2 research outputs found
Holographic Measurement and Improvement of the Green Bank Telescope Surface
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.Comment: 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