Toraldo Pupils can improve the angular resolving power of an optical instrument beyond the classical diffraction limit (hence the term “super-resolution”) using a filter consisting of finite- width concentric coronae with different amplitude and phase transmittance. Toraldo Pupils represent a viable approach to achieve super-resolution on antennas and radio telescopes. In this work we present a summary of the electromagnetic simulations and laboratory tests of a prototype optical module based on a Toraldo Pupil that has been field-tested on the Medicina 32-m radio telescope

BOLLI, Pietro

CARBONARO, LUCA

CRESCI, Luca

MACCAFERRI, ANDREA

MACCAFERRI, GIUSEPPE

MARONGIU, Pasqualino

Mugnai, Daniela

NESTI, Renzo

OLMI, LUCA

ORFEI, ALESSANDRO

PANELLA, Dario

RIGHINI, SIMONA

OA@INAF - Istituto Nazionale di Astrofisica

2018Publication Year2021-01-26T11:59:49ZAcceptance in OA@INAFFIRST TESTS OF A TORALDO PUPIL OPTICAL MODULE FOR THE 32M MEDICINA ANTENNATitleOLMI, LUCA; BOLLI, Pietro; CARBONARO, LUCA; CRESCI, Luca; MACCAFERRI, ANDREA; et al.Authorshttp://hdl.handle.net/20.500.12386/30002HandleFIRST TESTS OF A TORALDO PUPIL OPTICAL MODULE FOR THE 32M MEDICINA ANTENNA  L. Olmi(1), P. Bolli(1), L. Carbonaro(1), L. Cresci(1), A. Maccaferri(2), G. Maccaferri(2), P.Marongiu(3), D. Mugnai(4), R. Nesti(1), A. Orfei(2), D. Panella(1), S. Righini(2) (1) INAF – OAA, Largo E. Fermi 5, I-50125 Firenze, Italy, o  lmi.luca@gmail.com(2) INAF – IRA, Via Fiorentina 3513, I-40059 Villafontana, Italy(3) INAF – OAC, Via della Scienza 5, I-09047 Selargius, Italy(4) CNR – IFAC, Via Madonna del Piano 10, I-50019 Firenze, Italy AbstractToraldo Pupils can improve the angular resolving power of an optical instrument beyond theclassical diffraction limit (hence the term “super-resolution”) using a filter consisting of finite-width concentric  coronae with  different  amplitude and phase transmittance.  Toraldo  Pupilsrepresent a viable approach to achieve super-resolution on antennas and radio telescopes. Inthis work we present a summary of the electromagnetic simulations and laboratory tests of aprototype optical module based on a Toraldo Pupil that has been field-tested on the Medicina32-m radio telescope.Index Terms – Astronomical Optics, Diffraction, Super-resolution, TelescopesI. INTRODUCTION One  of  the  fundamental  properties  of  filled-aperture  telescopes  is  their  angularresolution, i.e., their ability to separate points of an object that are located at a smallangular distance. Apart from other effects which can be either cancelled or mitigated(e.g.,  aberrations  and  “seeing”),  the  diffraction  of  electromagnetic  (EM)  waves isgenerally considered to be a fundamental limit for any imaging device. The concept ofsuper-resolution (SR) refers to various methods for improving the angular resolution ofan optical imaging system beyond the classical diffraction limit. In optical microscopyseveral techniques have already been developed with the aim of narrowing the centrallobe  of  the  illumination  Point  Spread  Function  (PSF).  However,  microscopy  SRtechniques  cannot  be  easily  applied  to  astronomical  instrumentation,  and  thus  fewefforts have been made to overcome the diffraction limit of filled-aperture telescopes. Variable transmittance pupils represent one  viable approach to achieve SR inRadio Astronomy. Toraldo di Francia suggested in 1952 [1] that the classical limit ofoptical  resolution  could  be  improved  interposing  a  filter  consisting  of  finite-widthconcentric annuli of different amplitude and phase transmittance in the entrance pupil ofan optical system, now also known as Toraldo  pupils (TPs, hereafter). In this work wepresent a summary of the most recent results obtained in the context of the “PupilleToraldo” (PUTO1) project, which is devoted to a more complete analysis of TPs andhow they could be used to implement SR on a radio telescope. During the first part ofthis project we have performed both EM numerical simulations [2] and laboratory tests[3] at a frequency of 20 GHz that have confirmed and expanded the first measurementscarried out in the microwave range [4]. 1 http://www.ifac.cnr.it/PUTO/index.htm1Fig. 1 FEKO simulations at  20 GHz  showing the (normalized) field amplitude at thefocus of the collimator without (left panel) and with (right panel) TP. The collimator isilluminated by the same corrugated feed horn of the K-band receiver at Medicina.II. TP OPTICAL MODULE: EM SIMULATIONS AND TESTS Following these initial experimental tests we have then designed [5] and tested in thelaboratory [6] an optical module based on simple TPs that we planned to mount on theMedicina 32-m radio telescope2. The current design is based on a two-lens collimatorplaced after the Cassegrain focus and before the receiver dewar (see Fig.1). The firstlens of the collimator generates an image of the primary reflector which is then broughtto a subsequent focus by the second lens. The TP is placed at the image of the entrancepupil where it can modify the incident wavefront. The EM simulations of the optical module were conducted using the commercialsoftware  tool  FEKO.  In  the  most  recent  simulations,  the  3D model  is  analyzed  intransmission rather than in reception. In this model the source was a corrugated  feedhorn identical to that employed in the K-band receiver operating on the Medicina radiotelescope. The results are schematically shown in Fig.1, where the SR effect is visible asthe narrower PSF at the focus of the collimator. We then improved the EM model byeffectively including the antenna optics in the EM simulation. This was performed byusing the equivalent parabola to simulate the Medicina Cassegrain telescope (smaller insize  but  with  same  focal  ratio).  Figure  2  shows  that  while  we  achieve  the  correctillumination of the reflector with the basic collimator optics (left panel),  we note ananomalous illumination when the TP is applied (right panel). This incorrect illuminationis a consequence of the fact that the presence of the TP within the collimator wouldrequire a different, optimized feed horn (see Sect. III).III. LABORATORY AND FIELD TESTS The two-lens collimator has been extensively tested in the anechoic chamber of theOsservatorio  Astrofisico  di  Arcetri  (Firenze)  [6].  During  these  measurements  weilluminated the collimator with the same corrugated feed horn as the one employed on 2    http://www.med.ira.inaf.it/ Fig. 2 FEKO simulations at 20 GHz with the equivalent parabola illuminated by the K-band feed-horn through the two-lens collimator,  without (left  panel) and with (rightpanel) the TP applied. The figure shows the (normalized) surface current distribution onthe antenna. The inset in the left panel shows the expected far-field antenna pattern.Fig. 3 Left panel. Laboratory setup showing the two-lens collimator, illuminated by a K-band corrugated feed horn. The focus fields are sampled with a an open waveguide.Right panel. Gaussian-beam propagation in the collimator. the K-band receiver at Medicina, while the fields at the focus of the collimator weresampled using a waveguide probe (see left panel of Fig. 3). The results from these testswere quite consistent with the FEKO simulations shown in Fig. 1. However, when theopen waveguide at the focus is replaced by the corrugated horn (to simulate the realdetection process with the K-band receiver, as shown in the right panel of Fig. 3), thesignificant mismatch between the PSF at the focus of the collimator and the expectedfields on the aperture of the horn completely “washes out” the SR effect when the TP ispresent. This mismatch is the reason of the anomalous illumination of the reflector inthe right panel of Fig. 2.In March 2018 we finally field-tested the two-lens collimator on the Medicinaantenna. The collimator was mounted externally on the dewar of the K-band receiver(see Fig.  4) and then we performed two main series of tests:  (i) after  mounting thecollimator we had to find the new focal position by executing focus scans on pointsources, which allowed to optimize the position of the subreflector (see Fig. 3); (ii) thenwe carried out cross-scans on the point sources, and compared the measured antennabeam with the results obtained without the collimator. During these measurements weused both astronomical point sources and a geostationary satellite (Eutelsat Ka-sat 9a). 3Fig. 4 Left panel. Mounting of the collimator at the Cassegrain focus of the Medicinaantenna.  Middle panel.  The collimator mounted on the dewar of the K-band receiver.Right  panel.  Example  of  almost  identical  antenna beam-patterns  obtained during anazimuth scan without (top) and with (bottom) the collimator, but with no TP applied.The raw horizontal scale is not corrected for the different elevations. The vertical scaleis raw antenna temperature.These tests showed that the satellite signal was very difficult to use for focus andpointing  scans,  especially  with  a  new  optical  system to  be  checked.  However,  thepointing astronomical sources were strong enough to determine the new focus positionwith the collimator mounted on the receiver and the cross-scans showed that the antennabeam was basically identical with and without the collimator (see the right panel in Fig.4). When the TP was applied to the collimator it was not possible to make a reliablemeasurement of the antenna beam, due to the mismatch with the receiver feed-horn, aspreviously shown during our laboratory tests. An ad-hoc receiver and feed horn shouldbe used to fully exploit the SR effect.ACKNOWLEDGEMENTWe  gratefully  acknowledge  the  partial  support  of  the  Ente  Cassa  di  Risparmio  diFirenze (Italy). We also wish to thank all personnel of the Medicina Radio AstronomicalStation whose support was fundamental before and during the tests at the antenna.REFERENCES[1] G. Toraldo di Francia, ``Nuove pupille superrisolventi'', Atti Fond. Giorgio Ronchi, Vol.7, 1952. [2] L. Olmi,  et al., ``Super-resolution with Toraldo pupils: analysis with electromagneticnumerical simulations'', Proc. of SPIE, Vol. 9906, Astron. Telescopes and Instrumentation, 2016.[3] L. Olmi, et al., ``Laboratory measurements of super-resolving Toraldo pupils for radioastronomical applications'', Experim. Astron., Vol. 43,  p. 285, 2017.[4] D.  Mugnai,  A.  Ranfagni   and  R.  Ruggeri,  ``Pupils  with  super-resolution'',  PhysicsLetters A, Vol. 311, p. 77, 2003.[5] L.  Olmi,  et  al.,  “Design  of  super-resolving  Toraldo  Pupils  for  radio  astronomicalapplications”, URSI GASS,  XXXIInd,  2017. [6] L. Olmi,  et al., “Design and Test of a Toraldo Pupil Optical Module for the MedicinaRadio Telescope”, paper accepted for the 2nd URSI AT-RASC, 28 May - 1 June 2018 

FIRST TESTS OF A TORALDO PUPIL OPTICAL MODULE FOR THE 32M MEDICINA ANTENNA

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FIRST TESTS OF A TORALDO PUPIL OPTICAL MODULE FOR THE 32M MEDICINA ANTENNA

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