Subreflector Position Research Articles

Influence and Correction of Wavefront Primary Aberrations for Radio Telescopes using Aberration Theory

Dual-reflector antennas are widely used in astronomical observations and satellite communication. Structural deformations of the reflectors for radio telescopes are inevitable in outside working conditions due to exterior environment loads, which will cause distortion in the surface of the primary reflector and displacement of the subreflector, then lead to gain degradation and misalignment. In this paper, the influence and correction of misalignment in a dual-reflector antenna have been studied. From the perspective of wavefront aberration, a method is proposed to correct the wavefront primary aberration by adjusting the subreflector position. The characteristics of wavefront errors caused by structural deformation of the reflector have been analyzed, and relationships between the position motions of the subreflector and the Seidel wavefront aberrations are derived. The adjustment quantities of the subreflector are also derived. The results show the appropriate positional change of the subreflector in the lateral and axial directions can effectively correct the effects of the tilt and defocus in the primary aberrations caused by antenna structural deformations.

Active sub-reflector research for a large radio telescope

This paper proposes an active sub-reflector suitable for large radio telescopes, which can compensate both of the deformation of the main reflector and sub-reflector position offsets. The mathematical formula of the main reflector deformation compensated by the sub-reflector is deduced based on Cassegrain and Gregory antenna structures. The position of the sub-reflector is adjustable to compensate for defocusing errors on high and low elevations, which are mainly caused by the deformation of the sub-reflector supporting legs. In this paper, the method of obtaining the optimum position of the sub-reflector from the aperture phase by the interferometric method is introduced. The actual measurement is verified on the Tianma 65m radio telescope, which provides a new way to diagnose the position error of the sub-reflector.

Terahertz Near-Field Focusing Using a 3-D Printed Cassegrain Configuration for Backscattered Side-Channel Detection

This paper presents the use of terahertz near-field focusing for backscatter side-channel detection. Near-field focusing is done by using a Cassegrain reflector configuration. The focuser is designed to produce the focused beam 28 cm away from the antenna aperture. The focusing is done in the near-field region by axially moving the subreflector from the focal point. It is observed that the subreflector position has to shift approximately 11 wavelengths along the axis to create the focus at the required location. The focused antenna gain is 46 dBi, while the 3 dB focus width and depth of the designed antenna is ~4 mm and 10 cm, respectively. It is found that the focal plane position is sensitive to the subreflector shifts and it is observed that 1 mm change in the subreflector position can shift the focal plane by ~2 cm. The simulations are compared with the measurement results of a fabricated prototype and good agreement is observed. The antenna is fabricated by using 3-D printing technology, which allows rapid prototyping. Finally, we have demonstrated the detection of backscatter side channel from the board placed at 28 cm away from the designed antenna. The received power level of the backscatter signal increases by 6 dB as compared to horn antenna.

Gravitational deformation of ring-focus antennas for VGOS: first investigations at the Onsala twin telescopes project

The receiving properties of radio telescopes used in geodetic and astrometric very long baseline interferometry (VLBI) depend on the surface quality and stability of the main reflector. Deformations of the main reflector as well as changes in the sub-reflector position affect the geometrical ray path length significantly. The deformation pattern and its impact on the VLBI results of conventional radio telescopes have been studied by several research groups using holography, laser tracker, close-range photogrammetry and laser scanner methods. Signal path variations (SPV) of up to 1 cm were reported, which cause, when unaccounted for, systematic biases of the estimated vertical positions of the radio telescopes in the geodetic VLBI analysis and potentially even affect the estimated scale of derived global geodetic reference frames. As a result of the realization of the VLBI 2010 agenda, the geodetic VLBI network is currently extended by several new radio telescopes, which are of a more compact and stiffer design and are able to move faster than conventional radio telescopes. These new telescopes will form the backbone of the next generation geodetic VLBI system, often referred to as VGOS (VLBI Global Observing System). In this investigation, for the first time the deformation pattern of this new generation of radio telescopes for VGOS is studied. ONSA13NE, one of the Onsala twin telescopes at the Onsala Space Observatory, was observed in several elevation angles using close-range photogrammetry. In general, these methods require a crane for preparing the reflector as well as for the data collection. To reduce the observation time and the technical effort during the measurement process, an unmanned aircraft system (UAS) was used for the first time. Using this system, the measurement campaign per elevation angle took less than 30 min. The collected data were used to model the geometrical ray path and its variations. Depending on the distance from the optical axis, the ray path length varies in a range of about pm ,1,hbox {mm}. To combine the ray path variations, an illumination function was introduced as weighting function. The resulting total SPV is about - ,0.5 mm. A simple elevation-dependent SPV model is presented that can easily be used and implemented in VLBI data analysis software packages to correct for gravitational deformation in VGOS radio telescopes. The uncertainty is almost 200,upmu hbox {m} (2sigma ) and is derived by Monte Carlo simulations applied to the entire analysis process.

Optimal subreflector position determination of shaped dual-reflector antennas based on the parameters iteration approach

A new method based on the parameters iteration technique has been developed to determine the optimal subreflector position for shaped Cassegrain antennas, that are distorted by gravity, to improve their electromagnetic (EM) performance. Both the features of shaped surface and the relationship between optical path difference (OPD) and far field beam pattern are employed. By describing the shaped dual-reflector surface as a standard discrete parabola set, we can utilize the optical features of the standard Cassegrain system in the classical OPD relationship. Then, the actual far field beam pattern is expressed as the synthesis of ideal beam and error beam by decomposing subreflector adjustment parameters using a mechanical-electromagnetic-field-coupling-model (MEFCM). Furthermore, a numerical method for determining optimal subreflector position is presented. The proposed method is based on the iteration technique of subreflector adjustment parameters, and the optimal far field pattern is used for the iteration. The numerical solution of optimal adjustment parameters can be obtained rapidly. Results for a 25 m shaped Cassegrain antenna demonstrate that the adjustment of the subreflector to the optimal position as determined by the proposed method can improve the EM performance effectively.

Main reflector gravity deformation and sub-reflector position measurement for radio telescope based on interferometer

This paper focuses on the measurement of the main reflector gravity deformation and sub-reflector position change on Tianma 65 m (TM65 m) radio telescope. Firstly the paper summarizes the methods of principal measurement. Microwave holography and cross scan pattern are disscussed. Then the gravity deformation of the main reflector over the entire elevation measured by VLBI method is discribed. The measurement accuracy is also estimated. At last, the gravity model is constructed and compared with the simulation. They agree with each other. The formulas along with the simulation give a description between the offsets of feed, sub-reflector and the aperture phase. The sub-reflector position ofTM65 m is obtained by the least square method that applied on the aperutre phase and the formulas. The sub-reflector positions measured are consistent with that of the traditional amplitude method in X , Y and Z directions. This method provides a different way to measure the position model of the sub-reflector.

Compensation for gravity deformation via subreflector motion of 65 m shaped Cassegrain antenna

Compensation for degrading electromagnetic performances is one of the most important concerns for engineers involved in the structural design of a large antenna. A method is proposed to compensate the degrading electromagnetic performances induced by the main reflector deformation of the large and shaped Cassegrain antenna by adjusting the subreflector position in real time. Segmented parabola fitting based on the least squares method of the theoretical discrete data of the main shaped reflector yields a standard group of parabolic torus that is later used to fit the deformed main reflector, with all the parabolic toruses confocal in the fitting. The optimised best-fit parameters and the matching between the main reflector and subreflector calculate the adjustment quantity of the subreflector. The adjustment quantities, stored in the database of the subreflector, working under various conditions can be called in real time to compensate the gravity deformation of the main reflector. The application of the method to a 65 m aperture-reflector antenna was successful, thus, laying a good foundation for its application in engineering practice.

PERFORMANCE OF THE SRAO 6-METER RADIO TELESCOPE

We introduce and describe performance of the 6-meter telescope of Seoul Radio Astronomy Observatory (SRAO). All the softwares and instruments except the antenna structure and its driving system are developed for ourselves. The SIS mixer type receiver resulted in the receiver noise temperature less than 50 K (DSB) over the whole 3-mm radio window. An autocorrelation spectrometer, developed first in Korea, provides maximum 50 MHz band width over 1024 channels. Antenna surface is measured and adjusted using template method and radio holography which resulted in a superb surface accuracy bet-ter than 30. Accordingly, the aperture and beam efficiences amount to and , respectively, largely independent of frequency in the 85 - 115 GHz range. It is also found that telescope pointing errors are less than 10 in both azimuth and elevation and that antenna gain is almost constant against elevation greater than , without adjusting sub-reflector position. The SRAO 6-meter telescope is now fully operational and all these characteristics verify that observations are carried out with high precision and fidelity.

Surface accuracy of Cassegrain antennas

For a Cassegrain antenna system, a simple and general procedure is presented for computing the effective surface root mean square (rms) and beam deviations considering primary surface distortions and relative translations and rotations of the antenna components. It is shown that certain primary reflector distortions can be compensated for by an appropriate adjustment of the subreflector position. Different methods in use for calculating surface rms are reviewed. It is shown that the method in which the deformed configuration of the primary reflector is best fitted with another paraboloid yields a surface rms almost equal to that obtained by the optimal positioning of the antenna components. The actual deformation patterns of several large Cassegrain antenna systems with different structural concepts are reviewed. It is shown that for a class of antenna structures, the gain can be significantly improved by an optimal positioning of the subreflector.

An infinitely expandable space radiotelescope

Great rewards for radio astronomy lie in increasing antenna collecting areas and the distance between them. Both these possibilities, which determine sensitivity and angular resolution, can be developed essentially without limit if radiotelescopes can be built in space. Radio interferometers, with base lines up to 10 AU, assembled from large reflectors 1 to 10 km in diameter, could operate in the 1 mm to 1 m range with a sensitivity of 3 × 10 −37 W m −2 Hz −1 and an angular resolution of 1.5 × 10 −10 sec of arc. These parameters are a million times better than those of modern radio astronomical systems. Such instruments could solve many urgent problems: the search for artificial radio signals and other evidence of extraterrestrial civilizations; detection and study, within the RF range, of solar-like stars and planets; and holography of astronomical sources and direct measurements of their distance. The maximum distances of objects which could be studied would be comparable with the radius of curvature of the Universe. A possible space radiotelescope (SRT) configuration, with a modular spherical reflector and feeds providing multibeam operation, is analyzed here. An extendable reflector capable of operating at any intermediate stage is assembled from 200-m modules. These consist of a three-dimensional spar framework on which flat hexagonal subreflectors of about 4 m are mounted. The precise geometry required of such a reflector is provided by automatic adjustment of subreflector position relative to the framework and by adjustment of connections between modules. In this way, radiotelescopes for the 1 mm to 1 m range may be built with diameters up to 10 km. The problems of system delivery and assembly in orbit are assessed. Preliminary estimates for developing and constructing an SRT that is more than 1 km in diameter are lower than for a similar collecting area on Earth. State-of-the-art technology already allows the development and construction of a 200-m space module.