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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:
Broad bandwidth also needed for sensitivity to thermal continuum and phase calibration
7.Limitations of existing and future arrays
Small FoV mosaicing: FWHM of 10 m antenna @ 230 GHz is ~ 30’’
Limited uv-coverage, small number of elements (improved with CARMA, remedied with ALMA)
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
Broad bandwidth also needed for sensitivity to thermal continuum and phase calibration
7.Limitations of existing and future arrays
Small FoV mosaicing: FWHM of 10 m antenna @ 230 GHz is ~ 30’’
Limited uv-coverage, small number of elements (improved with CARMA, remedied with ALMA)
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