Issue |
Astron. Astrophys. Suppl. Ser.
Volume 122, Number 3, May I 1997
|
|
---|---|---|
Page(s) | 547 - 557 | |
DOI | https://doi.org/10.1051/aas:1997155 | |
Published online | 15 May 1997 |
Phase calibration and water vapor radiometry for millimeter-wave arrays
Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena CA 91125, U.S.A.
Received:
17
May
1996
Accepted:
2
August
1996
Correcting for the fluctuations in atmospheric path length caused by
water vapor is a major challenge facing millimeter- and
submillimeter-wave interferometers, and one that must be overcome to
obtain routine sub-arcsecond resolution. Using the model for the
power spectrum of phase fluctuations developed in Lay (1997), the
existing technique of phase referencing to a bright calibrator object
is analysed. It is shown that the phase errors after calibration have comparable
contributions from both the target and calibrator measurements.
The technique of water vapor radiometry, where the amount of emission
from water vapor in the beam of each antenna is used to estimate a path
correction, is also examined. It is found that there are two levels on
which a correction can be made. The simplest corrects just the
fluctuations within each on-source period; the calibration requirements
for the radiometers are modest, and this partial
correction can give a substantial improvement in the resolution and
coherence time of an interferometer. The atmospheric fluctuations on longer
timescales remain uncorrected, however, and are significant. To remove
these, a full correction is required, which measures the change in the
path difference that occurs when moving between the calibrator and the
target, in addition to the on-source fluctuations. Since there can be
a large difference in airmass between the calibrator and the target,
measuring this change requires that the radiometers have the same
response to a given column of water vapor to within %. Two
possible methods of achieving this very stringent limit are outlined.
For reasonable observing conditions at 230 GHz, it is predicted that
the effective atmospheric "seeing" (the apparent smearing of the
sky brightness distribution due to the atmosphere) is improved
from
(phase referencing every 25 minutes) to
(phase referencing and partial radiometric correction). A full
radiometric correction would, in principle, restore perfect seeing.
Key words: atmospheric effects / instrumentation: interferometers / site testing / techniques: interferometric
© European Southern Observatory (ESO), 1997