The observations were made during the standard pointing sessions which are performed approximately every two weeks at the 30 m telescope. In general, three SIS receivers were used simultaneously, namely at 87.7, 142 and 230GHz tuned to DSB mode. The performances of the receivers are given in Table2 (click here) (see also Wild 1995). Each pointing session provides a data set which allows an update of the pointing parameters of the telescope from measurements of the pointing errors of radio sources uniformly distributed over the sky (e.g. Greve et al. 1996). An additional important result of these observations is the determination of the source flux densities. This is done in the following way:
1) The sources are observed with cross-scans in azimuth and elevation
direction.
The temperature scale of these scans is 
, which is determined
via the standard chopper wheel method (e.g. Mauersberger et al. 1989).
2) Gaussian fits are applied to the individual scans and the temperature scale in each direction is corrected according to the pointing error in the direction perpendicular to it.
3) The corrected antenna temperature is converted into flux density by using the instantaneous conversion factors derived either for Mars or for the HII regions/planetary nebulae W3(OH), K3-50A, NGC7538 and NGC7027. In Table2 (click here) we give peak flux densities of these secondary calibrators at the most commonly used frequencies (Reuter et al., in preparation). These fluxes were derived from measurements of Mars assuming disk brightness temperatures of Mars as given by Ulich (1981) and Griffin et al. (1986).
Table 2: Receivers used at the telescope (1993-1994)
Table 3: Secondary calibrator flux densities
The mean and long-term variation (1
) of the calibration
factors (JyK
) at 88, 142 and 226GHz are found
to be 
, 
and 
, respectively. We
note that these values refer to measurements after March, 1994,
when the sensitivity of the telescope improved after a
re-alignment of the subreflector.