David Woody , Mark Holdaway , Oliver Lay , Colin Masson , Frazer Owen , Dick Plambeck , Simon Radford , Ed Sutton 
October 3, 1995
Keywords: atmosphere, phase stability, phase calibration, fast switching, radiometric phase correction, paired array
A major challenge for the MMA will be obtaining high quality images in the presence of less than perfect seeing conditions. Even at the best known sites some technique for correcting for the tropospheric phase fluctuations will be required, especially for the highest resolution configurations. Without atmospheric phase correction or improved mapping algorithms the projected fraction of time available for 0.1arcsec imaging is ~4% on Mauna Kea, Hawaii, and ~10% at Cerro Chajnantor, Chile. Significantly more time is usable for lower resolution imaging using shorter baselines. Special mapping techniques such as self-cal or correction for known decorrelation will increase the fraction of useable time at the highest resolution but this will not apply to all projects and the sensitivity loss from decorrelation will not be recovered. Some method for improving the atmospheric seeing will be required for the MMA to achieve the full potential of the MMA.
Several methods are under development that should adequately correct for the atmospheric errors which are likely to be encountered by the MMA if it is located at a high mountain site. The most viable methods are those already being pursued by various members of this working group. These are fast phase calibration (FPC) and radiometer phase correction (RPC).
The idea of FPC is to observe an astronomical calibrator on time scales short enough that the atmosphere doesn't change significantly between calibrations and to use calibrators close enough to the target source that the path through the lower troposphere responsible for the phase fluctuations is essentially the same. This method will work very well if the array has sufficient sensitivity and can position switch fast enough to phase calibrate on sources within ~1.5deg of the target source in a few seconds of lapsed time. At this point these requirements seem possible, but have not yet been proven.
RPC is based upon the facts that the dominant source of path length variations at millimeter wavelengths is caused by fluctuations in the water vapor along the line of sight and that this water vapor can be measured by its continuum or line emission. This method puts the burden on producing radiometer/receiver systems with sufficient stability and on developing robust algorithms for converting emission temperature to path delay.
These two methods correct for somewhat complementary regions in the power spectrum of fluctuations with FPC correcting for fluctuations on time scales longer than the time period between calibrations while RPC works on time scales of ~1sec up to a time scale set by the radiometer stability. Implementing both techniques should cover most observing conditions and operating modes, but a successful use of either technique would mean that the MMA will live up to its proposed imaging capability.
 U. Illinois
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