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Description
Precision calibration is fundamental in radio astronomy, as the accurate conversion of instrumental signals into physical units underpins reliable measurements of celestial radio sources. Among the available calibration strategies, internal noise diodes provide a stable and repeatable reference for determining receiver gain, system temperature, and flux density scaling.
In this study, we present a comprehensive noise diode–based calibration analysis for precise flux density measurements of the radio galaxy Virgo A (M87) observed at 6.75 GHz with the 32 m Ghana Radio Telescope. A standard three-state calibration sequence—comprising zero-signal, cold-sky, and noise-diode reference levels—was implemented to derive the system temperature ($T_\mathrm{sys}$), calibration temperature ($T_\mathrm{cal}$), and source antenna temperature ($T_\mathrm{source}$) for both left and right circular polarizations (LCP and RCP). The measured noise diode temperatures were 4.79 K (LCP) and 4.99 K (RCP), corresponding to conversion factors of $4\times10^{-6}$ K count$^{-1}$ and $6\times10^{-6}$ K count$^{-1}$, respectively. System temperatures of 80.55 K (LCP) and 85.88 K (RCP), averaging 83 K, indicate stable receiver performance.
Drift-scan observations of Virgo A produced peak antenna temperatures of 4.06 K (LCP) and 4.24 K (RCP). Using the telescope’s effective collecting area of 619.3 m² and an aperture efficiency of 0.77, the corresponding flux densities were $18.09\pm0.15$ Jy (LCP) and $18.92\pm0.22$ Jy (RCP), giving a total flux of $37.01\pm0.27$ Jy at 6.75 GHz. The low polarization fraction of 2.2 % indicates minimal instrumental or intrinsic polarization.
These results demonstrate that the Ghana 32 m telescope achieves calibration precision at the ∼1 % level, comparable to leading international facilities. The consistency with standard flux scales validates the noise diode calibration method and establishes a robust framework for future continuum, polarization, and variability studies of active galactic nuclei within the African VLBI Network (AVN) and the broader SKA ecosystem.
| Stream | Science or Engineering |
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