2. Antenna Structure

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2.3 Quadrupod and Secondary Reflector

The primary reflector backup structure substains the secondary mirror, placed at a distance of 9 m, through 4x45 inclined beams (quadrupod).

The secondary mirror is a hyperbolic reflector, 3.2 m in diameter, made of a single aluminium panel (rms = 0.35 mm). On the backup structure 3 mechanical actuators are installed and allow the mirror to tilt around the 3 axis. Besides the whole system can translate along the x and y axis.

Fig. 2.4 : Secondary mirror

The mirror must be completely retracted along the y axis when the primary focus is used.

Fig. 2.5 : Configuration for Cassegrain focus usage (drawn line) and primary focus usage (dotted line)

The mirror and the quadrupod induce an obstruction on the primary reflector of nearly 4%.

Cause

Obstruction

Sub-reflector  2 %
Quadrupod  2 %

Total

4 %

Tab. 2.2 : Primary reflector obstruction

2.3.1 Wobbling

The system that rotates the secondary mirror has been optimized in order to enhance the number of receivers that can be installed at the Cassegrain focus.

For the receivers installed in the external circumference, the same movement can be used for the Wobbling technique. Typical shifting times, quite advantageous compared to the Position Switching technique, are listed in the Table 2.3.

Frequency

(GHz)

Beam HPBW

()

Mirror rotation

2.5 beam

()

Required time

(sec)

Mirror rotation

5 beam

()

Required time

(sec)

5 450 2.56 1.16 5.12 2.12
6 390 2.22 1.03 4.44 1.86
22 120 0.68 0.45 1.37 0.71

Tab. 2.3 : Wobbling  time for 2.5 beam and 5 beam

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