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.

olmi.luca@gmail.com

http://www.ifac.cnr.it/PUTO