[alma-config] Imaging Simulations

[John Conway]

I have been doing some CLEAN imaging simulations comparing
snapshot and long track imaging with a spiral zoom array, 
for the case of two realistic images.

The results can be found at
http://www.oso.chalmers.se/~jconway/ALMA/SIMULATIONS/ 
Let me know if you have any problems reading it or if
reads too slow.

The title and abstract are given below.

  Cheers
      John.


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TITLE  Imaging Simulations with a Spiral Zoom Array; 
Comparisons with Ring Arrays 


ABSTRACT 

Imaging simulations have been carried out with a version of the spiral
zoom array presented in ALMA memo 283. Two test images were
used, one based on a VLA image of Cygnus A and another based on an OVRO CO
map of M51. Both snapshot and long track CLEAN
simulations were carried out. The results were compared with those
obtained for a ring array giving similar resolution. In each case the
spiral array gave better imaging performance. The difference was
particularly large for the case of the long-track observations when the
rms errors were 10 times larger for the ring than for the spiral arrays.
This large difference arises because the large near-in sidelobes of ring
arrays, caused by the sharp edge to the uv-coverage, persists even after a
long track. 

These simulations illustrate that it is important to consider that
deconvolution requires both extrapolation of the uv data as well as
interpolation between uv points. A naturally tapered uv coverage such as
that provided by a spiral zoom array provides more constraints on
the necessary extrapolation than for the case of ring arrays. More imaging
simulations must be done before final conclusion can be drawn;
however these initial tests show that, at least for a broad class of
realistic images, spiral zoom arrays can have superior imaging
performance as well as superior observing efficiency as compared 
to fixed ring arrays.