Title: The GBT Beam Shape at 109 GHz,
Abstract: With the installation of the Argus 16-pixel receiver covering 75-115 GHz on
the Green Bank Telescope (GBT), it is now possible to characterize the antenna
beam at very high frequencies, where the use of the active surface and
out-of-focus holography are critical to the telescope's performance. A recent
measurement in good weather conditions (low atmospheric opacity, low winds, and
stable night-time thermal conditions) at 109.4 GHz yielded a FWHM beam of
6.7"x6.4" in azimuth and elevation, respectively. This corresponds to
1.16+/-0.03 Lambda/D at 109.4 GHz. The derived ratio agrees well with the
low-frequency value of 1.18+/-0.03 Lambda/D measured at 9.0 GHz. There are no
detectable side-lobes at either frequency. In good weather conditions and after
applying the standard antenna corrections (pointing, focus, and the active
surface corrections for gravity and thermal effects), there is no measurable
degradation of the beam of the GBT at its highest operational frequencies.