The GBT at KBand Current Receiver n Two
The GBT at K-Band
Current Receiver n 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 ¨ n n Dual Polarizations FWHM Beamwidth : 34” at 22 GHz Feed separation ~ 3 arcmin (~5 beamwidths) in cross elevation Instantaneous Bandwidth : 4 GHz
Backends n DCR ¨ ¨ n Spectral Processor ¨ n Pointing and focus Mapping strong sources (1/F noise) Very limited -- narrow bandwidths, few spectral windows, coarse resolution Spectrometer ¨ 800 MHz n n n ¨ 200 MHz n n n ¨ 1 or 2 spectral windows Velocity Coverage per window : 2700 at 22 GHz Velocity Resolution : 0. 08 to 0. 3 km/s 50 MHz n n n ¨ 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 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 n n n 1, 2, or 4 spectral windows Velocity Coverage per window : 170 km/s Velocity Resolution : 0. 001 to 0. 04 km/s
Observing n Doppler Tracking ¨ ¨ n Switching modes ¨ ¨ n Hardware – 1 st spectral window Software – all others 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 n n 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 n Opacity ¨ ¨ ¨ Raises Tsys Reduces TA Effective System Temperature n n n Winds ¨ n Tsys’ = Tsys * exp(Tau*Air. Mass) Proportional to sqrt(integration time) Reduces effective aperture efficiency Day/Nightime ¨ Differential Heating/Cooling n ¨ Focus/Pointing/Surface Errors Not an issue for K-band
Precipitable Water/Cloud Statistics
Atmospheric Conditions - Opacity
System Temperatures 25% Weather Conditions
Forecasting
Relative Effective Tsys n Effective System Temperature ¨ n Tsys’ = Tsys * exp(Tau*Air. Mass) sqrt(integration time) Relative Effective Tsys = REST = Tsys’/Tsys’Best=sqrt(t/t 0)
Winds for a 5% loss of Efficiency
RFI 22. 21 – 22. 5 GHz : Shared protected band n 17. 7 – 20. 2 GHZ n ¨ Fixed Satellite Service ¨ Fixed Terrestrial Service ¨ Mobile Satellite Service n Car Avoidance Radar
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