Green Bank Telescope: Overview and analysis of metrology systems and pointing performance. (arXiv:2111.12636v1 [astro-ph.IM])
<a href="http://arxiv.org/find/astro-ph/1/au:+White_E/0/1/0/all/0/1">E. White</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ghigo_F/0/1/0/all/0/1">F. D. Ghigo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Prestage_R/0/1/0/all/0/1">R. M. Prestage</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Frayer_D/0/1/0/all/0/1">D. T. Frayer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Maddalena_R/0/1/0/all/0/1">R. J. Maddalena</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wallace_P/0/1/0/all/0/1">P. T. Wallace</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Brandt_J/0/1/0/all/0/1">J. J. Brandt</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Egan_D/0/1/0/all/0/1">D. Egan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nelson_J/0/1/0/all/0/1">J. D. Nelson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ray_J/0/1/0/all/0/1">J. Ray</a>
With a 100mx110m off-axis paraboloid dish, the Green Bank Telescope (GBT) is
the largest fully steerable radio telescope on Earth. A major challenge facing
large ground-based radio telescopes is achieving sufficient pointing accuracy
for observing at high frequencies, up to 116 GHz in the case of the GBT.
Accurate pointing requires the ability to blindly acquire source locations and
perform ad hoc corrections determined by observing nearby calibrator sources in
order to obtain a starting position accurate to within a small margin of error
of the target’s location. The required pointing accuracy is dependent upon the
half-power beamwidth, and for the higher-frequency end of GBT observing, this
means that pointing must be accurate to within a few arcseconds RMS. The GBT’s
off-axis design is advantageous in that it eliminates blockage of the dish and
reduces sidelobe interference, and there is no evidence that the resulting
asymmetric structure adversely affects pointing accuracy. However, factors such
as gravitational flexure, thermal deformation, azimuth track tilt and
irregularity, and small misalignments and offset errors within the telescope’s
structure cause pointing inaccuracies. A pointing model was developed for the
GBT to correct for these effects. The model utilizes standard geometrical
corrections along with metrology data from the GBT’s structural temperature
sensors and data from measurements of the track levels. In this paper we
provide a summary of the GBT’s pointing model and associated corrections, as
well as a discussion of relevant metrology systems and an analysis of its
current nighttime pointing accuracy.
With a 100mx110m off-axis paraboloid dish, the Green Bank Telescope (GBT) is
the largest fully steerable radio telescope on Earth. A major challenge facing
large ground-based radio telescopes is achieving sufficient pointing accuracy
for observing at high frequencies, up to 116 GHz in the case of the GBT.
Accurate pointing requires the ability to blindly acquire source locations and
perform ad hoc corrections determined by observing nearby calibrator sources in
order to obtain a starting position accurate to within a small margin of error
of the target’s location. The required pointing accuracy is dependent upon the
half-power beamwidth, and for the higher-frequency end of GBT observing, this
means that pointing must be accurate to within a few arcseconds RMS. The GBT’s
off-axis design is advantageous in that it eliminates blockage of the dish and
reduces sidelobe interference, and there is no evidence that the resulting
asymmetric structure adversely affects pointing accuracy. However, factors such
as gravitational flexure, thermal deformation, azimuth track tilt and
irregularity, and small misalignments and offset errors within the telescope’s
structure cause pointing inaccuracies. A pointing model was developed for the
GBT to correct for these effects. The model utilizes standard geometrical
corrections along with metrology data from the GBT’s structural temperature
sensors and data from measurements of the track levels. In this paper we
provide a summary of the GBT’s pointing model and associated corrections, as
well as a discussion of relevant metrology systems and an analysis of its
current nighttime pointing accuracy.
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