 |
2.Antenna Structure |
|
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2.Antenna Structure |
2.6 Quadrupod and Secondary Reflector
The primary reflector backup structure substains the secondary mirror, placed at a distance of 24 m, through 4x45° inclined beams (quadrupod).
The secondary mirror is a shaped elliptical reflector, 8 m diameter, made of 49 aluminium panels (rms ≤ 50mm) . A positioner anchored to the mirror allows the use of the primary feeds system (see tab. 3.4)
|
Normal 0 Km/h Wind (μm) |
Normal 30.5 Km/h Wind (μm) |
Precision 11.5 Km/h Wind (μm) |
Remarks |
|
| Gravity | 3.6 | 3.6 | 3.6 | |
| Wind | 0 | 4 | 0.6 | |
| Thermal gradients | 9.3 | 9.3 | 9.3 |
±30°C around 20° |
Tab. 2.4.a : RMS from structural analysis (Row 2)
|
PANEL N° |
QUANTITY |
SURFACE |
WEIGHT |
|
( - ) |
( - ) |
m2 |
Kg |
|
1 |
1 |
0.78 |
33 |
|
2 |
12 |
0.75 |
16 |
|
3 |
12 |
1.45 |
26 |
|
4 |
24 |
1.26 |
23 |
Tab. 2.4.b : Panels of subreflector mirror
Fig. 2.20 : Secondary mirror and feeds positioner (side)
The mirror, the quadrupod, the stiffening steel cables and the positioner induce an obstruction on the primary reflector of nearly 6%.
|
Cause |
Obstruction |
| Sub-reflector | 2.1 % |
| Quadrupod | 2.4 % |
| Cables | 1.3 % |
| Positioner | 0.5 % |
|
Total |
6.3 % |
Tab. 2.5 : Primary reflector obstruction
Fig. 2.21.a : Positioner Fig. 2.21.b : Extended positioner Fig. 2.21.c : Retracted positioner
DIFFRACTION ANALYSIS (.pdf - English only)
The positioner is equipped with electromechanical systems that move the feeds along the 3 reference axis, allowing the pursuit of the optimum focal position.
There will be the possibility of orienting the secondary mirror through 7 actuators (see tab. 3.8) :
± 140 mm along z axis (primary reflector axis)
± 140 mm along x axis (parallel to the elevation axis) ; ± 5° around x axis
± 140 mm along y axis (perpendicular to x aixs) ; ± 5° around y axis
The high tilt angle values, and an actuators speed of 12 mm/s, allow the wobbling technique, with shifting time ≤ 1 sec for frequencies ≥ 20 GHz.
|
Frequency (GHz) |
Beam HPBW (“) |
Mirror's rotation 2.5 beam (°) |
Time required |
Mirror's rotation 5 beam (°) |
Time required |
| 4.3 | 270.0 | 0.806 | 2.41 | 1.613 | 4.33 |
| 5.7 | 203.7 | 0.608 | 1.94 | 1.217 | 3.39 |
| 7.5 | 154.8 | 0.462 | 1.60 | 0.925 | 2.69 |
| 10.3 | 112.7 | 0.337 | 1.30 | 0.673 | 2.10 |
| 14.4 | 80.6 | 0.241 | 1.07 | 0.482 | 1.64 |
| 19 | 61.1 | 0.182 | 0.93 | 0.365 | 1.37 |
| 26 | 44.6 | 0.133 | 0.82 | 0.267 | 1.13 |
| 35 | 33.2 | 0.099 | 0.73 | 0.198 | 0.97 |
| 70 | 16.6 | 0.050 | 0.51 | 0.099 | 0.69 |
| 90 | 12.9 | 0.039 | 0.45 | 0.077 | 0.60 |
Tab. 2.6 : Wobbling time for 2.5 beam and 5 beam
Since 1996 the wobbling technique is available and testable on the 32 m Medicina Radiotelescope.
MEDICINA RADIOTELESCOPE :SECONDARY MIRROR’S MOVEMENTS ANALISYS (.pdf - Italian only) AND RELATED MEASURES (.xls - Italian only)
