[asac][almanews] ALMA Memo 489 Released

[Stacy Oliver]

ALMA MEMO #489 

Problems with the Homogeneous Array Simulations in ALMA Memo 488 


M.A. Holdaway 

2004-03-23


ALMA Memo 488 seeks to justify the building of the ACA (an array of 12
7~m antennas designed to improve the short baseline (u,v) coverage)
based on its purported improvement in ALMA's imaging quality.
Specifically, simulations of four different model sources are performed
with the 64-element ALMA plus total power on the one hand (called
ALMA+SD in Memo 488, also known as the homogeneous array), and the
64-element ALMA plus the ACA plus total power on the other hand. In
several cases, adding the ACA appears to improve the image fidelity by a
factor ranging from about 2 to 10 over the image quality of the
64-element ALMA without the ACA. 

This micro-memo points out two problems with the way these simulations
were performed: first, the total power data was treated differently for
the ALMA+SD and the ACA cases, with insufficient total power data being
added to the ALMA+SD case; second, the observations are for a source
right at the zenith, which will be the absolute worst case for
homogeneous array mosaicing. 

We stress that homogeneous array mosaicing will often offer some
advantages over mosaicing with ALMA supplemented by ACA data, and
homogeneous array mosaicing will be a useful observing mode for ALMA.
Adding well-calibrated ACA data will never hurt ALMA imaging, but the
quoted improvement is overstated, and in many cases, thermal noise and
other errors will result in essentially no improvement from the ACA
data. 

View a pdf version of ALMA Memo #489. 

http://www.alma.nrao.edu/memos/html-memos/alma489/memo489.pdf

 

 

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