A detailed study of the polarisation convention of the Giant Metrewave Radio Telescope

TL;DR

This study investigates the polarisation convention of the GMRT, revealing that due to dish reflection, the circular polarisation sense is reversed, necessitating a correction to align with standard IAU/IEEE conventions.

Contribution

The paper provides a detailed analysis of GMRT's polarisation convention and proposes a correction method for its circular feeds to match standard conventions.

Findings

GMRT channels 1 and 2 are true R and L polarisation.

Dish reflection reverses the sense of circular polarisation in GMRT.

Correction involves reassigning channels 1 and 2 to L and R, respectively.

Abstract

Our work aims to investigate the polarisation convention of the Giant Metrewave Radio Telescope (GMRT) radio telescope and understand whether the telescope follows the standard IAU/IEEE convention. The GMRT antennas are prime focus antennas, i.e. the radiation falling on the antenna feed reverses its circular polarisation. If this reflection is not taken into account, it will result in a reversal of circular polarisation. We carried out several tests to understand the GMRT polarisation convention. The observations were carried out on several strong and highly polarised pulsars with known polarisation properties at GMRT wavelengths, mainly covering frequency bands 2, 3 and 4. In addition, we tracked the signal from the feed to the fibre optic system, and fibre optic system to the user end. We also tracked satellites of known polarisations as well as utilised right and left circularly polarised helical antennas to study the polarisation convention. Our study shows that the GMRT channels 1 and 2 are true R and L, however, GMRT being a prime focus instrument, the reflection due to the dish reverses the sense of polarisation and converts the right circular polarisation (RCP) into left circular polarisation (LCP) and vice versa. This has not been taken into account and thus the GMRT Stokes V and U signs need to be reversed to make them consistent with the IAU/IEEE convention. This objective can be achieved by reassigning channel 1 to L and channel 2 to R for all circular feeds, i.e. bands 2, 3, and 4. The study remains inconclusive for the GMRT band 5, which has linear feeds.