Innovative Technologies for Radio Astronomy

Italy has a well established history in the field of radio astronomy and INAF is deeply involved in many of the most advanced next-generation astronomical facilities, such as the Sardinia Radio Telescope (SRT) and the Square Kilometer Array (SKA). These projects are clearly showing that radio astronomy is going through a critical phase transition, where novel electromagnetic (EM) technologies and approaches are replacing the standard radio astronomical methods for radiation detection. These new technologies are mainly based on the use of arrays, the control and modification of the incident wavefront and also the use of metamaterials. Implementing super-resolution (SR) on a filled-aperture (radio)telescope is one of these new areas of research. It is thus fundamental for the Italian scientific community to support these new technological advancements in order to remain active at the forefront of radio astronomical research.

The concept of SR refers to various methods for improving the angular resolution of an optical imaging system beyond the classical diffraction limit. In optical microscopy several techniques have been successfully developed with the aim of narrowing the central lobe of the illumination Point Spread Function (PSF). In Astronomy, however, no similar techniques can be used. A feasible method to design telescopes with angular resolution better than the diffraction limit consists of using variable transmittance pupils. The simplest pupils are discrete binary phase masks with finite phase-jump positions, also known as Toraldo Pupils (TPs), since the first time such pupils were discussed was at a lecture delivered by G. Toraldo di Francia at a colloquium on optics and microwaves in 1952. Toraldo di Francia suggested that the classical limit of optical resolution could be improved interposing a filter consisting of either infinitely narrow concentric rings or finite-width concentric annuli of different amplitude and phase transmittance in the entrance pupil of an optical system.

In the microwave range, the first successful laboratory test of a TP was performed in 2003. These first results suggested that TPs could represent a viable approach to achieve super-resolution in Radio Astronomy. In 2015 we have therefore started a project ( PUTO, PUpille TOraldo) devoted to a more exhaustive analysis of TPs, in order to assess their potential usefulness to achieve SR on a radio telescope. During the first part of this work we have carried out a series of extensive EM numerical simulations, and we have then used these simulations to prepare more comprehensive laboratory tests at 20 GHz involving different types of discrete TPs. We have subsequently designed a prototype TP optical system in K-band, which has been successfully tested in the laboratory, and which has already been field-tested on a satellite antenna.

While project PUTO is still active, we are currently exploring other novel technology and methods in order to achieve an astronomically attractive SR technique on a radio telescope.

Super-resolution with Toraldo Pupils

Methods for designing super-resolving pupil masks that use either variable transmittance pupils for optical telescopes have been widely discussed in the optical literature (see, e.g., Cagigal et al. 2004). TPs are simple binary phase masks which are a special case of the more general case of variable transmittance pupils (Olmi et al. 2017, 2018). They consist of either infinitely narrow concentric rings or finite-width concentric annuli of different amplitude and phase transmittance at the entrance pupil of an optical system (see Fig. 1). As an example, we show in Fig. 2 the calculated diffracted amplitude by a simple 3-coronae pupil (or TP3).

Fig. 1 Left panel. Imaging system with standard PSF. Right panel. Imaging system with phase mask at the entrance pupil resulting in a narrower PSF and higher sidelobes.

Fig. 2 Left panel. A phase mask composed of a TP with 5 coronae (TP5, hereafter). The yellow areas indicate the coronae where a 180◦ phase shift to the incident wavefront is applied through the appropriate thickness of the dielectric material. Right panel. Plot of the far-field at 20 GHz for a TP with 3 coronae (TP3, hereafter; solid line) compared with the field of an open pupil (red dashed line). The narrower full width at half-maximum (FWHM) of the PSF indicates the SR effect and we also note the higher amplitude of the sidelobes.

In order to interface with a (radio) telescope the TP, like any other optical device designed to modify the incident wavefront on the telescope, should ideally operate at the entrance pupil of the telescope (usually, the primary mirror). Alternatively, an image of the entrance pupil (formed, e.g., by a two-lens collimator) can be used to place a transmittance filter (see Fig. 3). The first lens of the collimator generates an image of the primary which is then brought to a subsequent focus by the second lens. The transmittance filter is placed at the image of the entrance pupil where it can modify the incident wavefront. Therefore, we have designed and built a standard two-lens collimator that would allow to interface the TP with the telescope. The measured performance of the collimator is also shown in the right panel of Fig. 3.

In order to field-test the two-lens collimator we designed a K-band measurement setup that consisted of a commercial satellite parabolic antenna 125cm in diameter with a secondary mirror to implement a Cassegrain telescope configuration having the same focal ratio (f 3) as the Medicina radio telescope (see Fig. 4). This specific design allowed us to employ the same corrugated feed as the one mounted in the K-band dual-feed receiver of the Medicina antenna.

The radiation pattern has been measured in receive mode using a transmitting source in such a way that the fields incident upon the antenna are approximately plane waves. The alt-azimuth mount of the antenna has been used to obtain pattern cuts in both the elevation and azimuth directions. Two different sources have been used: (i) a geostationary satellite, and (ii) the Medicina K-band corrugated feed. Due to the low dynamic range with the satellite we have performed most of the measurements with the transmitter on the ground located at a distance in the range D²/(4λ) to D²/λ. The SR effect has been observed and an example is shown in Fig. 4 where the elevation cut of the beam is shown.

Fig. 3 Left panel. Location of entrance and exit pupil in an imaging system. The wavefront can be modified either at the entrance pupil (as shown in Fig. 1) or at the exit pupil, through a collimator. Right panel. Normalized measured amplitude at the output beam-waist of the collimator for the open pupil (red dashed curve) and the TP5 (black solid curve). Both amplitudes were measured at their peak positions.

Fig. 4 Left panel. The two-lens collimator mounted on the test satellite antenna. Right panel. Measured radiation pattern without (left, FWHM ~ 0.67 deg) and with (right, FWHM ~ 0.52 deg ) TP. Because of the presence of many scattering effects, these preliminary measurements need to be confirmed.

Alternative methods to achieve super-resolution

Our current implementation of a TP optical module at the Cassegrain focus of a radio telescope is not the only possible solution to achieve the desired SR effect. Other phase-controlling techniques should be explored in order to find the optimum method, also according to the wavelength range.

In particular, metamaterials based on dielectrically-embedded metal mesh structures can be used to design and manufacture TPs as well as variable transmittance pupils in general. This technique has been extensively used in the past to develop mesh filters and mesh retarders, such as quarter-wave plates and half-wave plates (Pisano et al. 2013). A metal-mesh TP (MMTP, hereafter) can be designed to work either in transmission or reflection. A reflective MMTP is much easier to fabricate, and as an initial test, a reflective MMTP for W-band has already been designed, manufactured and tested (see Fig. 5, Pisano et al. 2018).

As shown in Fig. 1 the ideal location of a TP would be at the entrance pupil of the imaging system. On large radio telescopes with fixed-shape surfaces this would normally be quite cumbersome. However, reflectors with an active primary surface have the ability to modify the phase of the incoming wavefront and thus it is worth investigating whether the spatial resolution of the active surface is good enough to implement the phase changes required by at least the simplest TPs.

As a first test of this method, we have used the geometrical parameters of the active surface of the Noto 32m antenna to simulate the implementation of a TP3 at the entrance pupil of the telescope. The problem has been simplified by assuming an ideal active surface, with separately moving panels, and using in the EM simulations the equivalent parabola. The right panel of Fig. 5 shows the simulated PSFs at the focus of the equivalent parabola, and one can clearly see the SR effect when the (ideal) active surface emulates a TP3. However, when the simulation is repeated using the real motions of the active panels the SR effect disappears. This is due to the fact that adjacent panels on the active surface are connected at their corners by the actuators and thus they both rotate and translate when the actuators are moved. On the other hand, the SRT active surface represents an enlarged version of the Noto surface, with a better spatial resolution. Therefore, we will repeate the EM simulations using the geometry of the SRT active surface and determine if and under what specific conditions the SRT active surface can effectively be used to implement a TP at the entrance pupil of the telescope.

Fig. 5 Left panel. Prototype reflective TP5 with metal-mesh technology for the W-band (courtesy G. Pisano). Right panel. Normalized simulated PSF, obtained with GRASP, for the Noto nominal equivalent parabola (+ symbols) and for the modified surface simulating a TP3 (dotted line).