[alma-config] Imaging simulations for ACA

[Stephane Guilloteau]

-----Original Message-----
From: Kouichirou MORITA <morita at nro.nao.ac.jp>
To: alma-config at nrao.edu <alma-config at nrao.edu>;
mmaimcal at polaris.cv.nrao.edu <mmaimcal at polaris.cv.nrao.edu>
Date: Tuesday, December 12, 2000 2:31 AM
Subject: Re: [alma-config] Imaging simulations for ACA


Hello Koh-Ichiro and Melvyn,

    Just a few quick comments on the points you mentionned.
>
>Thank you for your comments.
>
>> I like your conclusion that 6 m
>> antennas would provide better performance on wide field imagings
>> than with 8 m antennas. !

    Well, you may have to input calibration precision before concluding
completely
on that. It will be somewhat worse for 6-m than 8-m... Also, the close
packing
constraint may be slightly better for say 7-m than 6-m (relative values).

>>
>> some comments and questions.
>>
>> 1. You show several configurations of 6m antennas -
>> which configuration appears to be best ?
>
>Among configurations used these simulations, 18 x 6 m ACA
>showed the best performance.  But I have an indication
>that imaging performance of ALMA+ACA depends on sidelobe
>level of ACA.  The shape of these configurations is too
>regular, so some randomizations to reduce the
>sidelobe level are needed, I think.

    Could Leonia Kogan work out some optimization starting from
the regular hexagonal packing for 7, 12 and 18 antennas ? These
are likely numbers for 8-m, 7-m and 6-m diameters ? That would be helpful
for all of us.


>
>
>> 2. " smoothed
>> total power data instead of raw total power data from 12 m
>> antennas. Improvements of imaging quality have been obtained
>> by this method."
>> -- so do you efffectively use only up to ~ 6m spacings from the
>> 12m antennas ?
>
>Yes.  But, I should test different beam width for the smoothing.

    In fact, this is the only way to minimize the effect of pointing errors
from the single-dish measurement. These result in phase errors proportional
to the UV distance, so improvement is obtained by only retaining the
shortest
spacings from the 12-m single-dish.  With 12-m SD, 6-m ACA, 12-m ALMA,
forgetting about close packing constraints, you would cover 0-4 m from the
12-m
antennas, 4-8 m from 6-m ACA, and 8-12 m from the 12-m ALMA. Adding
close-packing
at 1.25 D, this becomes 0-5 m, 5-10 m, 10-14 m.
    So I guess smoothing to the equivalent of a 5-m dish will give you nice
results.



>
>> 3.  In calibrating the compact array, I think it's reasonable to use
>> nearby 12m antennas as part of a calibration.
>
>I agree with you.

    Me too. Having say 3 or 4 12-m antennas that you cross-correlate with
the ACA
will help considerably the calibration. Rafael Moreno and myself have a memo
ready
to go (Christmas present...) on these calibration issues.  You would then
use the 12-m
antennas as Single-dish for the short spacings while the ACA is observing.
This also
provides a nice sensitivity match.

        Stephane