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The GBT at K-Band

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

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The GBT at K-Band

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  1. The GBT at K-Band

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. Precipitable Water/Cloud Statistics

  8. Atmospheric Conditions - Opacity

  9. System Temperatures 25% Weather Conditions

  10. Forecasting

  11. Relative Effective Tsys • Effective System Temperature • Tsys’ = Tsys * exp(Tau*AirMass)  sqrt(integration time) • Relative Effective Tsys = REST = Tsys’/Tsys’Best=sqrt(t/t0)

  12. Winds for a 5% loss of Efficiency

  13. RFI • 22.21 – 22.5 GHz : Shared protected band • 17.7 – 20.2 GHZ • Fixed Satellite Service • Fixed Terrestrial Service • Mobile Satellite Service • Car Avoidance Radar

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