| III. Review Paper: SITE CHARACTERIZATION FOR mm/SUBmm ASTRONOMY Simon Radford |
At millimeter and submillimeter wavelengths, pressure
broadened molecular spectral lines make the atmosphere a
natural limitation to the sensitivity and resolution of
astronomical observations. Tropospheric water vapor is the
principal culprit. The translucent atmosphere both decreases
the signal, by attenuating incoming radiation, and increases
the noise, by radiating thermally. Furthermore, inhomogeneities in the water vapor distribution cause
variations in the electrical path length through the
atmosphere. These variations result in phase errors that
degrade the sensitivity and resolution of images made with
both interferometers and filled aperture telescopes.
Because they often show only a poor correlation,
measurements of both atmospheric transparency and stability
are necessary for site characterization. Transparency is
inferred from the atmospheric brightness measured with
tipping heterodyne radiometers, broad band photometers, and
Fourier transform spectrometers are used to measure the
atmospheric brightness. Atmospheric stability can be
measured directly from the phase fluctuations seen when
observing a convenient beacon outside the beacon, i. e., on
a geostationary satellite, with a simple interferometer.
Stability can also be inferred from atmospheric brightness
fluctuations.
To evaluate its candidate site near Cerro Chajnantor,
Chile, the ALMA project has conducted an extensive testing
campaign over the past five years. These measurements
indicate Chajnantor is an excellent location for millimeter
and submillimeter wavelength astronomy. |
| III.1 220 GHz TIPPING RADIOMETER FOR MONITORING ATMOSPHERIC OPACITY AT HANLE OBSERVATORY P.G.Ananthasubramanian |
At Hanle a high altitude Himalayan site, an Indian Astronomical Observatory (IAO) is being set up
by Indian Institute of Astrophysics (IIA). The GPS co-ordinates of Hanle are Latitude - 32Deg 46' 46.3"(N),
Longitude - 78Deg 57' 51.1"(E) & Altitude- ~ 4470 m. A 2 m aperture Himalayan Infrared Optical Telescope (HIROT)
is being set up now. In order to characterize this site for mm and sub-mm telescopes, a 220 GHz tipping radiometer
was installed towards the end of December, 1999. This instrument is continuously sampling the sky once every
10 mins since then. We plan to collect the opacity data continuously for about two years. The radiometer has been
set up in collaboration with University of Tokyo, Raman Research Institute and Indian Institute of Astrophysics.
The radiometer consists of an off-axis 80 mm aperture parabolic mirror, with the front end-receiver at its focus and a personal computer for control and data acquisition. The received sky emission from the atmosphere at 220 GHz is
down converted to 1.5 GHz in a sub-harmonic mixer, total power detected in about 500 MHz bandwidth and
digitized for storing in personal computer. The parabolic mirror with a beam of about a degree, scans the sky from
nearly one horizon to the opposite horizon through zenith, at a particular azimuth position. At one horizon is a reference absorber (black body) for calibration. A forward scan and a reverse scan is taken once every 10 minutes and the data is averaged to remove the effect due to small gain variations in the system. This averaged data is used to fit for zenith opacity. From the detected total power voltage as a function of zenith angle, one fits for the zenith opacity assuming a sec(z) variation for the atmospheric opacity and the temperature of the absorber to be the same as the atmospheric medium temperature.
Preliminary results for December 1999 and January 2000 indicate a zenith opacity value in the range of about
(0.05 to 0.1), +/- 0.02. In this forum we plan to present the opacity results from December 1999 to September 2000.
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| III.2 COMPARING RADIOSONDE AND OTHER TEST DATA FROM CHAJNANTOR B.J. Butler, S.J.E. Radford, A. Otarola & G. Delgado |
The Atacama Large Millimeter Array (ALMA), is scheduled
to begin construction in the next few years. A high site
(5000 m) in the northern Chilean Andes (near Cerro
Chajnantor) has been selected for the location of the
array, based on site testing data which has been acquired
since 1995. Since October 1998, we have been launching
radiosondes in order to probe the atmospheric structure
(temperature, water vapor, and winds) above the site. I
will discuss these sonde launches, the data which comes
from them, their comparison to other data taken during the
launches, and their overall utility in terms of site
evaluation. The comparison data include a 220 GHz tipper,
a 350 micron tipper, two site testing interferometers,
and various surface weather instruments. |
| III.3 COMPARISON OF SATURATED WATER VAPOR COLUMN FROM RADIOSONDE, mm AND SUB-mm RADIOMETER OPACITIES AT THE SOUTH POLE Richard Chamberlin |
The geographic South Pole is being developed as an outstanding site for submillimeter astronomy. In the 1990's,
millimeter and sub-millimeter measurements of the atmospheric opacity have been made throughout the winter months covering the wavelength range from 1.3mm to 0.35 mm. (R.A Chamberlin, J. Bally, App Optics, 33(6), p. 1095-1099 (1994); R. A.Chamberlin, A. P. Lane, A. A. Stark, Astrophy. J.,476, p. 428-433 (1997); S. Radford, NRAO, unpublished 0.35mm measurements,(1998-2000) ).
These ongoing opacity measurements span almost a decade. However, they are not continuous: there is very little information about the South Pole sub-mm opacity prior to 1992; and, not all wavelength ranges are covered by opacity measurements for each year.
As part of characterizing the South Pole site quality, we made comparisons with saturated Precipitable Water Vapor (sPWV) derived from concurrent radiosonde measurements and found significant correlations with mm and sub-mm opacity data sets in every case. (sPWV is based only the atmospheric temperature profile and represents the maximum water vapor that a saturated atmosphere could hold. Using sPWV instead of PWV avoids problems associated with inconsistent hygrister response often found in different radiosonde instruments.)
Since the record of radiosonde observations at the South Pole goes back to 1961, and since we can solidly correlate our existing sub-mm opacity data sets with sPWV, we are able to infer historical site quality. We find that the outstanding submillimeter site quality observed in the 1990's prevails for every year on record going back to 1961. We note that this year-to-year consistency of outstanding site quality is unusual when compared to middle-latitude high
mountain sites. |
| III.4 CROSS-CALIBRATION OF A GPS WATER VAPOUR MONITOR M.R. Kidger, N. Pinilla, J.M. Rodriguez-Espinosa |
We describe the programme of calibration measurements of the GPS-based water vapour monitor which has been operating in the Roque de los Muchachos Observatory (La Palma, Canary Islands) since December 1999. GPS water vapour measurement is a relatively recent technique and has so far be used exclusively in regimes of very large zenital column, usually in excess of 20-mm. By comparing the GPS results with data taken from systematic water vapour monitoring with absolutely calibrated visible photometers and a Kitt Peak radiometer we are able to test the accuracy of the GPS system. We find that the GPS water vapour monitor provides accurate results even in extremely dry mountaintop conditions where the total column of precipitable water vapour is often less than 2-mm. We thus demonstrate its potential as a cheap system that can be used in remote mountain and desert areas to monitor water vapour automatically, without the need for an operator.
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| III.5 RELATION BETWEEN 183GHz WATER VAPOR LINE AND WATER CONTINUUM ABSORPTION MEASURED WITH FTS Satoki Matsushita |
For the phase correction in the millimeter and submillimeter-wave,it is important to estimate the effect of water vapor in the atmosphere.
We have analyzed Fourier Transform Spectrometer (FTS) measurement data of atmospheric opacity (150-1600GHz) taken at Pampa la Bola (4800m altitude) in northern Chile on September 1997 and June 1998.
We have identified water vapor and liquid water components in measured spectra, and successfully separated the two components.
The 183GHz water vapor line is also covered in the measured spectra.
Although frequency resolution is not enough to measure the line shape, we could measure the integrated line intensities.
The separated water vapor opacity component shows good correlation with the 183GHz line opacity, but the liquid water opacity component shows no correlation.
These clearly support the success of the separation.
The correlation between the water vapor component and the 183GHz line opacities indicate the saturation of the 183GHz line even in this high altitude site.
The separated liquid water component show its effect only on millimeter-wave region.
These results indicate that the wing opacity of the 183GHz line (avoid the saturated line center) or submillimeter continuum opacity is useful for the phase correction.
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| III.6 THE ATMOSPHERE LONGWAVE SPECTRUM: STATE-OF-ART IN THE HORIZON OF ALMA Juan Pardo |
The Atacama Large Millimeter Array will be the key instrument
for mm-submm Astronomy in the coming years. Although a high, dry site in the Chilean Atacama desert has been selected, the knowledge and precise modeling of the atmospheric effects on the incoming electromagnetic
field is crucial for the quality of the research that can be done with
the instrument. Millimeter-wave observatories up to date have been using
atmospheric corrections mostly relying on old models that can in no way
be applied to higher frequencies. |
| III.7 METEOROLOGIVAL CONDITIONS MEASURED AT THE IRAM 30m OBSERVATORY, SKY OPACITY AND ANOMALOUS REFRACTION Juan Penalver, Albert Greve & Wilhelm Altenhoff |
The IRAM 30m Telescope is located in the Sierra Nevada, Spain, at 3000
meters altitude. Since 1985 it is dedicated to carry out
radioastronomical observations at wavelengths from 3 mm. to 1.3 mm.
The meteorological conditions are very important for the working
frequencies of the Telescope, since the quality of radioastronomical
observations is very dependent on the cleanliness of the atmosphere
(clouds, H2O, anomalous refraction). The best results are obtained at
low contents of water vapor in the atmosphere.
During the years of operation of the Telescope we have been continuously
recording the meteorological parameters that permit us to characterize
the quality of the site for the astronomical observations carried out.
Comparisons between meteorological data are done with the opacity
measured astronomically by dedicated skydip observations.
The anomalous refraction is a phenomenon that disturbs the pointing of
the Telescope. We present the technique developed that permits us to
measure and quantify the magnitude of the pointing variations produced
by anomalous refraction. |
| III.8 FTS MEASUREMENTS OF BROADBAND SUBMILLIMETER OPACITY TERMS Eugene Serabyn |
Using a Fourier transform spectrometer on Mauna Kea, a data base of atmospheric opacity curves covering the full submillimeter domain accessible from the ground is being collected. Careful attention to the details of the calibration procedure and the inclusion of various correction terms allows the acquisition of transmission curves calibrated absolutely to 1-2% across the full instrumental passband. The calibration steps involved will be discussed, as will our driest data sets, which allow for the unambiguous separation of the dry and wet quasi-continuum terms.
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| P.III.1 (poster) PROSPECTS FOR A NEW VLBI ANTENNA AT THE ISLAND OF MADEIRA Pedro Augusto |
A telescope at Madeira is a great opportunity to dramatically increase the quality of Very Long
Baseline Interferometry (VLBI) observations, those of the European VLBI Network, in particular.
It would allow the closing of an old infamous 'mid-Atlantic gap' that exists in current VLBI
observations.
We have selected three good sites for more detailed local studies in the future,
one of which should be chosen as the place to build a new radio telescope. A summary of preliminary
results from the studies being undertaken will be presented (meteorological, radio interference, soil
and ground surveys and horizon profiles). |
| P.III.1 (poster) OPERATION OF THE IAC GPS WATER VAPOUR MONITOR IN OBSERVATORY CONDITIONS N. Pinilla, M.R. Kidger, J.M. Rodriguez-Espinosa |
We describe the operation of the GPS-based water vapour monitor in the Roque de los Muchachos Observatory (La Palma, Canary Islands). The installation of the monitor is described as well as its practical operation and the steps that have been taken to automate its operation. We describe in detail the programme of verification and tests that have been carried out. We describe some of the problems that have been found with its operation, along with the steps taken to solve them in an isolated observatory environment, showing a sample of results from the water vapour monitoring campaign with the monitor that started in February 2000.
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