Fabrizio Villa INAF / IASF Bologna

Transcription

1 Fabrizio Villa INAF / IASF Bologna villa@iasfbo.inaf.it

2 Directivity dbi % DEP 0.29 FWHM arcmin e 1.41 Spill-over 0.16%

3 Legge di Snell Angolo di incidenza = angolo di riflessione Principio di Fermat Se un raggio propaga tra due punti A e B, attraverso C su un riflettore, la lunghezza di cammino ABC e stazionaria rispetto allo spostamento di C

4 Estensione della GO (Keller, 1962) Calcolo della diffrazione dovuta ai bordi dei riflettori I raggi diffratti sono linearmente dipendenti dall onda incidente. La dipendenza e espressa mediante una matrice di COEFFICIENTI DI DIFFRAZIONE

5 Il campo elettromagnetico incidente induce delle correnti sul riflettore che a sua volta re irradia un campo elettromagnetico Il singolo elemento del riflettore e approssimato da una superficie piana infinita

6 Estensione della PO Calcolo della diffrazione dovuta ai bordi dei riflettori

7 MrGTD computes the scattered field from the reflectors performing a backward ray tracing. The main difficulty was to understand the contributions that had to be considered, or in other words, to identify the sequence of diffractions and/or reflections on each scatterer which produce a significant power level in the resulting radiation pattern.

8 Left panel: 4pi map of the field due to the rays coming from the feed (direct contribution), together with the field due to the rays reflected on the subreflector and not intercepted by the main reflector (Rs contribution). Most of the map is empty (grey colour) since most of the rays are blocked by the baffle. The power peak of the direct contribution is about dbi whereas the power peak of the Rs contribution is about dbi. This latter is the brightest contribution in the far sidelobes. Right panel: sketch of the optics in the symmetry plane, together with the ray-tracing of these two contributions in the plane at φ bf = 22 (these rays lie in the white dashed cut indicated in the map, where the field is present).

9 Left panel: 4pi map of the field due to the rays coming from the feed that are reflected by the baffle (Rb contribution) is shown. The power peak is about dbi. Right panel: sketch of the optics in the symmetry plane is shown together with the ray-tracing of this contribution in the plane at φ bf = 22 (these rays lie in the white dashed cut indicated in the map, where the field is present).

10 Left panel: 4pi map of the field due to the rays that are reflected onto the sub reflector and diffracted by the main reflector (RsDm contribution) is shown. The power peak is about dbi, in the main beam direction. Right panel: sketch of the optics in the symmetry plane is shown together with the ray-tracing of this contribution in the plane at φ bf = 22 (these rays lie in the white dashed cut indicated in the map, where the field is present).

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12 Co- and cross- polar components of the 4pi beam at 30 GHz (feed horn #27 Y polarised) The maximum level of the main spillover is about -4.6 dbi at φ ~ 17 and θ ~ 85 for the co- polar component, and it is about -8.0 dbi at φ ~ 18 and θ ~ 86 for the cross- polar component. GSC 7.2 microk (peak-to-peak) and 1.3 microk (rms)

13 Because of different response of feed horns with frequency different telescope diameter with frequency (normalized to wavelength) the main beam shape is expected to be frequency dependent within the bandwidth of each detector (20% for LFI). Directivity dbi % DEP 0.35 FWHM arcmin e 1.30 Spill-over 0.68% Directivity dbi % DEP 0.29 FWHM arcmin e 1.38 Spill-over 0.16% Directivity dbi % DEP 0.51 FWHM arcmin e 1.52 Spill-over 0.17%

14 27 GHz 33 GHz AVERAGE

15 blue rays pink rays Main reflector Sub reflector

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17 Approssimazione Dell Ottica Fisica

18 Up => Vp => COMP1 MAX => dbi COMP2 MAX => dbi % DEP => 0.44 FWHM X => 9.33 arcmin FWHM Y => arcmin FWHM AVE => arcmin e => 1.27 Up => Vp => COMP1 MAX => dbi COMP2 MAX => dbi % DEP => 0.44 FWHM X => 9.31 arcmin FWHM Y => arcmin FWHM AVE => arcmin e => 1.27

19 Frequenza (Lunghezza d onda) Sistemi di Coordinate Riflettori Superficie Contorno Sorgenti Elettromagnetiche Feed (Gaussiano, Standard) Oggetto PO Output (Pattern)

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21 Tipici valori di F#: Angoli: gradi

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23 Illuminazione non Simmetrica

24 On Axis Off Axis D [mm] Xc [mm] F [mm] ET [db] -20?

25 On Axis Off Axis θ θ θ θ C 28.07

26 D=1500 mm ; f = 30GHz (100GHz) ET FWHM 0 db db db db 32.5 (9.8 )

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