Motivation
EHT observations of Sgr A* and M87 to date have been carried out at 230 GHz (1.3 mm wavelength). We expect to add 345 GHz (0.87 mm) as an observing frequency in the near future.
The effort to extend millimeter VLBI observations to higher frequencies is motivated by several factors:
Scatter broadening: Scattering due to to interstellar electrons causes objects in the Galactic center to appear blurred. At traditional VLBI frequencies of 86 GHz (3.5 mm) and lower, the measured size of Sgr A* is dominated by scattering, obscuring details of the accretion flow. In contrast, the measured size of the 230 GHz emission (43 μas) is only slightly larger than the inferred actual size (37 μas), since the interstellar scattering scales as ν-2. Nevertheless, the anisotropic scattering obscures details smaller than about twice the size of the Schwarzschild radius for Sgr A*. Scatter broadening will be smaller by more than a factor of 2 at 345 GHz and negligible at higher frequencies. reference
Optical depth effects: The intrinsic size of the emission from Sgr A* and M87 is observed to decrease as the observing frequency increases due to the optical depth of the synchrotron emission in the plasma. While 230 GHz is a sufficiently high frequency to observe the inner accretion flow in Sgr A* (left), observations at 345 GHz would permit a deeper view of the accretion flow as it is lensed around the photon orbit (right).
Resolution: The angular resolution of a baseline is given by λ/B, where B is the projected separation between the antennas. The telescopes in the EHT array cannot easily be moved to other locations, and in any case the size of the Earth imposes a maximum baseline length unless very expensive space-based radio telescopes are launched. Going from 230 to 345 GHz will effectively increase the resolution of the EHT by a factor of 1.5, providing a resolution as small as 15 μas (1.5 Schwarzschild radii) for Sgr A*.
Challenges
Figure: Atmospheric transparency at Mauna Kea with 1 and 5 mm of precipitable water vapor in the atmosphere.
Weather: Water vapor in the atmosphere absorbs radiation from cosmic sources and causes the sky to appear brighter, leading to loss of sensitivity. Fluctuations in tropospheric water vapor also cause rapid phase variations that lead to decoherence. Both of these effects increase with observing frequency. Most EHT sites have suitable weather for 345 GHz observing under normal winter conditions.
Frequency stability: Hydrogen masers are typically used as the frequency standard to downconvert signals from the observing frequency to the gigahertz range where they are processed. Drifts in the frequency standard cause signal loss, with higher loss at higher observing frequencies. The EHT is investigating the suitability of better frequency standards including modern, ultrastable hydrogen masers and cooled sapphire oscillators. reference at http://adsabs.harvard.edu/abs/2011PASP..123..582D