Sensitivity analysis of optical systems in radio telescopes

Abstract

We have demonstrated an approach to predict the overall efficiency and system noise regarding optical systems in radio telescopes, based on a systematic evaluation of both the optical propagation efficiency through all optical components, from the receiver horn to the main reflector, and of their noise contributions. The analysis employs several conventional techniques, including the principle of multimode Gaussian optics and calculations of losses associated with optical components. A detailed analysis was performed for cases of ALMA band 4 (125-163 GHz) and 8 (385-500GHz). The analysis predicts a reasonable efficiency of 0.75-0.76 (0.57-0.62) and a system noise of ∼ 70 K (300-400 K) for band 4 (band 8). The calculated efficiency of the band 4 optics was found to be consistent with the efficiency measured by astronomical observations with the ALMA/ACA 12-m antenna. In addition to a confirmation that the bands 4 and 8 optics were designed to maximize the overall efficiency, the analysis for the bands 4 and 8 frequency ranges also confirmed that there is no difference in the optimum edge levels at the subreflector between the two different ways of maximization, i.e., toward the overall efficiency or the sensitivity, which is defined by the ratio of the overall efficiency over the sum of noises generated from all possible sources. We also applied a sensitivity analysis to the band 1 frequency case, and revealed a slight, but clear, difference in the optimum edge levels between the sensitivity and the efficiency (difference is 1-2 dB). This corresponds to a sensitivity loss of 1% if we optimize the optics to maximize the efficiency. © 2010. Astronomical Society of Japan.