[asac] [almanews] ALMA Memo #332 Released

[Carolyn White]

ALMA Memo No. 332

PHASE CORRECTION OF INTERFEROMETER DATA AT MAUNA KEA AND CHAJNANTOR 

Guillermo Delgado, Angel Otárola, Lars-Ake Nyman, et al. 

11/00

We describe here a method to correct interferometer phase variations due
to fluctuations in the atmospheric water vapour. The fluctuations are
estimated by fitting an atmospheric water line model to antenna
temperature values measured at frequencies near the water vapour line at
183 GHz. The fitting is done with the atmospheric water vapour density as
a variable, iterating until agreement between observation and prediction
is achieved. From the computed value of water vapour density, an
integrated value for the precipitable water vapour (PWV) contained in the
atmospheric column is derived.

The variations on the amount of PWV are directly related to electrical
path length variations of an electromagnetic wave travelling through the
atmosphere. This quantity, at millimetre and sub-millimetre frequencies,
can be linearly related to a phase variation in an astronomical signal
arriving to the elements of an interferometer.

This method to estimate the PWV has been used before to analyse the data
from a 183 GHz radiometer operating at Llano de Chajnantor, the selected
site for the Atacama Large Millimeter Array (ALMA), creating a time series
of the PWV in the area. The derived values have been previously correlated
with observations of atmospheric opacity at 225 GHz and radiosonde
measurements, with good results [1].

Here we apply this radiometric method of phase variation estimation to two
real cases. One is the data from interferometric observations at Mauna Kea
between the James Clark Maxwell Telescope (JCMT) and the Caltech
Submillimeter Observatory (CSO), operating at 356 GHz. The second case is
at Chajnantor, where we have a 300-m baseline interferometer operating at
11.2 GHz. In both cases we compare the phase estimated by radiometry and
the interferometer phase. Figures for correlation and achieved phase
correction are given. Some error sources are discussed.

View a PDF version of ALMA Memo #332 at
   http://www.alma.nrao.edu/memos/html-memos/alma332/memo332.pdf

Download postscript version of ALMA Memo #332 from
   http://www.alma.nrao.edu/memos/html-memos/alma332/memo332.ps
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