The 550 AU Mission: A Critical Discussion
  We have studied the science rationale, goals and requirements for a mission aimed at using the gravitational lensing from the Sun as a way of achieving high angular resolution and high signal amplification. We find that such a mission concept is plagued by several practical problems. Most severe are the effects due to the plasma in the solar atmosphere which cause refraction and scattering of the propagating rays. These effects limit the frequencies that can be observed to those above $\sim$100-200 GHz and moves the optical point outwards beyond the vacuum value of $\geq$ 550 AU. Density fluctuations in the inner solar atmosphere will further cause random pathlength differences for different rays. The corrections for the radiation from the Sun itself will also be a major challenge at any wavelength used. Given reasonable constraints on the spacecraft (particularly in terms of size and propulsion) source selection as well as severe navigational constraints further add to the difficulties for a potential mission. 
Slava G. Turyshev and B-G Andersson