m, a
goal for interferometry at 1 mm wavelength. An examination of linear
and nonlinear error sources shows that residual amplifier or detector
nonlinearity is likely to be the most significant instrumental limit
for atmospheric phase correction.
Fluctuating amounts of water vapor over an antenna in an
interferometer add phase shifts to the instrument, shifting the fringe
pattern on the sky to decrease signal correlation. Optically thin
line radiation from the same water vapor that causes the decorrelation
can be accurately measured, however, allowing an estimate of the
column of water above individual antennas. Knowledge of the water
column allows an equivalent but opposite phase to be inserted in the
data processing; this is the radio equivalent of adaptive optics.
This memo explores the instrumental requirements for a multi-channel
radiometer capable of measuring pathlength differences to 35
m, a
goal for interferometry at 1 mm wavelength. An examination of linear
and nonlinear error sources shows that residual amplifier or detector
nonlinearity is likely to be the most significant instrumental limit
for atmospheric phase correction.
View a PDF version of ALMA Memo #307.
Download a 84 kB gzipped postscript version of ALMA Memo #307 from
Last modified: May 04, 2000
kweather@nrao.edu