Calibration of Ooty Radio Telescope Module Response using Crab Nebula Transit Observations

Abstract

The Ooty Radio Telescope (ORT) is a large equatorially mounted cylindrical radio telescope operating near 326.5 MHz. Reliable module-to-module response assessment is necessary because gain variations can introduce systematic differences into long-duration transit observations. This study evaluates the relative response of selected ORT modules using repeated Crab Nebula transit measurements acquired over approximately five months. For each observing session, ON-source and OFF-source power sequences were aligned, visually inspected, converted to a logarithmic scale where required, and compared through peak excess and baseline behaviour. Time-series and module-wise plots were generated in GNUPLOT to identify repeatability, anomalous sessions, and relative gain differences. The processed observations consistently exhibited the expected rise–peak–fall transit morphology, while the measured peak excess varied across dates and modules. For the illustrated November subset, Module 21 showed values of approximately 1.59–3.28 dB, indicating measurable session-to-session variability. Because the available dataset does not include a fully documented absolute flux-density transfer, system-temperature measurement, uncertainty budget, or independent reference calibration, the analysis supports relative response normalisation rather than an absolute sensitivity determination in Jy K⁻¹ or system-equivalent flux density. The workflow provides a practical screening method for identifying comparatively weak or unstable modules and for selecting observations suitable for subsequent calibration. Future work should incorporate calibrated noise injection, contemporaneous weather and radio-frequency-interference logs, automated baseline fitting, uncertainty propagation, and comparison with a source of traceable flux density.