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

A. I. Boothroyd¹, K. Blagrave¹, F. J. Lockman², P. G. Martin¹, D. Pinheiro Gonçalves³ and S. Srikanth⁴

¹ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, Ontario, M5S 3H8, Canada
e-mail: boothroy@cita.utoronto.ca; blagrave@cita.utoronto.ca; pgmartin@cita.utoronto.ca
² National Radio Astronomy Observatory, PO Box 2, Green Bank, WV 24944, USA
e-mail: jlockman@nrao.edu
³ Department of Astronomy and Astrophysics, University of Toronto 50 St. George Street, Toronto, Ontario M5S 3H4, Canada
e-mail: goncalves@astro.utoronto.ca
⁴ NRAO Technology Center, 1180 Boxwood Estate Rd., Charlottesville, VA 22901, USA
e-mail: ssrikant@nrao.edu

Received: 7 July 2011
Accepted: 6 October 2011

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 × 10¹⁸ cm^-2.