[asac][almanews] Revision of ALMA Memo 442 Released

[Stacy Oliver]

ALMA MEMO #442

The Intensity Calibration Framework for HIFI
Volker Ossenkopf, (SRON Groningen)

Revised 2003/01/08
Original 2002/12/05

Keywords: Calibration


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.

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.

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.

ALMA Memo 442 has been revised as of January 8, 2003.

To view or downloaded the revised version ( January 8, 2003), please use the
following links:

View a pdf version of ALMA Memo #442.1.
http://www.alma.nrao.edu/memos/html-memos/alma442/memo442-1.pdf

Download a postscript version of ALMA Memo #442.1.

http://www.alma.nrao.edu/memos/html-memos/alma442/memo442-1.ps
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