Gain and Polarization Properties of a Large Radio Telescope from Calculation and Measurement: The John A. Galt Telescope

TL;DR

This study characterizes the gain and polarization properties of the John A. Galt Telescope through analysis and measurements, providing calibration data and insights into polarization performance for sky surveys at 1280-1750 MHz.

Contribution

It combines electromagnetic simulations and empirical measurements to evaluate aperture efficiency and polarization effects, offering a comprehensive calibration approach for the telescope.

Findings

Aperture efficiency varies smoothly between 0.49 and 0.54 across frequencies.

Simulations and measurements agree within 3% overall error.

Ground emission modeling helps estimate spurious polarized signals.

Abstract

Measurement of the brightness temperature of extended radio emission demands knowledge of the gain (or aperture efficiency) of the telescope and measurement of the polarized component of the emission requires correction for the conversion of unpolarized emission from sky and ground to apparently polarized signal. Radiation properties of the John A. Galt Telescope at the Dominion Radio Astrophysical Observatory were studied through analysis and measurement in order to provide absolute calibration of a survey of polarized emission from the entire northern sky from 1280 to 1750 MHz, and to understand the polarization performance of the telescope. Electromagnetic simulators CST and GRASP-10 were used to compute radiation patterns of the telescope in all Stokes parameters, and aperture efficiency. Aperture efficiency was also evaluated using geometrical optics and was measured using Cyg A. Measured aperture efficiency varied smoothly with frequency between values of 0.49 and 0.54; GRASP-10 yielded values 6.5% higher but with closely similar variation with frequency. Overall error across the frequency band is 3%, but values at any two frequencies are relatively correct to ~1%. Dominant influences on aperture efficiency are illumination taper of the feed radiation pattern and shadowing by the feed-support struts. A model of ground emission was developed based on measurements and on empirical data from remote sensing of the Earth from satellite-borne telescopes. This model was convolved with the computed antenna response to estimate conversion of ground emission into spurious polarized signal. The computed spurious signal is comparable to measured values, but is not accurate enough to be used to correct observations. A simpler model, in which the ground is considered as an unpolarized emitter with a brightness temperature of ~240 K, is shown to have useful accuracy when compared to measurements.