<html xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:st1="urn:schemas-microsoft-com:office:smarttags" xmlns="http://www.w3.org/TR/REC-html40">
<head>
<meta http-equiv=Content-Type content="text/html; charset=us-ascii">
<meta name=Generator content="Microsoft Word 11 (filtered medium)">
<o:SmartTagType namespaceuri="urn:schemas-microsoft-com:office:smarttags"
name="place"/>
<o:SmartTagType namespaceuri="urn:schemas-microsoft-com:office:smarttags"
name="City"/>
<!--[if !mso]>
<style>
st1\:*{behavior:url(#default#ieooui) }
</style>
<![endif]-->
<style>
<!--
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0in;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman";}
h1
{mso-margin-top-alt:auto;
margin-right:0in;
mso-margin-bottom-alt:auto;
margin-left:0in;
font-size:24.0pt;
font-family:"Times New Roman";
font-weight:bold;}
h2
{mso-margin-top-alt:auto;
margin-right:0in;
mso-margin-bottom-alt:auto;
margin-left:0in;
font-size:18.0pt;
font-family:"Times New Roman";
font-weight:bold;}
a:link, span.MsoHyperlink
{color:blue;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{color:purple;
text-decoration:underline;}
p
{mso-margin-top-alt:auto;
margin-right:0in;
mso-margin-bottom-alt:auto;
margin-left:0in;
font-size:12.0pt;
font-family:"Times New Roman";}
span.EmailStyle18
{mso-style-type:personal-compose;
font-family:Arial;
color:windowtext;}
@page Section1
{size:8.5in 11.0in;
margin:1.0in 1.25in 1.0in 1.25in;}
div.Section1
{page:Section1;}
-->
</style>
</head>
<body lang=EN-US link=blue vlink=purple>
<div class=Section1>
<h1><b><font size=3 face="Times New Roman"><span style='font-size:12.0pt;
font-weight:normal'>ALMA Memo # 505<o:p></o:p></span></font></b></h1>
<h2><b><font size=3 face="Times New Roman"><span style='font-size:12.0pt;
font-weight:normal'>Bandpass Calibration for <st1:City w:st="on">ALMA</st1:City>
<br>
<br>
A. Bacmann (ESO), <st1:place w:st="on">S. Guilloteau</st1:place> (IRAM/ESO) <br>
<br>
2004-09-30<o:p></o:p></span></font></b></h2>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Keywords: Calibration, Bandpass <o:p></o:p></span></font></p>
<p><font size=3 face="Times New Roman"><span style='font-size:12.0pt'>This memo
contains a detailed evaluation of the expected performance of the bandpass
calibration for <st1:City w:st="on"><st1:place w:st="on">ALMA</st1:place></st1:City>.
We evaluate the limitations on the bandpass accuracy due to the imperfect
knowledge of the atmospheric transmission. We show that the best bandpass
accuracy is obtained when the amplitude calibration is applied in a single-load
scheme. The proposed scheme is a variant of the so-called bandpass
normalization technique. It uses normalization by the difference between the
sky emission and the load emission, rather than normalization by the
auto-correlation spectrum in the usual case. Bandpass calibration should be
performed at the observing frequency. It will be limited by the knowledge of
the sideband opacity difference. This knowledge can be based on a model, but a
direct measurement is also possible, although time consuming.<br>
<br>
The bandpass calibration must be performed on strong point-like sources, of
known spectral index. The knowledge of the spectral indexes will be a limiting
factor in the wide band modes. Building up a database of suitable sources will
be necessary, since the knowledge of these spectral indexes could be improved
by bootstrapping techniques among several sources. Since astronomical sources
have to be used, the required integration time varies substantially as a
function of frequency. High accuracies can be reached in a minute of time at mm
wavelengths, but integration time as large as an hour can be needed at sub-mm
wavelengths. Occasional direct measurements of the sideband opacity difference
could also be used to improve the accuracy of the atmospheric modelling. <o:p></o:p></span></font></p>
<p style='margin:0in;margin-bottom:.0001pt'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>View a pdf version of ALMA Memo #505. <o:p></o:p></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/alma505/memo505.pdf">http://www.alma.nrao.edu/memos/html-memos/alma505/memo505.pdf</a></span></font></u><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
</div>
</body>
</html>