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Particularly at the shorter wavelengths, the ALMA will need to do mosaic
observing to cover large fields of view. Along with the mosaic pointing,
there will need to be total power maps to fill in the interferometric
short spacings and produce complete images. It is well known that this is best
done with a single antenna that is two to three times the diameter of the
interferometer antennas (Vogel,S.
,
ApJ, 1984, 283, 655).
However, that will not be possible for ALMA; there are no 24m - 36m antennas
available that will work well to 0.35mm wavelength. As
long as a mosaic of pointings is employed in the interferometry, a single
antenna map made with one of the interferometer antennas will suffice in
principle (Ekers,R. and Rots,A, 1979, In Image Formation, etc., Dordrecht, Reidel).
This is rarely done,
largely because the interferometer antennas are not equipped to do it. Tests done
at the VLA at cm wavelengths (Cornwell, T. 1988, A&A, 202, 316.) indicate
that it should work, and at mm wavelengths in the CO(1-0) line, Marc Pound
made a good map of the Eagle Nebula combining a Mosaic interferometer map made
with the BIMA array and a single antenna map made with the Bell Labs 7m antenna
(Pound, 1998, ApJ, 493L, 113). This capability must be in place for the ALMA
antennas. How it is best done may be studied with the prototype antennas.
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Al Wootten
2000-04-04