Millimeter Wavelength
VLBI at millimeter wavelengths becomes progressively more challenging as the wavelength gets shorter. Demands on the stability of the frequency standards grow more stringent, the sensitivity of available telescopes and receivers grows poorer, the need for good weather grows more acute, the phase fluctuations due to the atmosphere grow more severe, and in many cases, the strength of the target sources on the sky grows less. The scientific reward is ever increasing angular resolution, and the ability to observe phenomena inaccessible to any other technique.
Some of these problems, such as atmospheric stability and opacity, are fundamental. Their effects, however, can be mitigated by boosting the sensitivity of the VLBI system above the threshold where the effects of the atmosphere can be calibrated out. This requires achieving a minimum signal to noise ratio in the shortest possible time, before atmospheric fluctuations have a chance to smear out the signal.We can achieve this by increasing the bandwidth of the observations, through increases in the rate at which data are sampled and recorded. At short wavelengths and high frequencies, large sky bandwidths are readily available from the antenna, so there is much scope for technological progress, recording rate improvements, and millimeter VLBI sensitivity gains.
Opening up high-sensitivity millimeter VLBI has been and continues to be a long term goal at Haystack. In coming years, multiple key developments will together lead to implementation of extreme bandwidths for high-value experiments at the highest frequencies.
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