The GBT at KBandK波段GBT.pptVIP

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2018-06-19 发布于福建
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The GBT at KBandK波段GBT.ppt

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The GBT at KBandK波段GBT

The GBT at K-Band Current Receiver Two Dual-Feed Receivers in a single dewar 18.0 – 22.4 GHz 22.0 – 26.5 GHz Feeds arranged in a square Current cabling prevents using as a 4-feed 22-22.4 GHz Rcvr Dual Polarizations FWHM Beamwidth : 34” at 22 GHz Feed separation ~ 3 arcmin (~5 beamwidths) in cross elevation Instantaneous Bandwidth : 4 GHz Backends DCR Pointing and focus Mapping strong sources (1/F noise) Spectral Processor Very limited -- narrow bandwidths, few spectral windows, coarse resolution Spectrometer 800 MHz 1 or 2 dual-polarization spectral windows Velocity Coverage per window : 11000 at 22 GHz Velocity Resolution : 1.3 to 5 km/s at 22 GHz 200 MHz 1 or 2 spectral windows Velocity Coverage per window : 2700at 22 GHz Velocity Resolution : 0.08 to 0.3 km/s 50 MHz 1, 2, or 4 spectral windows Velocity Coverage per window : 680 km/s Velocity Resolution : 0.005 to 0.17 km/s 12.5 MHz 1, 2, or 4 spectral windows Velocity Coverage per window : 170 km/s Velocity Resolution : 0.001 to 0.04 km/s Observing Doppler Tracking Hardware – 1st spectral window Software – all others Switching modes Total power Frequency switching (few MHz) NOD – using telescope drives Subreflector Nodding (not yet deployed) Observing types On-Off Track On-The-Fly (crosses, rectangular maps, strips, …) Point maps Antenna Active Surface using an elevation-dependent lookup table 390 micron surface accuracy Aperture Efficiency: 65% - 58% at 18 and 26.5 GHz Beam Efficiency : 89% – 79% Gain : 1.8 – 1.6 K/Jy Probably flat with elevation Atmospheric Conditions Opacity Raises Tsys Reduces TA Effective System Temperature Tsys’ = Tsys * exp(Tau*AirMass) Proportional to sqrt(integration time) Winds Reduces effective aperture efficiency Day/Nightime Differential Heating/Cooling Focus/Pointing/Surface Errors Not an issue for K-band Precipitable Water/Cloud Statistics Atmospheric Conditions - Opacity System Temperatures Forecasting Relative Effective Tsys Effective System Temperature Tsys’ = Tsys * exp(Tau*A