Aperture Radio Telescope Research Articles

A way to improve the field of view of large aperture radio telescopes

There is a limitation on the field of view (FOV) of a large aperture radio telescope: the FOV decreases with increasing resolution and sensitivity of the telescope and the greater the aperture, the smaller the FOV. In order to enlarge the FOV, we may place several discrete feeds on the focal plane of the telescope. When these feeds work simultaneously, the FOV and therefore the operating efficiency of the telescope will be increased by several times. A disadvantage of this solution is the discontinuous character of the resulting FOV. Moreover, the number of feeds mounted on the focal plane is restricted by the F/D ratio of the telescope. We investigate in this paper an innovative technique for realization of simultaneous and continuous large FOV for the FAST (Five-hundred-meter Aperture Spherical radio Telescope). The proposed new feed is a dense focal-plane array of microstrip antenna elements, the so called Vivaldi antennas. The electro-magnetic properties of the Vivaldi element and Vivaldi array are briefly introduced, which are essential for this application. The focal-field distributions (FFD) of FAST are simulated by using the software package GRASP8 for different illuminations. The simulation results give important information for the layout design of the array, especially for the weighting system of the Vivaldi array. A preliminary configuration of the array structure is suggested, and the illumination patterns are estimated. Finally, some remarks on the feasibility of applying the Vivaldi array to FAST are made.

Systematic observations of anomalous refraction at millimeter wavelengths

It is well known that the water vapor in the troposphere plays a fundamental role in radio propagation. The refractivity of water vapor is about 20 times greater in the radio range than in near-infrared or optical regimes. As a consequence, phase fluctuations at frequencies higher than about 1 GHz are predominantly caused by fluctuations in the distribution of water vapor, and thus radio seeing at these frequencies is predominantly caused by tropospheric turbulence. Radio seeing shows up on lled-aperture telescopes as an anomalous refraction (AR), i.e. an apparent displacement of a radio source from its nominal position, corrected for large-scale refractive eects. The magnitude of this eect, as a fraction of the beam width, is bigger on larger telescopes and thus its impact on the pointing is likely to become critically important in the next generation of electrically large lled- aperture radio telescopes (D= > 10 4 ) and in particular on the Large Millimeter Telescope. AR eects are expected to reduce the total eective observing time at the highest frequencies and will aect on-the-fly mapping. Here we present the results of systematic AR measurements carried out with the 13.7-m telescope of the Five College Radio Astronomy Observatory. The measured AR pointing errors range from 1 00 3 00 (winter) to about 20 00 (summer) and most of the events last less than about 4 s. We analysed the structure function, power spectrum and Allan variance of the data and we have carried out a statistical analysis to identify correlations of the statistical functions with selected observing parameters such as precipitable water vapor, time of day, season and elevation angle. Our results suggest that uncompensated AR may be the most important dynamic environmental source of pointing errors on the new large radio telescopes (ALMA, GBT, LMT, SRT) and may guide the design of active AR-compensation devices and help allocating suitable observing time through dynamic scheduling.

The Brightness Temperature of Venus at 8.6 MM.

Observations of the planet Venus were obtained at 8.6-mm wavelength during the 3 weeks following the inferior conjunction of September, 1959. By means of integrated scanning of the planetary radiation with a 10-foot aperture radio telescope, the brightness temperature of Venus at this wavelength was found to be 410 K with an uncertainty of +30 and 200 probable error, which is in agreement with the center value obtained in an earlier, less exact, 1958 measurement.

Large radio telescopes and their use in radio astronomy

After referring to the need for directivity and sensitivity in radio astronomical investigations, the paper discusses the fundamental features of radio telescopes. Examples are then given of contemporary methods of obtaining directivity, using radio telescopes of various designs. A description is given of the 218-ft aperture radio telescope at Jodrell Bank. The results obtained with this instrument in the investigation of the radio emission from the Milky Way and also of the extragalactic radio emissions, are described. The paper concludes with a description of the large 250-ft aperture steerable radio telescope which is now being built at Jodrell Bank.