That is the minimum possible resolution* (hence the lower bound on the diameter). Atmospheric stuff will make it worse and be variable from day to day (and depe

That is the minimum possible resolution* (hence the lower bound on the diameter). Atmospheric stuff will make it worse and be variable from day to day (and depends on the length of the exposure, which would be a bigger problem looking at stars than the earth). I think that is around 2 microradians for astronomical telescopes. *there are tricks to get super-resolution, but ignore that stuff.