Astronomy & Astrophysics

Free access article

A&A 465, 685-693 (2007)
DOI: 10.1051/0004-6361:20065765

Out-of-focus holography at the Green Bank Telescope

B. Nikolic^{1, 2}, R. M. Prestage¹, D. S. Balser¹, C. J. Chandler³, and R. E. Hills⁴

¹ National Radio Astronomy Observatory, PO Box 2, Green Bank, WV 24944, USA
² National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, Va 22903, USA
e-mail: bnikolic@nrao.edu
³ National Radio Astronomy Observatory, PO Box 0, Socorro, NM 87801, USA
⁴ Mullard Radio Astronomy Observatory, Cavendish Laboratory, Cambridge CB3 0HE, UK

(Received 6 June 2006 / Accepted 4 January 2007)

Abstract
We describe phase-retrieval holography measurements of the 100-m diameter Green Bank Telescope using astronomical sources and an astronomical receiver operating at a wavelength of 7 mm. We use the technique with parameterization of the aperture in terms of Zernike polynomials and employing a large defocus, as described by Nikolic et al. (2007, A&A, 465, 679). Individual measurements take around 25 min and from the resulting beam maps (which have peak signal to noise ratios of 200:1) we show that it is possible to produce low-resolution maps of the wavefront errors with accuracy around $\lambda/100$ . Using such measurements over a wide range of elevations, we have calculated a model for the wavefront-errors due to the uncompensated gravitational deformation of the telescope. This model produces a significant improvement at low elevations, where these errors are expected to be the largest; after applying the model, the aperture efficiency is largely independent of elevation. We have also demonstrated that the technique can be used to measure and largely correct for thermal deformations of the antenna, which often exceed the uncompensated gravitational deformations during daytime observing. We conclude that the aberrations induced by gravity and thermal effects are large-scale and the technique used here is particularly suitable for measuring such deformations in large millimetre wave radio telescopes.

Key words: telescopes

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