The intensity calibration for HIFI

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 standingwave 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.