Accurate Galactic 21-cm H I measurements with the NRAO Green Bank Telescope

TL;DR

This paper presents a comprehensive calibration system for the Green Bank Telescope that significantly reduces systematic errors in 21-cm H I measurements, enabling highly accurate Galactic hydrogen spectra at 9 arcminute resolution.

Contribution

The study introduces a detailed all-sky response model and calibration procedures that correct for sidelobe contamination, improving measurement accuracy over previous methods.

Findings

Reduces systematic errors in 21-cm measurements by about tenfold.

Achieves 3 K km s^-1 accuracy in integrated H I line emission measurements.

Enables precise determination of Galactic H I column densities.

Abstract

Aims: We devise a data reduction and calibration system for producing highly-accurate 21-cm H I spectra from the Green Bank Telescope (GBT) of the NRAO. Methods: A theoretical analysis of the all-sky response of the GBT at 21 cm is made, augmented by extensive maps of the far sidelobes. Observations of radio sources and the Moon are made to check the resulting aperture and main beam efficiencies. Results: The all-sky model made for the response of the GBT at 21 cm is used to correct for "stray" 21-cm radiation reaching the receiver through the sidelobes rather than the main beam. This reduces systematic errors in 21-cm measurements by about an order of magnitude, allowing accurate 21-cm H I spectra to be made at about 9' angular resolution with the GBT. At this resolution the procedures discussed here allow for measurement of total integrated Galactic H I line emission, W, with errors of 3 K km s^-1, equivalent to errors in optically thin N_HI of 5 x 10^18 cm^-2.