversion layer, we have been investigating a second technique to evaluate the height of the turbulence using the 11 GHz site test interferometers. Two such instruments measure the atmospheric phase fluctuations by monitoring the CW signals from two different geostationary satellites. If the turbulence arises in a thin, "frozen" layer with a known wind velocity, then the time lag between the two phase measurements (found by cross-correlating the two data sets) can be used to estimate the height of the layer. We will present recent data from Chajnantor in a variety of weather conditions and an analysis of the derived turbulent scale heights. The implications for the design of the ALMA phase correction scheme will be discussed. |
| IV.7 ATMOSPHERIC PHASE CORRECTION USING 22GHz WATER LINE MONITORS David Woody |
Turbulence and fluctuations in atmospheric water vapor
limit the image quality and fidelity of maps produced
by millimeter interferometer arrays. Radiometers have
been developed and deployed on the Owens Valley Millimeter
Array to measure the emission from the 22 GHz water line
and are used to correct for the water vapor induced
phase errors. The radiometers have three bands
centered at 19, 22 and 25 GHz to isolate the emission
of the water vapor from contaminating sources of
continuum emission such as liquid water in clouds
and ground pickup. This technique has improved the
coherence from as low as 30 % in bad weather to
better than 80%. Tests have demonstrated that good
sub-arcsec images can be obtained during seeing
conditions that were previously unusable.
The first set of radiometers utilized room temperature
amplifiers and had residual delay errors of
100-200 microns. A new set of radiometers with cooled
amplifiers is under construction that will significantly
reduce the residual delay errors. Correction
techniques such as the one described here will be
required to obtain the highest quality maps at even
the best millimeter and sub-millimeter sites in the world. |
| IV.8 WATER LINE AT 183 GHz IN THE SPOT LIGHT Martina C. Wiedner |
Water Vapour Monitors operating at 183GHz (WVM) (Wiedner,
1998) were used successfully for phase correction in submillimetre
interferometry.
In order to further check the reliability of the
WVMs, to test the Atmospheric Transmission at Millimetric
and submillimeteric wavelengths (ATM) model (Pardo, 96),
and to investigate the shape of the 183 GHz line,
simultaneous measurements of the atmosphere were taken at
the Caltech Submillimeter Observatory with
the Water Vapour Monitor and the Fourier Transform
Spectrometer (FTS) (Serabyn and Weisstein, 96).
By fitting the ATM model to the FTS measurements the
amount of precipitable water vapour was retrieved and
the flux in three different frequency channels at the
wing of the 183GHz line calculated. These flux predictions
were compared to the measurements of the WVM.
Measurements and calculations agree roughly,
but not perfectly. The discrepancies might be explained
by different distributions of water vapor in the atmosphere. -Pardo, J. R. 1996, Etudes de l'atmosphere terrestre au moyen d'observations dans les longueurs d'onde millimetriques et submillimetriques, PhD Thesis, Universite Pierre et Marie Curie, France -Serabyn, E., Weisstein, E.W. 1996, Appl. Opt. 35, 2752 - M.C. Wiedner 1998, Atmospheric Water Vapour and Astronomical Millimetre Interferometry, PhD Thesis, University of Cambridge, UK |
| IV.9 IRMA - AN INFRARED WATER VAPOUR RADIOMETER FOR CORRECTION OF PHASE ERRORS IN MILLIMETER ASTRONOMICAL ARRAYS D. A. Naylor(1), G. Smith(1), P.A. Feldman(2), L.W. Avery(2) (1) University of Lethbridge, Alberta, Canada (2) National Research Council of Canada |
The phase distortions caused by rapid variations in atmospheric water vapour pose
serious problems for the operation of millimetre and submillimetre astronomical arrays.
It is important to monitor the atmospheric water vapour content with sufficient accuracy
and on short enough time scales to permit corrections to these distortions. This paper
describes an Infrared Radiometer for Millimetre Astronomy (IRMA), operating in the 20um
wavelength region, that is designed to meet these requirements. A prototype instrument,
deployed at the JCMT on Mauna Kea, is collecting data under remote control from the
University of Lethbridge. The instrument is compact, lightweight, mechanically robust,
and poses no threat of RF or submillimetre interference to astronomical receivers.
Initial analysis of the data from the prototype in Hawaii indicates IRMA is
achieving a (1-sigma) sensitivity of +/- 0.5um of pwv in 1 second of integration.
This corresponds to 3um of optical path difference within the IRMA beam, considerably
better than the current specification for ALMA phase correction of 11um optical path
difference.
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