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Telescope efficiencies

In order to estimate the observing time needed to detect a signal of a given intensity, its flux density (or `beam averaged brightness temperature' ) has to be converted in antenna temperature units. This is done e.g. by multiplying or by the telescope efficiency. Different efficiencies should be used, depending on the size of the source relative to the telescope beam: the aperture efficiency , the main-beam efficiency , the `Moon' efficiency . Another relevant efficiency is the `telescope forward efficiency' , which depends on the coupling of the `receiver' with the cold sky, and which can be measured during excellent weather conditions by tipping the antenna (SKYDIP --beware however that your is correct!).

The various efficiencies are listed in columns 4--6 of Table 1. The aperture efficiencies were derived from continuum cross scans of Mars and Uranus carried out between March, 17th and August, 15th, 1994. The planetary diameters during this period ranged between and . A (theoretical) correction was made for the antenna gain-elevation dependence. This correction was at most 7% for this data set.

The third and last columns of Table 1 give the telescope half-power beamwidth and the flux density to antenna temperature ratio for a point source, (in Jy/K). This latter is calculated from the formula , where the numerical factor stands for . Values for the Moon efficiency are in preparation. Note that in order to convert the `antenna' temperatures given by OBS into main-beam averaged `Raleigh-Jeans brightness temperatures' (or `radiation' temperatures), , the former have to be to be multiplied by (assuming the source does not extend outside the telescope main beam).



Table: 30m Telescope efficiencies

Robert Lucas
Thu Mar 9 12:14:01 MET 1995