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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 #490 <br>
<br>
Effects of Atmospheric Emission Fluctuations and Gain Fluctuations on Continuum
Total Power Observations with </span></font><font size=3><span
style='font-size:12.0pt;font-weight:normal'>ALMA</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>
2004-03-30</span></font></b></h2>
<p style='margin:0in;margin-bottom:.0001pt'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>Atmospheric water vapor emits strongly at millimeter
wavelengths. Fast temporal and spatial fluctuations in the atmospheric </span></font></p>
<p style='margin:0in;margin-bottom:.0001pt'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>water vapor results in problems in removing the
atmospheric emission, which is done by very quick position switching or beam</span></font></p>
<p style='margin:0in;margin-bottom:.0001pt'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>switching. Most phase fluctuations seen with the 11.2
GHz site testing interferometer are due to fluctuations in water vapor, </span></font></p>
<p style='margin:0in;margin-bottom:.0001pt'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>so we can use the statistics of the phase fluctuation
measurements to infer how well the cancellation of variable water vapor </span></font></p>
<p style='margin:0in;margin-bottom:.0001pt'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>emission will be for a given observational strategy
and a given atmosphere. In fact, we have found a (non-optimal but sufficient)</span></font></p>
<p style='margin:0in;margin-bottom:.0001pt'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>way to match observations at all </span></font>ALMA
bands (1-10) to atmospheric opacity and stability conditions such that OTF
total </p>
<p style='margin:0in;margin-bottom:.0001pt'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>power continuum observations are essentially always
thermal noise limited and never limited by atmospheric fluctuations. </span></font></p>
<p style='margin:0in;margin-bottom:.0001pt'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>In addition to the problem of atmospheric
fluctuations, the total power continuum observations also must contend with </span></font></p>
<p style='margin:0in;margin-bottom:.0001pt'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>gain fluctuations, which in fact will limit these
observations. Gain fluctuations of 1e-4 in one second will just barely limit</span></font></p>
<p style='margin:0in;margin-bottom:.0001pt'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>total power continuum observations. However, gain
fluctuations of 1e-3 will result in residuals which are an order of</span></font></p>
<p style='margin:0in;margin-bottom:.0001pt'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>magnitude worse than the thermal noise limit. While
other considerations might drive the decision for the receiver stability</span></font></p>
<p style='margin:0in;margin-bottom:.0001pt'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>specification, it should be noted that a specification
of 1e-3 in one second, or worse, will cause problems for </span></font>ALMA </p>
<p style='margin:0in;margin-bottom:.0001pt'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>continuum observations of large objects. All
simulations were performed in AIPS++, and several reusable glish tools have
been</span></font></p>
<p style='margin:0in;margin-bottom:.0001pt'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>written. </span></font></p>
<p style='margin:0in;margin-bottom:.0001pt'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'> </span></font></p>
<p style='margin:0in;margin-bottom:.0001pt'><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'>View a pdf version of ALMA Memo #4<font color=navy><span
style='color:navy'>90</span></font>. </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/alma490/memo490.pdf">http://www.alma.nrao.edu/memos/html-memos/alma490/memo490.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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