冒牌自然老師

1.Atmospheric opacity significant λ<1cm: raises Tsys and attenuates source
      Opacity varies with frequency and altitude
      Gain calibration must correct for opacity variations
2.Atmospheric phase fluctuations
      Cause of the fluctuations: variable H2O
      Calibration schemes must compensate for induced loss of visibility amplitude (coherence)   and spatial resolution (seeing)
3.Antennas
      Pointing accuracy measured as a fraction of the primary beam is more difficult to achieve:    PB ~ 1.22 l/D
      Need more stringent requirements than at cm wavelengths for: surface accuracy & baseline       determination
4.Instrument stability
      Must increase linearly with frequency (delay lines, oscillators, etc…)
5.Millimeter/sub-mm receivers
      SIS mixers needed to achieve low noise characteristics
      Cryogenics cool receivers to a few K
      IF bandwidth
6.Correlators
      Need high speed (high bandwidth) for spectral lines:                               

      ΔV = 300 km s-1  1.4 MHz @ 1.4 GHz, 230 MHz @ 230 GHz