Prototipo di pupilla Toraldo per test sull'antenna di Medicina
L. Olmi, INAF-OAA P. Bolli, INAF-OAAL. Carbonaro, INAF-OAAL. Cresci, INAF-OAAP. Marongiu, INAF-OACD. Mugnai, CNR-IFACE. Natale, INAF-OAAR. Nesti, INAF-OAAD. Panella, INAF-OAAS. Righini, INAF-IRAJ. Roda, INAF-IRAG. Zacchiroli, INAF-IRA
Implementation on (Radio) Telescopes
Telescope Collimator
Acting on wavefront at exit pupil is equivalent as acting on it at entrance pupil of telescope.
Ideally, place TP on entrance pupil
Dewar should be moved backward
Transparency function
Mounting on the Antenna
RX can't be (easily) moved
Axial shift of subreflector: Geometrical Optics
Δz = m2 ΔS = (Fc/Fp)2 ΔS = (3.04/0.32)2 ΔS = 90.3 ΔS
For every cm shift of the subreflector the Cass. Focus moves ~ 90 cm
Axial shift of subreflector: Phase Errors
Δz = m2 [ 1 + (1/m2)(4Fp)-2 ] / [ 1 + (4Fp)-2 ] ΔS = 56.4 ΔS
For the phase errors (caused by the secondary or feed axial shifts) to be the same (Baars 2007). Confirmed by P. Bolli (2002) with GRASP.
Collimator Design
Woinp Wo
out
dout
Cass. Focus
Horn Phase ctr.
Thick Lens
Optimization procedure takes input parameters and estimate all other output optical quantities, based on Gaussian-beam propagation and thick lenses.
Units are cm
Collimator Design: (potential) Problems
Cass. Focus
Horn Phase ctr.
Thick Lens
Fc changes
Coupling to horn (TP)
Relatively small D(lens)/λ
Lens geometrical aberrations
“Aperture plane excitation” simulates Cassegrain focus
Collimator Performance: FEKO
TP3
TP4
Clear aperture
Decrease in efficiency
THIN lenses
Electric currents
Face up Face down
Magnetic currents
FEKO Simulations IFEKO Simulations IFEKO Simulations I
TP3
Little energy in the forward direction
Most energy in the backward direction
~10dB
TP4 gets worse with thick lenses (?)
FEKO Simulations II
Thick vs. thin lens approximation
TP3 improves with thick lenses
TP3
TP4
FEKO Simulations III
Phase spatial distribution
FEKO Simulations IV
Hyperbolic thick lenses
AP TP3
Implementation on 32m Medicina Telescope
Nominal focus position (inside dewar)
New focus position
Medicina Tests
RUN 1
Point and track satellite and planets
Intensity of satellite signal and attenuation
RUN 2
Mount collimator on antenna
Focus (and align?) antenna
Scan beam w/o TP on satellite/planets
Scan beam with TP on satellite/planets
Get down collimator
EIRP ~50dbW
~ -56dbm at horn
~ 14dbm at RX output
Conclusions and Next Steps
The collimator with TP3 has been fully characterized.
TP4 is still being analyzed.
Fabrication of the components is underway (OAA, OAC, IFAC).
Proposal to Medicina for field-tests has been submitted.
Preliminary tests in anechoic chamber are being considered.
http://www.ifac.cnr.it/PUTO
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