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The major limitation in the accuracy of the single dish mode will be
due to differential spillover in the ``on''-``off'' comparison. It is
difficult to know in advance whether the nodding secondary or the OTF
scanning scheme will be more plagued by this. The antenna background is
likely to be of the order of 10K. Whether the modulation of this by the
moving secondary or scanning across the ground is worse can only be
determined by experiment. Thus, it will be important that the prototype
antennas be equipped for both kind of observing and tests be carried out.
Both good gain stability
and chopping secondaries must be installed. It may be possible to test
these options on existing systems. It may also be possible to test
further the
``homogeneous array'' operation (array with single dish one of the array
antennas) using one of the existing antenna systems.
Another activity that would be important in the near term would be to
test the calculations of HOE with atmospheric data taken at higher
frequencies. The atmospheric conditions on Mauna Kea are probably close
enough to those at Chajnantor that JCMT observations would be useful for
this.
Among all the possible methods to obtain the total power data for the
array, the OTF scheme is the simplest and least expensive,
and it appears that it should
work. The main requirement is a fractional receiver gain stability of about
1 x 10-4 in a one second time interval for all the antennas.
There should be no difficulty in achieving this.
Next: 183GHz Water Vapour Radiometers
Up: Total Power Observing with
Previous: OTF Mapping
Al Wootten
2000-04-04