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's vertex)

This focus has been studied to offer more 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

At this focus, 9 receivers can be mounted (a central one plus eight around)

Fig. 3.5 : Cassegrain receiver bays

Fig. 3.6 : Cassegrain focus, (dimensions : mm)

The secondary hyperbolic reflector operates 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

Presently the 5 GHz and 6 GHz receivers are installed at this focus.