Under the assumption of baseline independent Gaussian residual phase errors, such as might exist if Welch's total power monitor scheme or Woody's water vapor spectrometer scheme were employed, a simpler decorrelation correction might suffice. If the residual phase errors were antenna dependent or time dependent, then one of the decorrelation correction methods described here might improve the imaging.
In the case of fast switching, the residual phase errors are
equal to the square root of the phase structure function
for short baselines
and saturate at
a value of
for baselines longer than
the effective switching length
(Holdaway and Owen, 1995).
Since the decorrelation is baseline dependent under fast switching,
the decorrelation correction methods described above would be helpful.
Currently, it is believed that reasonable imaging with the 40 element mma should be possible with 30 degree rms phase errors, assuming the phase errors do not maintain some systematic value over long times. The 30 degree rms phase error per baselines specification comes from point source simulations (dynamic range = 200:1; Holdaway, 1992) and from sensitivity arguments (down to 0.87). These simulations show that the MMA will be able to make high fidelity, moderate dynamic range images of complex sources with rms phase errors of 70 degrees per baseline (the worst baselines in this simulation actually had rms phase errors of 100 degrees). The 70 degree phase errors will result in a stiff penalty in sensitivity since the decorrelation is down to 0.47 on the typical baseline. A modest resolution loss of 17% also occurs.
We propose that we have two levels of phase error specifications: