1) From the ground of the Earth, atmospheric turbulence limits the typical best image to a full-wide-at-half-maximum (FWHM) of ~0.5 arcsceond (this is an angle). Suppose we can determine positional accuracy of stars to ~1/10 of the FWHM, to how far in light years can we use the parallax method to measure the distance to stars? Now I am stuck on this question. My biggest trouble is that I don't know how to relate the 0.5 arcsceond to the parallax method or parallax angle. Can someone please help me out? Any help is greatly appreciated!