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<DIV><FONT size=2>ALMA MEMO #442<BR><BR>The Intensity Calibration Framework for
HIFI</FONT></DIV>
<P align=left><FONT size=2>Volker Ossenkopf, (SRON Groningen) <BR><BR>Revised
2003/01/08 <BR>Original 2002/12/05 </FONT></P>
<DIV><FONT size=2>Keywords: Calibration <BR><BR></FONT></DIV>
<P><FONT size=2>Existing schemes for the calibration of radio-astronomical
observations are insufficient for the application in forthcoming space missions
with heterodyne systems like HIFI. Many of the approximations used are not valid
for systems with large IF frequencies and they are too rough for the desired
calibration accuracy of a few percent. Moreover, they do not exploit the full
capabilities of instruments with two thermal load for the bandpass calibration.
We propose a new calibration scheme for the planning and reduction of HIFI
observations, taking advantage of the lack of an atmosphere and correcting for
the effects of standing waves in the combined observation of lines and continuum
with HIFI. <BR><BR>The new calibration scheme uses a separate OFF measurement to
determine the properties of standing waves between the subreflector and the
receiver. Two different effects are corrected. Standing wave ripples are removed
from the continuum level providing the spectral baseline of the observations and
the modulation of the line strengths by standing waves is corrected in the
absolute line calibration. However, at high frequencies the standing wave
correction imposes severe constraints on the integration time for the OFF
measurement. A reasonable use of the standing-wave calibration is thus only
possible if the standing wave pattern is stable over long time scales.
<BR><BR>We have also provided an accurate estimate of the error budget of the
calibration allowing to put clear limits on the accuracy of the different
instrumental parameters required to achieve a given intensity calibration
accuracy. </FONT></P>
<DIV><FONT size=2><B>ALMA Memo 442 has been revised as of January 8, 2003.
</B><BR><BR>To view or downloaded the revised version ( January 8, 2003), please
use the following links:</B> </FONT></DIV>
<DIV><FONT size=2><BR></FONT><FONT size=2>View a pdf version of ALMA Memo
#4<SPAN class=357183517-08012003>42.1</SPAN>.</FONT></DIV>
<P><A
href="http://www.alma.nrao.edu/memos/html-memos/alma442/memo442-1.pdf"><FONT
size=2>http://www.alma.nrao.edu/memos/html-memos/alma44<SPAN
class=357183517-08012003>2</SPAN>/memo44<SPAN
class=357183517-08012003>2-1</SPAN>.pdf</FONT></A></P>
<P><FONT size=2>Download a postscript version of ALMA Memo #<SPAN
class=357183517-08012003>442.1</SPAN>.</FONT></P>
<P><A
href="http://www.alma.nrao.edu/memos/html-memos/alma442/memo442-1.ps"><FONT
size=2>http://www.alma.nrao.edu/memos/html-memos/alma44<SPAN
class=357183517-08012003>2</SPAN>/memo44<SPAN
class=357183517-08012003>2-1</SPAN>.p<SPAN
class=357183517-08012003>s</A></SPAN></FONT></P></BODY></HTML>