2 SIMBA observations and data reductions

The observations were performed with the SEST Imaging Bolometer Array (SIMBA) at the 15 m Swedish-ESO Submillimeter Telescope (SEST), La Silla, Chile, during the periods November 17-23 and 27-29, 2001. SIMBA has been developed by the Max-Planck-Institut für Radioastronomie, Bonn, in collaboration with the Astronomisches Institut der Ruhr-Universität Bochum. The 37 liquid helium cooled semiconductor elements are n-doped silicon chips, mounted on a saphire substrate. Projected on the sky, the 37 horn antennae have each an HPBW of 23 $^{\prime \prime }$ and are spaced by 44 $^{\prime \prime }$ in a hexagonal arrangement, covering about 4$^{\prime}$. The spectral band pass is centered at 250 GHz (1200 $\mu$m) and has a full width at half maximum (FWHM) of 90 GHz.

As map centre, the equatorial coordinates of $\beta ~ {\rm Pic}$ were used, viz. 05$^{\rm h}$47$^{\rm m}$17  $.\!\!^{\rm s}$1, -51$^{\circ}$03$^{\prime}$59 $^{\prime \prime }$ (J 2000). Generally, the mapping was done in fast mode by scanning 600 $^{\prime \prime }$ in azimuth and 400 $^{\prime \prime }$ in elevation at the rate of 80 $^{\prime \prime }$ per second and with a step size in elevation of 8 $^{\prime \prime }$, oversampling the beam by about a factor of three. The zenith opacities were obtained by means of frequent sky dips and were on the average 0.22 at the beginning of our observing run, but improved to about 0.15 after a couple of days. The pointing of the telescope was regularly checked using the extragalactic radio source 0537-441 and/or the planet Saturn and was found to be within a third of the beamwidth.

Flux calibrations were based on observations of the planet Uranus. The brightness temperature of 94.8 K at 250 GHz implies a Uranus flux density of 38 Jy/beam, with an uncertainty of 5% (Griffin & Orton 1993). As is evident from Fig. 1, the spatial flux distribution of this point source (diameter $3\hbox{$.\!\!^{\prime\prime}$ }5$) is consistent with a circular Gaussian telescope beam, with the measured FWHM( $~\parallel{,}~\perp ) =$ (24 $\hbox{$.\!\!^{\prime\prime}$ }$9 $\pm$ 0 $\hbox{$.\!\!^{\prime\prime}$ }$3, $24\hbox{$.\!\!^{\prime\prime}$ }6$ $\pm$ 0 $\hbox{$.\!\!^{\prime\prime}$ }3$). The deviation from a circularly symmetric Gaussian beam pattern occurs at the 1% level of the peak value (-20 db).

In total, we recorded 171 maps with 21 hours of integration. These data were reduced by making use of the MOPSI reduction software package developed by Robert Zylka. This involved correcting for atmospheric extinction, cosmic rays and the variable sky background, as well as producing maps from the fast scanning mode. The sky noise was greatly reduced by correlating and removing the simultaneous flux level variations of the different bolometer channels.