[asac][almanews] ALMA Memos 442 Released

[Stacy Oliver]

ALMA MEMO #442

The Intensity Calibration Framework for HIFI

Volker Ossenkopf, (SRON Groningen)

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.

View a pdf version of ALMA Memo #442.
http://alma.aoc.nrao.edu/memos/html-memos/alma442/memo442.pdf

Download a postscript version of ALMA Memo #442.
http://alma.aoc.nrao.edu/memos/html-memos/alma442/memo442.ps

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