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Efficiencies of the 30m Telescope

The table below summarizes efficiency data taken by the IRAM 30m staff in the period between March and August 1994. These data update and extend the comparable list of efficiencies presented in Newsletter No. 5 (September 1992). Measurements with the 1.3mm G2 SIS receiver are still pending.

While the forward efficiencies were deduced from skydips, the aperture and main beam efficiencies and the half power beam widths were derived from continuum cross scans on Mars and Uranus. The planetary diameters during this period ranged between and . Beam and aperture efficiencies are corrected for the theoretical antenna gain-elevation dependence; this correction is at most 7% for this data set. The values for the continuum sensitivity are on the scale (flux per Kelvin = 3.906 for the 30m telescope).

From the scatter of the day-to-day measurements we estimate errors of the efficiencies to be about %. We will continue to monitor the efficiencies.

The product of measured beam widths and frequency is nearly constant for frequencies up to 270 GHz. This indicates a constant illumination of the subreflector independent of the receiver. The good agreement allows to give a formula which may be used to estimate the beam width to an accuracy of about between 90 and 270GHz:

30m Telescope Efficiency Data (November 1994)

Note: The conversion from antenna temperatures to main beam brightness temperatures can be done via . We use the nomenclature of Downes, ``Radio Astronomy Techniques'', IRAM preprint 151 (also in Evolution of Galaxies, Astronomical Observations, Springer 1989).

Carsten KRAMER and Wolfgang WILD

Robert Lucas
Thu Mar 9 12:42:24 MET 1995