Holographic Measurement and Improvement of the Green Bank Telescope Surface

Abstract

We describe the successful design, implementation, and operation of a 12 GHz holography system installed on the Robert C. Byrd Green Bank Telescope (GBT). We have used a geostationary satellite beacon to construct high-resolution holographic images of the telescope mirror surface irregularities. These images have allowed us to infer and apply improved position offsets for the 2209 actuators which control the active surface of the primary mirror, thereby achieving a dramatic reduction in the total surface error (from 390 microns to ~240 microns, rms). We have also performed manual adjustments of the corner offsets for a few panels. The expected improvement in the radiometric aperture efficiency has been rigorously modeled and confirmed at 43 GHz and 90 GHz. The improvement in the telescope beam pattern has also been measured at 11.7 GHz with greater than 60 dB of dynamic range. Symmetric features in the beam pattern have emerged which are consistent with a repetitive pattern in the aperture due to systematic panel distortions. By computing average images for each tier of panels from the holography images, we confirm that the magnitude and direction of the panel distortions, in response to the combination of gravity and thermal gradients, are in general agreement with finite-element model predictions. The holography system is now fully integrated into the GBT control system, and by enabling the telescope staff to monitor the health of the individual actuators, it continues to be an essential tool to support high-frequency observations.

Accepted for publication in PASP. Contains 28 pages with 2 tables and 9 figures (several at reduced quality). The full resolution version is available at http://wwwlocal.gb.nrao.edu/ptcs/papers/Hunter2011/gbtholo.ps.gz (34MB gzip file unpacks to 134MB postscript)

References in corpus (5)

• The On The Fly Imaging Technique
• MUSTANG High Angular Resolution Sunyaev-Zel'dovich Effect Imaging of Sub-Structure in Four Galaxy Clusters
• Out-Of-Focus Holography at the Green Bank Telescope
• Measurement of Antenna Surfaces from In- and Out-Of-Focus Beam Maps using Astronomical Sources
• MUSTANG 3.3 Millimeter Continuum Observations of Class 0 Protostars

Cited by in corpus (11)

• Green Bank Telescope: Overview and analysis of metrology systems and pointing performance
• The conceptual design of the 50-meter Atacama Large Aperture Submillimeter Telescope (AtLAST)
• Confirmation of Enhanced Long Wavelength Dust Emission in OMC 2/3
• The GBT 67 -- 93.6 GHz Spectral Line Survey of Orion-KL
• Evaluating a strategy for measuring deformations of the primary reflector of the Green Bank telescope using a terrestrial laser scanner
• Antennas and Receivers in Radio Interferometry
• Tied-array holography with LOFAR
• The Simons Observatory: HoloSim-ML: machine learning applied to the efficient analysis of radio holography measurements of complex optical systems
• The National Science Foundation's AST Portfolio Review of 2012 is Not Relevant to the Green Bank Telescope of 2017: A White Paper
• The GBT Gain Curve at High Frequency
• Maser observations with new instruments