[alma-config] Imaging Simulations

[Min Yun]

Hi John,

It is a nice and substantial work.  I still have not gotten back
to you on my comments to your ALMA draft memo, and I will return
those comments along with more detailed comments on the current
one.  Just repeating Bryan's point, the comparison will be more
meaningful if you used a double ring configuration rather than
a single ring.  If you do not have an example of such a configuration,
you can pick it up as www.aoc.nrao.edu/~myun/mma/64_DONUT_0.25 --
this is an ascii table with 64 antennas arranged in two rings,
one with a 3km diameter and one with 1.5km diameter.  It is part
of the summary of my URSI Toronto talk 
(www.aoc.nrao.edu/~myun/mma/straw_config.html).  I am glad to hear
that you are also addressing the issue on snapshot vs. long track.
It is one of the topics I wanted to cover for tomorrow's telecon.
I hope you will be able to join us.  Cheers!



					-- Min



> From owner-alma-config at kochab.cv.nrao.edu Tue Jan 18 05:12 MST 2000
> Date: Tue, 18 Jan 2000 13:09:01 +0100 (CET)
> From: John Conway <jconway at oso.chalmers.se>
> To: alma-config at cv3.cv.nrao.edu
> cc: jconway at oso.chalmers.se
> Subject: [alma-config] Imaging Simulations
> MIME-Version: 1.0
> 
> 
> 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.
> 
> 
> ---------------------------------------------------------------
> 
> 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. 
> 
>