Problems with Homogeneous Array Simulations in ALMA Memo 488 Abstract
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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.
