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<h1><b><font size=3 face="Times New Roman"><span style='font-size:12.0pt;
font-weight:normal'>ALMA</span></font></b><font size=3><span
style='font-size:12.0pt;font-weight:normal'> MEMO #489 <br>
<br>
Problems with the Homogeneous Array Simulations in ALMA Memo 488 </span></font></h1>
<h2><b><font size=3 face="Times New Roman"><span style='font-size:12.0pt;
font-weight:normal'>M.A. Holdaway <br>
<br>
2004-03-23</span></font></b></h2>
<p><font size=3 face="Times New Roman"><span style='font-size:12.0pt'>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 </span></font>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. <br>
<br>
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. <br>
<br>
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. </p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>View a pdf version of ALMA Memo #489. </span></font></p>
<p class=MsoNormal><u><font size=2 color=blue face="Courier New"><span
style='font-size:10.0pt;font-family:"Courier New";color:blue'><a
href="http://www.alma.nrao.edu/memos/html-memos/alma489/memo489.pdf">http://www.alma.nrao.edu/memos/html-memos/alma489/memo489.pdf</a></span></font></u></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> </span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> </span></font></p>
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