3.2 Cassegrain Focus

The secondary mirror (9 m from the primary mirror) allows the usage of the Cassegrain focus (at nearly 20 cm above the reflector vertex)

This focus has been designed to offer several adjacent focal positions, which can be obtained through the angular movement of the secondary mirror (see fig. 3.4).

Fig. 3.4 : Cassegrain focal plane

Unlike the Medicina antenna, the focal positions do not host different receivers and the frequency change requires the installation of the necessary receiver into the central bay. The Wobbling technique is used only to realize the Beam Switching (e.g. On Source-Off Source) and to carry faster radiometric measurements.

The secondary hyperbolic reflector yelds a magnification i₂ which depends on the ratio between the focal length and the distance from the prime focus (nearly 9 m and 1 m, respectively).

The total focal length can be estimated as follows  :

The focal ratio is therefore:  F₂ / D ≈ 3.04

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